Role of PVC in Swirl Combustion Systems

A review of oscillation mechanisms and the role of the precessing

vortex core (PVC) in swirl combustion systems

Nicholas Syred *

School of Engineering, Cardiff University, Queens Buildings, The Parade, Cardiff, Wales CF24 0YF, UK

Received 3 September 2004; accepted 13 October 2005

Available online 19 January 2006

Abstract

This paper reviews the occurrence of the precessing vortex core (PVC) and other instabilities, which occur in, swirl combustion

systems whilst identifying mechanisms, which allows coupling between the acoustics, combustion and swirling flow dynamics to

occur.

Initially, the occurrence of the PVC in free and confined isothermal flows is reviewed by describing its occurrence in terms of a

Strouhal number and geometric swirl number. Phase locked particle image velocimetry and laser doppler anemometry is then used

to describe the three-dimensional flow fields, which are generated when swirling flow is discharged into an open environment. This

shows the presence of a rotating and precessing off centred vortex and associated central recirculation zone (CRZ), extending up to

one burner exit diameter. The presence of axial radial eddies close to the burner mouth, in and around the CRZ, is clearly shown.

Typically one large dominant PV is found, although many harmonics can be present of lower amplitude. The occurrence of these

phenomena is very much a function of swirl number and burner geometry.

Under combustion conditions the behaviour is more complex, the PVC occurrence and amplitude are also strong functions

of mode of fuel entry, equivalence ratio and level of confinement. Axial fuel entry, except at exceptionally weak mixture ratios,

often suppresses the vortex core precession. A strong double PVC structure is also found under certain circumstances.

Premixed or partially premixed combustion can produce large PVC, similar in structure to that found isothermally: this is

attributed to the radial location of the flame front at the swirl burner exit. Provided the flame is prevented from flashing back to the

inlets values of Strouhal number for the PVC were excited by w2 compared to the isothermal condition at equivalence ratios

around 0.7. Confinement caused this parameter to drop by a factor of three for very weak combustion.

Separate work on unconfined swirling flames shows that even when the vortex core precession is suppressed the resulting

swirling flames are unstable and tend to wobble in response to minor perturbations in the flow, most importantly close to the burner

exit.

Another form of instability is shown to be associated with jet precession, often starting at very low or zero swirl numbers. Jet

precession is normally associated with special shapes of nozzles, large expansions or bluff bodies and is a different phenomenon to

the PVC. Strouhal numbers are shown to be at least an order of magnitude less than those generated by the PVC generated after

vortex breakdown.

Oscillations and instabilities in swirl combustion systems are illustrated and analysed by consideration of several cases of stable

oscillations produced in swirl burner/furnace systems and two where the PVC is suppressed by combustion. The first cases is a low

frequency 24 Hz oscillation produced in a 2 MW system whereby the PVC frequency is excited to nearly six times that for the

isothermal case due to interaction with system acoustics. Phase locked velocity and temperature measurements show that the flame

is initiated close to the burner exit, surrounding the CRZ, but is located inside a ring of higher velocity flow. Downstream the flame

has expanded radially past the high velocity region, but does not properly occupy the whole furnace. This allows the flame and

Progress in Energy and Combustion Science 32 (2006) 93–161

www.elsevier.com/locate/pecs

0360-1285/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.pecs.2005.10.002

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N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–16194

swirling flow to wobble, exciting instability. The next family of oscillations reviewed occur in a 100 kW swirl burner/furnace

systems whereby oscillations in the w40 Hz range are excited with flow fields akin to those found in pulsating combustors where

the flow is periodically stopped in the limit cycle of oscillation. The phase locked velocity and temperature measurements show a

number of mechanisms that can excite oscillation including substantial variations in shape and size of the CRZ during the limit

cycle of oscillation, and wobble of the whole flame and flow as shown by negative tangential velocities close to the centre line.

Analysis is then made of a high frequency w240 Hz oscillation in the same 100 kW swirl burner/furnace system, this oscillation

being caused by minor geometry changes. The flame was shown to not fully occupy the furnace, allowing irregular wobble and

precession of the flow and flame to develop, being especially noticeable close to the outer wall. The addition of an exit quarl to the

swirl burner is shown to substantially reduce the amplitude of oscillation by eliminating the external recirculation zone (ERZ),

reducing flow/flame wobble and variations in the size and shape of the CRZ. The quarl used was designed to largely occupy the

space normally taken up by the ERZ.

Two gas turbine combustor units firing into chambers are then considered, strong PVCs are developed under isothermal

conditions, these are suppressed with premixing in the equivalence number range 0.5–0.75. PVC suppression is attributed to the

equivalence ratios used, the burner configuration, location of the flame front and associated combustion aerodynamics. Other work

on an industrial premixed gas turbine swirl burner and can showed the formation of strong helical coherent structures for

equivalence ratios greater than 0.75. LES studies showed the PVC contributed to instability by triggering the formation of radial

axial eddies, generating alternating patterns of rich and lean combustion sufficient to reinforce combustion oscillations via the

Rayleigh criteria.

Finally, it was concluded that coupling between the acoustics and flame/flow dynamics occurs through a number of mechanisms

including wobble/precession of the flow and flame coupled with variations in the size and shape of the CRZ arising from changes in

swirl number throughout the limit cycle. Remedial measures are proposed.

q 2005 Elsevier Ltd. All rights reserved.

Keywords: Swirl combustors; Precessing vortex core (PVC); Reverse flow zones; Oscillation mechanisms

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

2. Vortex core and jet precession . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

2.1. General characteristics of the PVC under isothermal conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

2.2. Effect of confinement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

2.3. Precessing jets and jet burners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

3. Combustion and the PVC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

4. Vortex breakdown, modelling of the PVC and related phenomena, comparison with experiment . . . . . . . . . . . . . 126

5. Oscillations in swirl burner furnace systems, related systems and associated driving mechanisms . . . . . . . . . . . . 135

5.1. Driven PVC oscillations in the 2 MW swirl burner/furnace system, 100% axial fuel entry . . . . . . . . . . . . 135

5.2. Helmholtz and other resonances and vortex wobble /precession in a 100 kW swirl burner/furnace system,

partial premixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140

5.3. Characterisation of high frequency oscillations in a 100 kW swirl burner furnace system, partial

premixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

5.4. Combustion oscillations in a swirl burner combustion chamber systems and suppression of the PVC . . . . 148

5.4.1. Instabilities generated in industrial premixed gas turbine combustor systems . . . . . . . . . . . . . . . 149

6. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154

6.1. Interaction between the above effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158

7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159

1. Introduction

The use of swirl-stabilised combustion is wide-

spread, including power station burners, gas turbine

combustors, internal combustion engines, refinery and

process burners [1]. The mechanisms and benefits of

swirl stabilised combustion are well documented and

depend in most instances on the formation of a central

toroidal recirculation zone which recirculates heat and

active chemical species to the root of the flame, allows

flame stabilisation and flame establishment to occur

in regions of relative low velocity where flow and the

Nomenclature

A constant in modified Strouhal number for PJ

burner

B constant in modified Strouhal number for PJ

burner

CRZ central recirculation zone formed by swir-

ling flow

d inlet orifice diameter of precessing jet

burner

D upstream PJ inlet orifice diameter, mm

De exhaust diameter of swirl burner, m

Dfe diameter of central exhaust of furnace

Do diameter of furnace or confinement vessel,

m

ERZ external recirculation zone

f frequency

Iis intensity of oscillation, w/cm2, derived from

pressure measurements for the isothermal

PVC

I intensity of oscillation relative to Iis, the

value for the isothermal PVC

Isothermal used to describe operation of a swirl

combustor without combustion where the

unit is fed air at ambient temperature

k kinetic energy of turbulence

LES large eddy simulation modelling

LDA laser Doppler anemometry

Lcontract length of contraction nozzle on end of

furnace

Lf flame length

Lfurn length of parallel section of furnace in swirl

burner furnace

Linlet length of inlet duct to swirl burner furnace

LPC lean premixed combustion

mair mass flowrate of air

NGV nozzle guide vanes in gas turbines

~p pressure

PIV particle image velocimetry

PJ precessing jet as characterised at University

of Adelaide

PVC precessing vortex core

Q volumetric flowrate, m3/s

QH thermal input, kW

r radius, m

re exit radius, m

ro radius of furnace or confinement, m

rs radius of bluff body in exit of Sydney swirl

burner, m

r* r/ro

rms root mean square of a signal

RANS reynolds averaged Navier–Stokes

modelling

Re reynolds number

Ri modified richardson number, (1/r)((r/(r)

(W2/r))/(U/(r)2

S swirl number, unless stated otherwise,

always derived from device geometry,

defined as ratio of axial flux of angular

momentum to axial flux of axial momentum,

non-dimentionalised by the exhaust radius

Scr critical swirl number when direction of

precession changes

Sg swirl number for the Sydney swirl burner—

ratio of integrated bulk tangential to

primary bulk axial air velocities measured

via LDA just above burner exit annulus.

The geometric swirl number S is 90% of this

value

SSN Strouhal number for the Sydney swirl

burner, 2frs/Ws

Strouhal number the common definition, fDe/ub, is

used throughout. ub is derived from the

isothermal burner flowrate and is based on

the burner exhaust area. Where the original

data used the definition fDe3/Q, this has

been converted

PJ Strouhal fd/ub—one definition of Strouhal num-

ber for the PJ nozzle number

u axial velocity, m/s

ub average bulk burner exit axial velocity, Q

outlet area available for flow (isothermal

conditions assumed), m/s

ub bulk flow velocity through the PJ inlet

orifice, m/s.

VBD vortex breakdown

w tangential velocity at a specific radius r, m/s

Ws bulk or average tangential velocity as

measured by LDA in exhaust annulus of

Sydney swirl burner, m/s

x axial distance

x 0 axial distance from exit of PJ nozzle

F equivalence ratio

g directional intermittency, % of negative

samples in ‘bin’ used to collect velocity

samples from LDA

3 turbulence dissipation rate

r gas density, m3/kg

N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 95

Fig. 1.1. Schematic diagram of processing leading to CRZ formation [1]: (1) tangential velocity profile creates a centrifugal pressure gradient and

sub-atmospheric pressure near the central axis; (2) axial decay of tangential velocity causes decay of radial distribution of centrifugal pressure

gradient in axial direction; (3) thus, an axial pressure gradient is set up in the central region towards the swirl burner, causing reverse flow.


turbulent flame velocity can be matched, aided by the

recirculation of heat and active chemical species [1,2].

These processes are illustrated in Fig. 1.1 and arise as

follows:

– Swirling flow generates a natural radial pressure

gradient due to the term w2/r.

– Expansion through a nozzle causes axial decay of

tangential velocity and hence radial pressure

gradient.

– This in turn causes a negative axial pressure gradient

to be set up in the vicinity of the axis, which in turn

induces reverse flow and the formation of a CRZ.

– Where the tangential velocity distribution is of

Rankine form [1] (i.e. free/forced vortex combi-

nation), the central vortex core can become unstable,

giving rise to the PVC phenomena.

– The formation of the CRZ is thus dependent on

the decay of swirl velocity as swirling flow expands.

A typical toroidal recirculation zone formed at the

exhaust of a swirl burner is shown in Fig. 1.2 for a swirl

number of 1.57 and shows the large bubble of time

mean recirculated flow that is formed with here 12% of

the flow being recirculated [3].

With confinement this process is modified, the rate

of decay of swirl velocity is considerably reduced,

hence the size and strength of the CRZ formed [1,2].

This is illustrated by results from a swirl burner

furnace system for the combustion of low calorific

values gases from carbon black plants [4]. The

combustion system is illustrated in Fig. 1.3 and

consists of a variable swirl number burner with

separate flow controls for axial and tangential

premixed air and fuel. This is fired into a refractory

lined chamber, the confinement ratio for the swirl

burner, Do/De is 2, whilst the Lfurn/Do ratio for the

furnace is 2.5. Isothermal velocity results are shown in

Figs. 1.4–1.6. The tangential velocity distribution,

Fig. 1.4, close to the burner exit at x/DoZ0.11 shows a

peak velocity of w17 m/s at r*Z0.55; by x/DoZ0.33

this peak velocity has been maintained whilst moving

radially inwards to r*Z0.35. These tangential velocity

profiles are then conserved until the end of the furnace.

This initial change in tangential velocity profiles

induces complex axial velocity profiles and reverse

flow zone patterns, Fig. 1.5, and also a PVC close to

the burner exhaust. Throughout the furnace a region

of forward axial exists on the axis, extending to

r*w0.3–0.4. An annular reverse flow zone, centred at

Fig. 1.2. Stream function distribution at swirl burner exhaust showing typical recirculation zone set up in the exhaust of a swirl burner, isothermal

conditions, SZ1.57. PVC is located on boundary of reverse flow zone [3].

Fig. 1.3. Schematic diagram of refractory lined swirl burner furnace system for combustion of low Calorific value gases from carbon black plants

[4]: (1) inlet for tangential premixed gas and air; (2) inlet for axial premixed gas and air. Swirl number variation achieved by varying proportions of

above. Do/DeZ2.:LfurnZ2.5: air flow rateZ2.85 kg/s: SZ1.36.

Fig. 1.4. Distribution of tangential velocity in system of Fig. 1.3 [4].


r*Z0.5, develops between x/DoZ0.11 and 0.69,

virtually disappearing by x/DoZ1, although there is

evidence of a weak intermittent zone to the end of

the furnace. Associated velocity vectors are shown

in Fig. 1.6 and show the development of the annular

CRZ.

The conservation of swirl velocity and hence,

angular momentum along the furnace length causes

the PVC formed near the burner exit to be of higher

frequency, but lower amplitude, than that formed by a

free, unconfined, expansion. Moreover, this conserva-

tion of swirl velocity also means that there is

potential for the formation of further PVCs in the

furnace exit downstream. This is discussed later in

Section 2.

Despite the advantages of swirl stabilised combus-

tion there is a well known propensity for instability to

Fig. 1.5. Distribution of axial velocity in system of Fig. 1.3 [4].


develop and again there is an extensive literature in this

area [5–19].

Recent focus has been on lean premixed combustion

(LPC) as used with many modern gas turbine systems

to ensure low NOx emissions. Premixed flames are

by nature more susceptible to static and dynamic

Fig. 1.6. Velocity vectors in system of Fig. 1.3 illustrating flow

patterns [4].

instabilities due to the lack of inherent damping

mechanisms. The resulting absence of diffusive mixing

times leaves flames sensitive to acoustic excitation

from sound waves with flame response dependent upon

the amplitude, frequency and nature of acoustic wave

impingement. If conditions are favourable, periodic

fluctuations in the heat release will match the natural

resonant frequency of one or more of the geometrical

components of the combustor, or related natural fluid

mechanic mechanisms, resulting in self-excited

thermo-acoustic instabilities. The mechanism respon-

sible for the maintenance of limit-cycle heat-driven

oscillations was originally proposed by Rayleigh [20]

and refers to the relationship between the pressure wave

and rate of heat release. This paper discusses natural

fluid dynamic and related instabilities, occurring in

swirl combustors and related systems, which can excite

or increase periodic heat release. A major focus here is

the influence of vortex core precession and precessing

vortex cores (PVC).

The actual mechanism of the coupling effect

between the flow/flame dynamics appears to arise

from flow instability feeding into unsteady heat

release/combustion processes, which then feed instabil-

ity via coupling with acoustic modes of oscillation

and amplification via the Rayleigh criterion. Associated

work has shown that in high pressure process plant

containing large ductwork runs and cyclone separators

low frequency high amplitude pressure oscillations can

arise from coupling between natural modes of acoustic

oscillation and the vortex core precession (PVC)

generated in the cyclone separator, Yazabadi et al.

[21,22]. Similarly, Kurosaka [23] has shown that the

cooling effect produced by the Ranque–Hilsch tube

relies on the presence of strong PVCs, with up to six

strong harmonics being readily detectable, typical

fundamental frequencies were 2–7 kHz, being a near

linear function of inlet velocity. Suppression of the

PVC could be achieved by fitting 12 quarter wave

damping tubes radially around the circumference of the

tube and tuning their frequency to that of the PVC so

that they worked in anti phase.

It is often difficult to analyse the role of the PVC, its

influence on instability and indeed its presence in

combustion systems. The occurrence of the PVC is a

function of swirl number (S) [1,2], the presence of a

CRZ (normally SO0.6–0.7 for vortex breakdown, the

PVC and a CRZ to occur [1,2]), as well as the mode of

fuel entry, combustor configuration and equivalence

ratio. It has been shown that axial fuel entry normally

suppresses the PVC amplitude substantially, whilst

premixed fuel and air can restore its presence and


indeed can considerably excite it [2]. This is of course

extremely important with premixed and partially

premixed combustors. Here again, the effect of

confinement ratio on the swirling flow is important as

discussed in Section 5.

This paper, thus commences with a review of

relevant work and then uses recent and new data to

analyse the role of the PVC and the associated CRZ,

relative to other factors which influence instability in

swirl combustion systems.

2. Vortex core and jet precession

The concepts to be discussed in this paper are

initially best illustrated by reference to the swirl

burner shown in Fig. 2.1. This is of simple

configuration with two circular inlets firing into a

circular chamber, which leads via a sudden contrac-

tion to the exhaust, normally 50% of the diameter of

the main chamber. The area of the tangential inlets

can be varied by removable inserts to give swirl

numbers in the range 0.75 upwards. Fuel can be

introduced by several methods, including axially

along the centre line and premixed with the air by

introduction via a premixing system just before the

tangential inlets. This device normally produces a

central recirculation zone (CRZ) in the exhaust over

its operational range giving excellent flame holding

capabilities.

This burner has been extensively used to describe

and illustrate the phenomena of the precessing vortex

core (PVC) [18,24–31]. Fig. 2.2(a) and (b) shows PIV

images obtained from this device under isothermal

conditions, operating with SZ2.6 [29]. Here, a laser

sheet fired horizontally close to the exhaust of the

burner in the tangential radial plane has illuminated

fine oil particles and with a double pulse laser unit

Fig. 2.1. Schematic diagram of generic swirl burner—swirl number adjusta

backwards into swirl chamber to prevent flashback. Nominal thermal input

enabled velocity vectors to be derived in the

tangential radial direction. The central axis is marked,

together with the diameter of the burner exhaust.

Fig. 2.2(a) shows the main vortex is displaced from

the central axis and is precessing about the central

axis of symmetry, here with a frequency of 140 Hz.

In this figure, the PIV image is superimposed on top

of the phase averaged tangential radial velocity

contours obtained from LDA at the same section.

The PVC can be seen to generate a central region of

negative tangential velocity due to the co-ordinate

system used for velocities in this plane. In this type of

unit the PVC phenomena persists for about 1–1.5 exit

diameters in free air. Fig. 2.2(b) shows another PIV

image showing evidence of the presence of a second

PVC [30]. A schematic representation of the flow

patterns associated with the PVC is shown in

Fig. 2.3(a), with a typical periodic signal obtained

from a pressure transducer inserted in the burner

exhaust flow shown in Fig. 2.3(b) [2]. Normally, the

PVC frequency increases quasi-linearly with flowrate.

A visualisation of PVC obtained under combus-

tion conditions is shown in Fig. 2.4(a) [30]. Here,

10% of the fuel is injected axially into the burner

where it is entrained into a low-pressure region in

the PVC centre. The remainder of the natural gas

fuel is premixed with the air upstream of the

tangential inlets and produces a non-luminous blue

flame. This consumes most of the available oxygen

and hence the fuel in the PVC burns fuel rich on the

PVC boundary as it is starved of oxygen. The

structure of the PVC extends to about 1.5 diameters

downstream of the burner exit before breaking up.

There is evidence from many sources that the PVC

is helical in nature [29–33], wrapping itself around

the reverse flow zone boundary, as shown in

Fig. 2.4(b) [33].

ble from 0.75 upwards via use of tangential inserts. Exhaust extends

100 kW [27–29].


2.1. General characteristics of the PVC under

isothermal conditions

For isothermal conditions, the frequency of the PVC

can be readily characterised by a Strouhal number and

Fig. 2.2. (a) Isothermal PIV instantaneous velocity vector plot just above ex

instantaneous velocity vector plot just above exit of swirl burner, Fig. 2.1,

the swirl number, S [1,2,24,25,32,33]. The Strouhal

number is a weak function of Re and asymptotic values

have been used for Fig. 2.5 (data has been gathered

from many sources, it should be noted that the Strouhal

number used on this and subsequent figures, fDe/ub, is

it of swirl burner, Fig. 2.1, showing one PVC [30]; (b) isothermal PIV

showing two PVCs [30].

Fig. 2.3. Processes associated with the PVC [30]: (a) schematic diagram of the flow patterns; (b) pressure fluctuation against time trace obtained

from pressure transducer located at lip of swirl burner, Fig. 2.1.


different by the factor {P/4} to that used in the source

data [1,2,21,22,26,32–39], fDe3/Q, to ensure common-

ality with other literature reviewed). The relationship is

clearly a function of burner/swirl flow system configur-

ation. There are a number of effects here; the data from

large power station boilers was obtained from large-

scale systems and indicates a scale effect in that high

values of Strouhal number are obtained for low swirl

numbers. Strouhal numbers of more than two are

produced for Swirl numbers of one. Swirl burners and

cyclone combustors with non divergent exhausts with

no centre bodies produce data which fit onto the same

curve, giving values of Strouhal number of w0.86 for a

swirl number of 1. The well known Ijmuiden movable

block swirl burner [1,2,39], gives a Strouhal number of

w0.37 for the same swirl number of 1, indicating the

unit is not that effective in generating swirling flow.

This type of unit also uses a large central fuel injector,

and produces CRZ down to low swirl conditions,

resulting in the occurrence of a PVC type structure at

low swirl levels. These results were obtained with zero

or very low fuel jet velocities and thus do not arise from

precession of the central fuel jet: this has been

confirmed by separate PIV studies [39]. Cyclone dust

separators similar values of Strouhal number as for the

Ijmuiden movable block swirl generator, indicating that

the vortex finder (facing backwards into the cyclone

chamber to prevent boundary layer egress of particles

into the exhaust) is having deleterious effects on swirl

generation and hence, the Strouhal number.

Fig. 2.4. (a) Visualisation of single PVC with separate axial fuel

injection into a premixed flame, swirl burner as Fig. 2.1 [30]; (b)

visualisation of helical nature of the PVC from Chanaud [33].


Interesting results have been reported from the group

at the University of Sydney where a swirl burner is

formed by forcing swirling flow through the thin

annulus formed when a large bluff body is inserted into

the exhaust of a swirl flow system, Fig. 2.6 [40–45].

Fuel is introduced via a small central jet, which can be

of high velocity. This configuration produces extremely

complex flows, with the occurrence of multiple

recirculation zones due to the interaction of the swirling

flow, bluff body and the high velocity central fuel jet.

As discussed in [1] this can result in several flame

types; work at the international flame research

foundation (IFRF), the Netherlands, show that there

are at least two main types, the so called Type I flame

where a high velocity fuel jet fires through the CRZ,

followed by a wide zone of instability as the jet velocity

was reduced, finally resulting in a stable, more

common, Type II flames. The reported occurrence of

precession in this type of burner, Fig. 2.6, is very

dependent on the swirl number, central jet velocity and

bulk flow velocities [40,41]. Precession is reported

down to swirl numbers of 0.28 with high central jet

velocities relative to the bulk inlet velocity, and is

associated with precession of the central fuel jet as

opposed to a PVC interacting with the outer boundary

of the CRZ. As discussed later this form of precession is

characterised by values of Strouhal number an order of

magnitude less than shown in Fig. 2.5.

The persistence of preccessional frequencies to low

swirl numbers in the power station burners, Fig. 2.5,

arises from the use of large central fuel injectors, bluff

body stabilisers, and is differentiated from jet preces-

sion by the values of Strouhal number, which indicate

PVC form. All the other data from a range of different

swirl burners and cyclone combustors lack fuel

injectors and large bluff bodies in the exhaust flow

[21,22,34–38]. These units only produce PVC signals

beyond the normally accepted level of swirl for the

formation of a CRZ and vortex breakdown for SO0.5.

The Strouhal number data for these units thus collapses

to one separate curve, Fig. 2.5, as does that for cyclone

separators.

The structure of the PVC has been quantified under

isothermal conditions using the swirl burner of Fig. 2.1,

and LDA techniques whereby the PVC pressure signal

is used to phase lock and overlap velocity data to

produce the rotating velocity field associated with the

PVC, Fig. 2.7(a)–(c) (x/DeZ0.007) and Fig. 2.8(a)–(c)

(x/DeZ0.78), SZ1.5 [25]. Three diagrams are shown

for the rotating tangential (a), axial (b) and radial

velocities (c). Each diagram shows the average rotating

velocity field over the full 3608 of the burner. Close to

the burner exit, Fig. 2.7(a), the rotating tangential

velocity shows considerable variation in the q direction,

with a small but significant area of negative tangential

velocity near to and around the axis of symmetry, due to

the effect of the PVC, this is also shown on Fig. 2.2(a).

This is an important effect due to the presence of the

PVC and arises from the convention used to designate

measured tangential velocities, reference to Fig. 2.2(a)

is useful here. High levels of tangential velocity are

confined to a banana shaped sector of 1208 close to the

outer wall. There is an area of low tangential velocity

diametrically opposite to the high velocity region of

w8 m/s, reflecting the inlet velocity. The angular

position of maximum rotating axial velocity,

Fig. 2.7(b), closely matches that of the rotating

tangential velocity, indicating that much of the flow

leaves the burner in a thin banana shaped segment

inclined upwards at an angle of 458. The reverse

flow zone has a relatively high axial velocity value of

K7 m/s and is displaced substantially from the central

axis, extending from r/reZ0 to 0.7 and over a phase

angle of 1008. The rotating radial velocity levels,

Fig. 2.5. Variation of Strouhal number with Swirl number, asymtotic high reynolds number values, data for four distinct groups of devices [1,2,21,

22,25,26,31,34,35,37–39].


Fig. 2.7(c), are somewhat lower than the other two

components. Of particular note is the kidney

shaped region of negative radial velocity (corre-

sponding to inwards flow), extending from the central

axis to r/rew0.77. Correlation of the tangential, axial

and radial velocities close to the burner exhaust,

Fig. 2.7, shows there is a large PVC present which

leads the reverse flow zone by about 90–1008 in phase.

These appear to be linked but distinct structures as most

of the volume of the PVC is in a region of forward flow.

By one exit diameter downstream the flow has nearly

returned to axisymmetry, although the PVC and off

centred reverse flow zone could still be detected at x/

DeZ0.78, Fig. 2.8(a) and (b). The region of negative

tangential velocity has shrunk considerably, although

its existence was still well defined. The maximum value

of tangential velocity had also decayed from 24

(Fig. 2.7(a)) to 9.5 m/s. Rotating axial velocity levels,

Fig. 2.8(b), were also more uniform, although the

banana shaped area of flow is still evident for both axial

and tangential velocities, although moved by about

2308 in the flow direction. Clearly, as indicated by

Fig. 2.4 the PVC and associated structures are helical in

nature, having been twisted by w2708 between the

sections shown in Figs. 2.7 and 2.8 (x/DeZ0.07 and

0.78); see also Fig. 2.4(b).

Phase locking of the PVC and associated phenomena

clearly looses some information and this is illustrated

by the two instantaneous PIV images shown in Fig. 2.2,

SZ1.7 [30]. Fig. 2.2(b) shows a state where two PVCs

can be distinguished, Fig. 2.2(a) shows a state where a

single PVC exists. The single PVC dominates this flow,

intermittently jumping to a two PVC state. This PIV

data has been subsequently analysed to give phase

locked axial radial velocity vectors, Fig. 2.9(a) and (b),

at two different cross-sections separated by 908.

Especially in Fig. 2.9(b), the presence of axial radial

eddies can be seen in and on the boundary of the CRZ

whilst, Fig. 2.9(a) shows that a phase angle change of

908 causes these eddies to diminish significantly. Other

work [2] using water models has shown the existence of

axial radial eddies produced by a swirl burner, SZ1.86.

Here, the eddies appear to be periodically shed from the

end of the CRZ and propagate downstream through the

expanding flow, accompanied by large scale motions or

flapping of the CRZ and shear layer. Other workers

have reported similar phenomena with swirling flames

including Roux et al. [45], Masri et al. [42], Syred et al.

Fig. 2.6. Swirl burner developed by sydney university group [41–44].


[46], but without the presence of the large PVC.

Dorrestein [47] found large radial axial eddies on the

edge of a swirling flame and attributed large amplitude

system oscillations to acoustic coupling with these

eddies.

2.2. Effect of confinement

The effect of confinement has an extremely

important effect upon the PVC and its related

instabilities. As discussed by Syred and Dahmen [48],

Syred and Beer [2], Gupta et al. [1], confinement can

dramatically alter the size and shape of the CRZ and

ERZ formed as the swirl burner flow expands into a

furnace or combustion vessel. It can also induce weak

regions of forward axial flow on the central axis inside

the CRZ [2,46]. Confinement ratio, Do/De, is the

dominant factor, the smaller this ratio the larger is the

effect. Other important factors include the level of

swirl, equivalence ratio and whether or not a quarl or

sudden expansion is used on the burner exit. As

discussed in Section 1 the CRZ formed by an

unconfined swirl burner arises because of the sudden

expansion and associated entrainment effects on the

edge of the swirling flow [1]. This causes decay in swirl

velocity profile, which in turn generates strong radial

and axial pressure gradients creating the CRZ.

Inevitably, any form of significant confinement will

affect this process and alter the size and shape of the

CRZ, whilst also normally causing an ERZ to form as

the flow sticks to the external wall. As the PVC is

closely associated with the boundary of the CRZ

confinement has considerable effects as discussed by

Fick [30].

Available results are summarised in Fig. 2.10(a) and

(b) for isothermal flow in the small 100 kW burner of

Fig. 2.1 (unconfined flow) and for confined flow in the

swirl burner/furnace system of Fig. 2.10(c). Fig. 2.10(a)

shows that for a confinement ratio of 2 (see Fig. 2.10(c))

Strouhal number is scarcely affected by this relatively

high level of confinement until a swirl number of about

1.3, when sudden difference occur, more than doubling

the unconfined value for SO1.5. This continues up to

the maximum swirl number characterised of 4.78.

There is still sufficient swirl left in the flow for another

vortex breakdown to occur, Fig. 2.10(a), in and just past

the exhaust of the furnace, Fig. 2.10(c), creating

another, separate PVC (this can be readily observed).

Chao et al. [49] reported a similar phenomena finding

two natural frequencies in different regions of an

undisturbed swirling flow field; a transition region was

also found where both instabilities co-existed, as found

by Fick [30]. Thus, in the exhaust of the furnace shown

in Fig. 2.10(c) a PVC was found whose value of

Strouhal number was considerably less than that found

in the main furnace just after the burner exit. There was

an effect of Reynolds number as especially at low swirl

numbers, SZ0.5, the effect did not appear until

high flow rates and furnace exit average axial velocities

of 12 m/s, SZ0.5 (ReZ48,000). This decreased to

4.5 m/s, for SZ1.5 (ReZ18,000). Fig. 2.10(b) shows

the relationships between the various frequencies at

high Re. For SO1.7 the effect of confinement is to

increase the frequency of the PVC formed in the swirl

burner exhaust by w2.1. The PVC then formed in the

furnace exhaust has a frequency w30% of that formed

just downstream of the burner exhaust. The occurrence

of this secondary PVC is unfortunate, as it can easily

Fig. 2.7. (a) Phase locked tangential velocity contours above swirl burner exit, x/DeZ0.07, burner as Fig. 2.1 [25]; (b) phase locked axial velocity

contours above swirl burner exit, x/DeZ0.07, burner as Fig. 2.1 [25]; (c) phase locked radial velocity contours above swirl burner exit, x/DeZ0.07,

burner as Fig. 2.1 [25].


Fig. 2.7 (continued)


become another mechanism for driving instability.

Other work on cyclone dust separators has shown that

the PVC can travel around bends [21,22]. Thus, where

instability is a problem the use of inline exhausts for the

burner and furnace/combustion chamber as shown in

Fig. 2.10(c) is probably undesirable.

Unfortunately, detailed results are only available for

a confinement ratio of 2, Do/DeZ2 [30]. Clearly, larger

confinement ratios would produce values of Strouhal

number somewhat in between those for the unconfined

and confined cases shown in Fig. 2.10(a) and (b).

Anacleto et al. [50] studied swirling flow with and

without combustion in a LPP combustor model as

shown in Fig. 2.11 using a number of techniques and a

variable angle swirl generator. Swirl vane variation

between 0 and 608 could achieved, giving a swirl

number range of up to 1.6, whilst a wide range of Re

could be covered. Flow passes through the vaned

swirler, with an outer diameter of 120 mm, and then is

converged to a 50 mm diameter, 110 mm long pre-

mixing section, before passing through a 40 mm

contraction to the final combustion chamber of

110 mm diameter, Fig. 2.11. The PVC was character-

ised under isothermal conditions just past the 40 mm

contraction, both with and without the final combustion

chamber. The Strouhal number is shown as a function

of swirl number and Re, Fig. 2.12(a) and the pressure

difference, central flow axis to the wall of the 50 mm

diameter chamber, Fig. 2.12(b) at a position just past

the tip of the fuel injector, Fig. 2.11. In region I for S!0.5 no PVC is detectable, vortex breakdown occurs in

region II with the formation of a PVC. Strouhal

numbers then decrease from the initial value with

increasing swirl until values of Sw0.9. Subsequent

increases in swirl number produces the expected

increase in Strouhal number as indicated for other

systems in Fig. 2.5. The effect of the final combustion

chamber on the PVC is small, Fig. 2.12(a), with the

largest deviation occurring for SZ0.88. Thus, the

processes determining the formation of the PVC in this

system are governed by those occurring in the first

50 mm diameter premixing chamber, Fig. 2.11. The

pressure difference curves, Fig. 2.12(b) shows the

changes in flow structure occurring with vortex

Fig. 2.8. (a) Phase locked tangential velocity contours above swirl burner exit, x/DeZ0.78, burner as Fig. 2.1 [25]; (b) phase locked axial velocity

contours above swirl burner exit, x/DeZ0.78, burner as Fig. 2.1 [25]; (c) phase locked radial velocity contours above swirl burner exit, x/DeZ0.78,

burner as Fig. 2.1 [25].




breakdown between states I and II and II and III. The

Strouhal number data for SO0.9 and high Re compares

well to the data of Fig. 2.5 and shows isothermal

Strouhal numbers just a little higher in value than those

produced by many other swirl burners and cyclone

combustors.

2.3. Precessing jets and jet burners

Studies have carried out at the University of

Adelaide on Precessing Jets (PJ) and their application

to burners in cement kilns and similar installations

[51–60]. Significant advantages in terms of reduced

NOx emissions have been shown on gas fired cement

kilns and promise is shown when firing pulverised coal

[52–54]. The associated fundamental work has

included studies of oscillating two-dimensional jets of

varying aspect ratios, triangular jets and most relevant

to this work oscillating or fluidic jets [51,55]. The

relevance to the PVC and swirl instabilities is that there

are many similarities in the mechanisms from which

the PJ originates and the characteristics of the jets so

generated. Fig. 2.13(a) and (b) schematically illustrates

the processes occurring with the PJ [51], with the entire

jet precessing about the axis of the system. Fig. 2.13(a)

shows a schematic of the processes occurring, whilst

Fig. 2.13(b) shows a water model visualisation,

obtained via a thin light sheet illuminating the central

axial radial plane. The unit consists of a cylindrical

chamber with a small axisymetric sharp edged inlet

orifice at one end and an exit lip at the other. Flow

enters the sharp edged orifice and expands into the

chamber where it attaches asymmetrically to the wall,

with substantial internal flow recirculation, Fig. 2.13(a).

The asymmetry causes the reattaching flow to precess

about the axis of the device, producing a precessing exit

flow. The lip and large transverse pressure gradients

near the outlet together steer the exit flow through a

large angle, towards the axis and across the face of the

nozzle outlet [51,56]. As a result the PJ entrains large

quantities of external fluid, some 5–6.8 that of an

equivalent free turbulent jet. Later versions of the PJ

nozzle have a centre body located just before the exit,

Fig. 2.14, to improve the regularity of the precession.

Fig. 2.15 shows phase locked LDA measurement of

axial velocity past the PJ nozzle exit at a PJ frequency

Fig. 2.9. (a) Phase locked PIV image in axial radial plane at exit of swirl burner, Fig. 2.1, plane 155–3358 [30]; (b) phase locked PIV image in axial

radial plane at exit of swirl burner, Fig. 2.1, plane 65–2458 [30].


Fig. 2.10. (a) Variation of Strouhal number with swirl number,

isothermal conditions, data for unconfined and confined flow [30]; (b)

frequency ratio for the PVC between confined and unconfined states

[30]; (c) schematic diagram of swirl burner furnace system, swirl

burner as Fig. 2.1 [30]. Inserts are used in the tangential inlets to alter

swirl number. DeZ75 mm. Burner exit protrudes backwards into

swirl chamber to prevent flashback.


of 7.5 Hz. Flow leaving the PJ nozzle, Fig. 2.15, at

x 0/dZ0.67, assumes a banana or kidney shape region

of high velocity, moving downstream the flow has

returned to near symmetry by x 0/dZ1.93. These results

are similar to those obtained with the PVC, Figs. 2.7(b)

and 2.8(b). However, in operation the frequency and

motion of the PJ is more irregular than the PVC with

considerable signal jitter. This lead to several studies

produced by a mechanically rotating PJ nozzle where

the flow field was much more regular [59,60]. Wong

[57] describes a new phase locking technique to analyse

the complex motion from conventional non-rotating PJ

nozzles using two separate hot wire probes to produce

triggering signals for LDA or PIV systems. Earlier

phase locked techniques were unable to detect the

direction of rotation of the PJ. In this way, different and

consistent and parts of the cycle can be phase averaged

to obtain detailed velocity maps. Fig. 2.16 shows the

general flow characteristics of the PJ analysed by this

technique derived from instantaneous PIV, phase,

precession direction resolved phase averaged PIV and

surface flow visualisations. This shows a number of

smaller vortices and structures which the earlier phase

locked LDA technique had missed, for instance on the

exit nozzle, centre body and in the flow around the

precessing jet.

Wong [57] also discusses in detail various methods

of non-dimensionalising the frequency data from

various designs of PJ to produce Strouhal number

data. He proposes a modified Strouhal number for PJ

systems based on the inlet orifice diameter, d,

precessional frequency, bulk flow velocity through the

inlet orifice and two constants, A and B, which are

representative of system length scales. The derived

Strouhal number range from 0.008 to 0.06. This

contrasts with previously derived Strouhal number

values of w0.001–0.005 [51] using precessional

frequency, inlet PJ orifice diameter and corresponding

bulk flow velocity (fd/ub). These values are much lower

those obtained for the PVC, ranging from 0.2 to 2 or

more, although there are differences in definition.

In combustion situations, the driving fluid is usually

high-pressure gaseous fuel, typically natural gas [52],

although pulverised coal versions have been developed

[54]. The PJ creates a rapid decay in mean velocity

close to the nozzle and generates large-scale flame

structures with reduced shear relative to a simple free

turbulent jet [56]. As a result, the natural gas initially

burns in an oxygen deficient region and produces a

flame of excellent stability and high emissivity, unusual

for natural gas flames. This enhances radiant heat

transfer and can reduce NOx emissions by between 20

and 60% [52,53]. The high entrainment rates ensure

that downstream as the large-scale structures breakup,

good mixing occurs with good final fuel burnout.

Extensive experimental work shows that combustion

Fig. 2.11. Vaned swirler, prevapourisation and combustion chamber of Acacleto et al. [50].


has little effect on the PJ structure and characteristics

as the initial processes which generate the PJ occur

upstream of the combustion process. There is parallel

here with the work of Ancacleto et al. [50], where the

PVC was generated upstream of the combustion

process and thus was not affected.

Fig. 2.12. Effect of Swirl Number on Strouhal Number and Pressure

Drop Parameter [50].

3. Combustion and the PVC

Combustion processes make the behaviour and

occurrence of the PVC more complex. The form of

the PVC and associated flows can be similar to that

found in isothermal flows [1,2,26,30,37,38,50]. The use

of axial or tangential fuel entry alone [1,2,26,37,38] can

suppress the amplitude of the PVC by an order of

magnitude or more and its frequency/occurrence

becomes a complex function of flow rate, equivalence

ration and mode of fuel entry. The PVC occurs more

readily with premixed combustion [1,2,30]. The

occurrence of the PVC is a very strong function of

the position where the flame is radially located and this

is highly dependent on the mode of fuel entry. This is

illustrated in Fig. 3.1(a)–(c) which summarises three

main flame types that can be found with a large 1 MW

rated unconfined swirl burner fired on natural gas, SZ1.86. The burner is schematically shown in Fig. 3.1(d)

and has no flashback protection to guard the eight

inlets. The first flame [2], Fig. 3.1(a), shows a result

with premixed air and natural gas where the flame is

actually located in the air/fuel inlets and the PVC is

considerably excited in frequency and amplitude. This

is an extreme result from a large unconfined flame

where the premixed flame has flashed back to the eight

slit tangential inlets through which the air/natural gas is

fired. The flame is thus mainly contained inside the

burner and is extremely noisy. A strong PVC signal was

readily seen and the results for a range of f are shown

for Strouhal numbers as a function of Re, Fig. 3.2(a).

Flame extinction occurred beyond fw0.68. The high-

est excitation of the PVC frequency occurred for

fw0.68 producing a value of Strouhal number

increased by a factor of 4 on the isothermal result.

This effect steadily decreases for reducing equivalence

ratios. Simple calculations indicate that this Strouhal

number increase can be described if allowance is made

for the acceleration of the gases due to combustion in

Fig. 2.13. (a) Schematic of processes occurring in the PJ burner [51]; (b) water model visualisation via an axial radial slit light of the processes

occurring in the PJ burner [51].

Fig. 2.14. Improved version of the PJ burner with centre body [56].


the tangential inlets as the value for bulk velocity, ub, in

the Strouhal number is based on the isothermal

flowrate. This premixed condition is unusual and

normally undesirable as considerable overheating and

distortion of the inlets can occur. Reference to Figs. 2.1

and 2.10(c) shows that an extension to the exhaust

nozzle is normally fitted to prevent flashback. This

produces very different flames, which are now

primarily stabilised downstream in the exhaust nozzle.

Fig. 3.1(b) shows the type of flame produced by axial

fuel injection in the same large burner; here the main

part of the flame is located downstream of the burner

exit, but parts of the flame surrounding the CRZ extend

Fig. 2.15. Phase locked LDA axial velocity contours in exhaust of PJ burner [56].


back into the burner mouth and indeed right down to the

burner back plate, as a thin tulip shaped column.

Similar flames are produced with axial fuel injection by

the burners of Figs. 2.1 and 2.10(c). The PVC

amplitude is nearly always suppressed by at least an

order of magnitude compared to the isothermal state

(until very small equivalence ratios [2,61]).

Fig. 3.1(c) shows another flame produced by

tangential fuel entry with the flame located at an

intermediate radial position between that shown for the

flames of Fig. 3.1(a) and (b). Again, the PVC amplitude

is suppressed by at least an order of magnitude.

Fig. 3.2(b) shows the effect of axial and tangential

fuel entry upon Strouhal number for the suppressed

PVC as a function of Reynolds number and equivalence

ratio for a 1/5 scale model of the burner of Fig. 3.1(d).

There are differences between axial and tangential fuel

entry, but at high Reynolds numbers the values of

Strouhal number are approaching 80–90% of the

isothermal state. At exceptionally low equivalence

ratios (f!0.02) a large PVC reappears with axial fuel

injection [2,51] and this is the configuration upon which

the stability analysis discussed later was carried

out. The radial location of the flame front close to the

burner exhaust is important as this can give rise to

unfavourable/favourable gradients of rwr and density

conducive to PVC formation/suppression.

Claypole [26,37,38] used a natural gas fired swirl

burner of similar configuration to that of Fig. 2.1, but

with four inlets. Fig. 3.3(a) shows the effect of

combustion upon PVC rms pressure amplitude for

centreline axial fuel injection via spectral analysis of

signals obtained from a pitot tube located at the burner

exhaust lip. The dramatic reduction in amplitude by up

to a factor of 15 can be observed. Premixed fuel and

air was shown to only slightly affect the PVC under the

stated conditions, Fig. 3.3(b). Fig. 3.4 shows the

occurrence of the PVC for a range of Swirl numbers

and flow rates (Re). PVCs only occur beyond a flow rate

of w600 l/min (ReZ40,000) and this is where vortex

breakdown occurs as there is no CRZ formed at lower

flowrates. For 0.8OSO1.8 two PVCs are observed

of approximately equal intensity. For higher Swirl

numbers a single PVC reappears, but with multiple

Fig. 2.16. General flow characteristics of the PJ, derived from instantaneous PIV, phase and precession direction resolved phase averaged PIV and

surface flow visualisations [57].


harmonics, whilst for S!0.8 only one PVC could be

detected. A visualisation of a two PVC state in the swirl

burner of Fig. 2.1 with axial fuel injection is shown in

Fig. 3.5. A stainless steel mirror enabled simultaneous

images to be obtained in the axial/radial and tangential/

radial directions. Inside the burner the two PVCs rotate

in mesh and then spiral outwards in a helical manner as

they leave the burner exit. The PVCs persist for about

1.5 exhaust diameters downstream of the exhaust, being

of similar length to the single PVC visualised in

Fig. 2.4.

For fZ0.89, Fig. 3.6, the Strouhal number ranged

from w0.8 (SZ0.63) to 0.32 (SZ1.26), 0.3 (SZ1.53).

The value for SZ0.9 shows a sudden jump as a double

PVC mode is established with values of Strouhal

number dropping from 0.86 to 0.2 as the mode switched

from single to double PVC. This behaviour is quite

different from the isothermal state [26,37,38] where

there is a steady increase of Strouhal number with swirl

number, Fig. 2.5. The variation of Strouhal number for

the second harmonic of the PVC shows the same trends,

Fig. 3.7. A radial fuel injector reduced the coherence of

the PVC somewhat.

Available data on PVC frequencies in combustion

systems has been assembled in Figs. 3.8 and 3.9, this data

has been derived from references [1,2,26,30,31,37,38].

Fig. 3.8 shows the variation of Strouhal number for the

PVC with 100% axial fuel entry with swirl number

varying from 0.73 to 3.43 as a function of equivalence

ratio, all the flames being unconfined. An equivalence

ratio of 0 conveniently corresponds to the isothermal

state. For the lowest swirl number of 0.732, there is a trend

of increasing PVC frequency with equivalence ratio,

changing as the swirl number increases due to the

occurrence and formation of double PVC structures with

changes in equivalence ratio.

Fig. 3.1. Effect of different modes of fuel injection, SZ1.8 [2]; (a)

premixed flame with excited PVC, premixed natural gas and air, fZ0.52; (b) effect of central axial fuel injection, fZ0.952; (c) effect of

tangential fuel entry, fZ0.952; (d) schematic diagram of burner.



For partially premixed conditions, Fig. 3.9 (100 kW

burner of Fig. 2.1, 10–50% of the fuel injected axially,

SZ0.76), a different situation pertains and although

higher harmonics of the PVC were present, the first

harmonic always dominates. This is why with

unconfined flames, SZ1.76, there is a steady increase

in PVC frequency with equivalence ratio, the value

doubling from the isothermal Strouhal number of 0.86–

1.64 at fZ0.71, but then dropping down again to

values between 0.56 and 0.96 as the burner is operated

up to fZ2. The level of axial fuel injection varied from

0 to 50%, depending on the equivalence ratio. For

unconfined flames the technique is limited by the blow

off limits of the combustor.

Fig. 3.9 also shows results from a large 2 MW swirl

burner furnace system, Fig. 3.10(a) and (b) (0.7!S!1.6, De/DoZ0.5, swirl burner four times geometric

scale up unit of Fig. 2.10(c)). Here, because the furnace

Fig. 3.2. The effect of Reynolds number and equivalence ratio f upon Strouhal number [2]: (a) premixed natural gas and air and isothermal state-

large combustor, SZ1.86; (b) 1/5 scale model combustor axial and tangential fuel entry, SZ1.86.


is refractory lined to investigate the combustion of low

calorific value gases, much wider blow off limits can be

investigated with different modes of fuel entry. Seventy

to one hundred % of the natural gas fuel was injected

axially, the rest was premixed in the inlets with the air.

The value of Strouhal number drops dramatically from

that of the isothermal state, 1.2 to w0.5 for f between

0.1 and 0.3, and then rises steadily back to the

isothermal value for fw1, then steadily increases

again with increasing f. There is only a small effect of

Fig. 3.3. Effect of combustion upon the PVC with (a) 100% axial fuel

injection; and (b) premixed, SZ1.98, DeZ75 mm, equivalence ratio

0.89 [26,37,38].

Fig. 3.4. Occurrence of the PVC for a range of swirl numbers as a

function of Swirl number and flow rate (Re). Hatched area shows

region of single PVC, square hatched area shows region of double

PVC [26,37,38].


Swirl number and mode of fuel injection, upon Strouhal

number for fw1. As the PVC frequency varied quasi-

linearly with flowrate changes it was evident that the

PVC was not driven by system acoustics.

The isothermal value of the Strouhal number for the

2 MW system is 1.2, Fig. 3.9. Reference to Fig. 2.10(b)

shows a frequency ratio confined to free (Do/DeZ2) of

1.5 and thus, the equivalent Strouhal number for the

2 MW unit firing into free air is w0.8. This is a

reasonable match to the isothermal result for the

unconfined SZ1.76, 100 kW unit, Strouhal number

0.88, when differences in swirl number are allowed for.

For partially premixed conditions only one dominant

PVC was normally found, although there was always

evidence of other harmonics.

The work of Anacleto et al. [50] provides an

interesting contrast here. In their LPP system,

Fig. 2.11, a vaned type swirl surrounds a central hub

containing a fuel injector/atomiser. The swirling flow is

converged to a small diameter, but long chamber,

where centrally injected liquid fuel is pre-vapourised.

Combustion occurs downstream of this section in a

larger diameter chamber. The mixing and flow

characteristics in the combustion chamber are shown

to be strongly influenced by the formation of a large

PVC in the first pre-mixing chamber. However, as the

PVC has had significant opportunity to develop in the

first chamber, there is no suppression, and the Strouhal

number with combustion is very similar to the

isothermal state.

The stability of rotating flow may be analysed via

the work of Rayleigh on flow stratification [62] and

consideration of stratification parameters such as

modified Richardson numbers, Ri, as proposed by

Beer et al. [63]. The stability criterion proposed by

Rayleigh was that a system is:

– stable if rwr increases with r (solid body rotation)

– neutrally stable rwr is constant with r (free vortex)

– unstable if rwr decreases with r

Syred et al. [61] characterised the flow containing a

single PVC with combustion via axial fuel injection in

the burner of Fig. 3.1(d) at very low equivalence ratios,

fZ0.02, using phase locked fluctuating temperature

measurements and flow analysis. The rotating tempera-

ture fields obtained are shown in Fig. 3.11(a) and (b).

Here, the natural gas was completely entrained into the

Fig. 3.5. Co-incident pairs (a and b), (c and d), (e and f) of double PVC images from natural gas fired swirl burner, SZ1.77, via high-speed video. Images obtained via inclined stainless steel mirror.

N.

Syred

/P

rog

ressin

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ergy

an

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om

bu

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93

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8

Fig. 3.6. Variation of Strouhal number for first harmonic PVC with

flow rate and swirl number equivalence ratio, 0.98, DeZ75 mm

(1 m3/minZreynolds number 67,500) [37,38].Fig. 3.8. Variation of unconfined strouhal number with equivalence

ratio, 100% axial fuel injection, 100 kW unit, Fig. 2.1 [1,2,26,30,31,

37,38].


PVC and burnt on the boundary, the precessional

motion developing in the last half diameter before the

exhaust. By the exhaust the PVC was about 39% of the

exhaust radius and trailed a wake of burning hot gas,

achieving maximum temperatures of 1250 8C on the

edge of the PVC. Phase locked distributions of angular

momentum (rwr), Fig. 3.12, showed negative gradients

in and around the PVC, followed by a neutral region

just past the PVC, then an outer annulus with positive

gradients at larger radii showing stable flow. Further

analysis of this flow was carried out using the modified

Richardson number, Ri, which is the ratio of the

centrifugal forces in a field with density gradients to

the shear forces. Stabilising effects occur for values of

Fig. 3.7. Variation of Strouhal number for the second Harmonic of the

PVC with flow rate and swirl number [26,37,38].

RiO0. As the radial density gradient distributions

showed large negative values in and around the PVC,

Fig. 3.13(a) and (b), Ri becomes negative and thus

confirms the unstable nature of the flow region around

the PVC. Re-examination of the flames produced by

axial fuel injection, Fig. 3.1(b) shows that at the burner

exit the flame is burning in and around the central

vortex core region at quite a small diameter, typically

w0.2 De. There is little opportunity for negative

gradients of rwr and temperature (hence, density

gradient) to develop and thus precession of the vortex

core is minimised. The flame can only expand radially

when velocities have decayed due to the downstream

expansion of the flow, allowing matching to occur

between the flow and flame speed. This also causes a

downstream displacement of the CRZ; conditions in

and around the CRZ are then not favourable to

significant PVC formation.

Recent work from several sources [42–46,64] has

shown some light on stability of swirling flames when the

PVC is suppressed. Roux et al. [45] modelled the flows

within an atmospheric complex swirl combustion system

using compressible large eddy simulation (LES), acoustic

analysis and experiments in both isothermal and reacting

flows with methane as fuel. A vaned type swirler fired into

a square combustion chamber was used whilst the fuel,

methane, was premixed with the air. Reasonable

agreement between predictions and experimental

measurements was found. Under combustion conditions

(fZ0.75, mairZ12 g/s, QHZ27 kW) a PVC found under

isothermal conditions was suppressed. Here the combus-

tion aerodynamics are strongly influenced by an acoustic

Fig. 3.9. Variation of Strouhal number with equivalence ratio with partial premixing, 2 MW and 100 kW units [1,2,26,30,31,37,38].

Fig. 3.10. Photograph and schematic diagram of swirl burner/furnace

system, four times scale up of 100 kW system, operated with 25%,

tangential inserts to give Swirl No. 1.155 [30]: (a) photograph; (b)

schematic.


588 Hz 3/4 wave mode. A snapshot of an LES prediction

of an instantaneous temperature isosurface is shown in

Fig. 3.14(a) with a compact but irregular flame located

close to the burner exhaust. Mean temperature profiles

show a regular flame form [45], and this is constructed

from the average of thousands of snapshots as shown in

Fig. 3.14(a). This irregular instantaneous flame form is

clearly susceptible to distortion and coupling with

acoustic and other natural modes of oscillation of the

system. Selle et al. [64] carried out a LES simulation of a

swirl stabilised Siemens industrial gas turbine burner

firing into a square combustion chamber under atmos-

pheric conditions firing natural gas with preheated air at

673 K. The burner was constructed from two sections, a

central axial swirler is used to inject some air, whilst the

majority of the air is injected by a so-called diagonal

swirler. Fuel is normally injected in the diagonal swirler

through holes located on both sides of the swirl vanes.

Measurements were taken of mean and rms velocities for

hot and isothermal cases, in addition thermocouples were

used to obtain temperature fields under combustion

conditions. Under isothermal conditions PVC was

predicted and measured, but completely suppressed by

combustion and a 1000 K temperature iso-surface from

the LES work is illustrated in Fig. 3.14(b). This clearly

shows again the turbulent nature of the flame/flow

interaction where pockets of fresh gas are periodically

shed from the main flame zone and burn downstream.

A central core of hot gas is stabilised along the burner axis

by the CRZ, this core is attached to the face of the axial

swirler. The pressure field structure with combustion

corresponds with and induces an acoustic mode of the

chamber not analysed.

Syred et al. [46] have shown that swirling flames

with a suppressed PVC are susceptible to irregular

disturbances and hence, coupling with acoustic or other

modes of oscillation and indeed re-establishment of

a large PVC structure in certain circumstances [30].



The burner of Fig. 2.1 is operated with axial fuel entry

alone at fZ0.456, whilst the PVC amplitude is

substantially suppressed so little residual trace can be

found. A salt solution is injected with the fuel so that

the high temperature regions of the flame are well

visualised by the fluorescence of the sodium chloride.

This volatilises early in the high temperature regions

inside the burner, enabling high frequency images to be

obtained at 1 kHz, with an exposure of w0.1 ms (no

laser is used, this is just natural flame luminescence).

Successive images are subtracted from each other to

give a measure of the change of intensity especially

towards the edge of the flame, Fig. 3.15. These

subtracted images are then analysed to give a mean,

rims and local intensity value. Both a side and top,

tangential/radial, views are obtained via the use of

stainless steel mirror. This plot thus gives information

on the fluctuation of flame intensity for frequencies up

to 1 kHz for a 1 s time frame (the storage limit of the

camera was 1000 frames, based on LDA experience

probably 10,000 images are needed to obtain better

statistics). In particular, it shows that the edge of the

flame is highly intermittent with instantaneous fluctu-

ations up to six times the mean, this occurring within

about 1–1.5 burner exit diameters. Circumferentially,

there is also a very large non-uniformity as shown by

the top view, and tangential radial mixing appears to

dominate on the edge and top of the flame. The data can

be further analysed, Fig. 3.16. Here, four successive

flame images, again each separated by 1 ms have been

analysed in terms of the flame shape, the 5 and 95%

areas of maximum intensity have been identified.

Analysis of the behaviour of volatilised sodium in

flames indicates that the flame boundary corresponds to

the outer contour and a temperature around 650 8C. As

to be expected the downstream flame shows substantial

variation in shape, but most interestingly the flame just

leaving the burner exit shows considerable variation in

its diametric location and it appears that the flame is

physically wobbling or precessing with no regular

frequency that could be detected, there are also

indications of this in Fig. 3.15. The top view of the

flame shows that it is non-circular in shape and

considerably distorted. there have been similar reports

of this phenomena by other workers [40–42,45].

Examination of the cine film and still images so

derived shows that the flame is sensitive to small

disturbances and is easily disturbed by flow or acoustic

perturbations, especially downstream of the burner exit.

Clearly, the presence of a quarl (or conical burner

outlet) which guides the expansion of the flow can also

serve to damp significant eddy movements on the

outside of the flame as it expands past the burner exit,

(discussed in more detail later). However, it does little

to suppress the irregular circumferential movements

shown in the top views of Figs. 3.15 and 3.16, or the

irregular end section of the flame. The 95% intensity

contour also suggests that the boundary of the highest

temperature regions of the flame and central reaction

zone is also varying considerably, Fig. 3.16, probably

also corresponding to an irregular fluctuation in the size

and shape of the CRZ and associated shear layer.

Fig. 3.17 shows flame boundaries derived from an

analysis of three separate successive side views. Here, it

is clear that between images 783 and 784 there has been

Fig. 3.11. Phase locked rotating temperatures (8C) obtained from compensated thermocouples in swirl burner of Fig. 3.1(d), fZ0.02 [61]: (a)

x/DeZK0.52; (b) x/DeZ0.


Fig. 3.12. Phase locked radial distribution of angular momentum

flux [61].


massive movement in 1 ms, corresponding to mean

flame front velocities of up to 40 m/s in places, more

commonly 20 m/s. The radial movement of the whole

flame close to the burner exit is evident between images

783 and 784, with a radial displacement in 1 ms of

5.2 mm, or 10% of the flame diameter at this point.

As discussed earlier one reason for the suppression of

the large PVC is the very small column of hot

recirculating flow in the exhaust of the burner, surrounded

by a thin annular flame, this being contained within a high

velocity annulus of swirling flow where the flame will not

stabilise. Flame expansion occurs downstream of the

burner exhaust and this flame then surrounds the CRZ

located well downstream of the burner exhaust (i.e. flame

of Fig. 3.1(b)) in a region not conducive to PVC

formation.

The work of Roux et al. [50] is noteworthy in that it

shows a swirl flow combustor with premixed fuel and

air where the PVC is suppressed. The reasons for this

appear to lie with the configuration of the system,

location of the flame front and the development of the

swirl flow system and CRZ. In particular, swirling flow

enters an annulus surrounding a centre body and then is

forced into a contraction before entering the combus-

tion chamber. The processes occurring are illustrated

by Figs. 3.18(a), (b) and 3.19(a), (b) (fZ0.75, mairZ12 g/s, QHZ27 kW), which show velocity and tem-

perature profiles just inside the combustion chamber.

The important features are as follows.

For isothermal flow the tangential velocity close to the

entrance to the combustion chamber follows a Rankine

distribution [1,2], Fig. 3.18(a), with a steady rise from the

central axis to a peak in the forward shear layer (forced

vortex) followed by a decay towards the walls (free

vortex). This type of distribution continues downstream

with steady decay of velocity levels, Fig. 3.18(a), with

transference of angular momentum to the external flow

and smoothing out of the profile by xZ25 mm.

In contrast, the tangential velocities with combus-

tion show, Fig. 3.18(b), that at the entrance to the

combustion chamber (xZ1.5 mm) there is little

tangential velocity in the central region of flow.

Significant transference of angular momentum to this

region does not occur until xZ35 mm, Fig. 3.15(b). All

the tangential velocity is concentrated in an annular

flow region on entry to the combustion chamber.

Essentially, as there is little angular momentum in the

central region of the flow there is no real vortex core

(normally this region has a forced vortex distribution

[1,2]) and nothing to precess.

The axial velocity profiles under combustion

conditions, Fig. 3.19(a), show that the initial annular

jet flow rapidly diverges and gives rises to a large

toroidal recirculation zone and is of high velocity

w25 m/s. The corresponding temperature profiles,

Fig. 3.19(b), shows recirculation of very hot combus-

tion products back to the root of the flame at xZ1.5 mm. These hot recirculated gases are extremely

viscous and appear to substantially reduce the

transference of angular momentum into the central

region, thus producing conditions not favourable to

PVC formation. The presence of a centre body restricts

the upstream location of the CRZ and due to the high

velocity levels the flame cannot flash back and allow a

PVC to develop as reported in [1,2]. The configuration

of the centre body is important here, it consists of a

tapering cone leading from the axial swirler and

terminating at small diameter at the entrance to the

combustion chamber. There is thus some restriction of

flow on the central axis but not enough to induce a

substantive bluff body flow and allow a PVC to form as

reported in [39].

Although the PVC has been suppressed with

premixed combustion an acoustic 3/4 wave for the

whole device is amplified at 588 Hz and interacts with


the instantaneous the flame structure. Obviously under

appropriate conditions considerable excitation can

occur.

Selle et al. [64] also showed suppression of the PVC

with combustion in an Industrial gas turbine swirl

stabilised combustor. There are considerable simi-

larities to the results of Roux [45] in terms of the

velocity and temperature profiles produced close to the

burner exhaust when firing into the combustion

chamber. Especially, noticeable is the experimental

LDA results showing that there is negligible tangential

velocity in the central region of flow at x/DeZ0.35, this

only develops for values of x/DeO0.6 (LES results

differ). Again the temperature profiles show a CRZ

completely filled with hot gas at the main combustion

chamber temperature (w1650 K), surrounded by an

initially cold, just starting to burn, annular jet of fuel

and air, thus creating similar conditions to Roux et al.

[45]. Thus, there is a situation where there is initially no

swirl velocity in the central region, hence, no vortex

Fig. 3.13. Phase locked distribution of radial density Gradient: (a) cross-se

various phase angles.

core to precess. Similarly, use of the Rayleigh criteria

for stratified flows [62] and consideration of the

modified Richardson number, Ri, shows that

– as angular momentum flux, rwr, is very low in the

central region of flow close to the burner axis,

positive gradients exist due to the strongly swirling

annular jet entering the combustion chamber, thus

promoting stability;

– in terms of the modified Richardson number, Ri,

density gradients and centrifugal force gradients are

positive from the central region outwards to the

annular swirling jet, again promoting stability;

– this analysis applies equally to the work of Roux

et al. [45]. Unfortunately, neither Roux et al. [45]

nor Selle et al. [64] define a swirl number for their

configurations.

Reddy et al. [66] used PIV with a 508 vaned swirler

(swirl number S estimated at about 1) firing

ction at burner exhaust, x/DeZ0; (b) radial distribution at x/DeZ0,



isothermally into a square combustion chamber and

found both a central CRZ, and also, not unexpectedly, a

series of corner eddies. The swirl vanes are fitted to a

large flat centre body. The PVC was clearly visualised

well downstream of the swirler at x/DeZ2.5. Unfortu-

nately, the effect of combustion was not investigated.

Paschereit and Gutmark [65] described and analysed

the effectiveness of passive combustion control

methods applied to a low-emission swirl stabilised

industrial combustor. Several axisymetric and helical

unstable modes were identified for fully premixed and

diffusion type combustion. The combustion structures

associated with the different unstable modes were

visualised using phase locked images of OH chemilu-

minescence and analysed using cross-correlation

between OH detecting fibre optics. Four different

thermo-acoustic instability modes were forced to

occur by adjusting the acoustic boundary conditions

for different operating conditions. Each of the four

modes was due to different acoustic and or flow modes.

Three of the modes reported were of helical form, both

with premixed combustion and diffusion flames. The

Strouhal numbers ranged from (all Strouhal numbers

corrected to that used in this text) 0.59 (axisymetric

structure, premixed), to 1.19 (helical structure, pre-

mixed, 2.05 (helical structure, diffusion) to 7.97 (helical

structure, premixed). The helical structures appear to of

PVC form. One form of instability that contributed to

the pressure oscillations was movement of the CRZ and

initiation of vortex breakdown. Three passive control

methods were discussed and reviewed in the paper:

† Miniature vortex generators installed around the

circumference of the burner exit to induce instability in

the Kelvin–Helmholtz vortices formed at that point.

These instabilities disrupted the roll-up of the vortices,

thus reducing the source of regular oscillating heat

release, and disrupting amplification via the Rayleigh

criteria [20]. This technique reduced high frequency

oscillations and at the same time suppressed low

frequency instabilities. Some nozzle designs yielded

Fig. 3.14. (a) Snapshot of an LES prediction of 1250 K instantaneous

temperature, iso-surface [45]. Note compact but irregular flame

located close to the burner exhaust; (b) Snapshot of an LES prediction

of 1000 K instantaneous temperature iso-surface produced by

industrial gas turbine combustor [64].


over 10 dB suppression of high and low frequency

instabilities.

† An elliptically shaped burner, which essentially

has two volute or scroll inlets, induces axis-switching

dynamics in the large-scale swirling vortices formed in

the combustor. These are characterised by several

azimuthal unstable modes that reduce the coherence of

the vortices. Such geometry prevented coupling with

acoustic modes and resulted in suppression of instabil-

ities by over 25 dB for a wide range of flame

temperatures and power levels. In addition, NOx and

CO were reduced due to enhanced mixing and

increased turbulence.

† Extended pilot fuel lance protruding into the

plenum of the burner was used to stabilise the point of

vortex breakdown (VBD). Tests in high and low

pressure combustion conditions showed the VBD was

highly sensitive to combustor pressure fluctuations,

thus leading to another mechanism for thermoacoustic

excitation. A longer lance prevents this interaction and

was implemented in gas turbines in the field.

Krebbs et al. [67] undertook a detailed acoustic

analysis of a swirl stabilised gas turbine combustor and

described design and modelling methodologies aimed

at evolving configurations which minimise acoustic

response and excitation, hence interaction with any

nascent PVC.

In contrast, Ancacleto et al. [50] used a system with

a vaned swirler firing through a convergence, followed

by a first stage premixing chamber and then the

combustion chamber, Fig. 2.11. The first stage-mixing

chamber allowed the PVC to develop and this then

continued through into the combustion chamber so

combustion had little effect on the PVC.

A summary of flame types with and without the PVC

is summarised in Table 1. Six different flame types are

identified; the state of the PVC is indicated and whether

or not it is suppressed or not. What is clear from the

above is that even when the large PVC is absent the

resulting swirl stabilised flame is very sensitive to small

disturbances and can follow an irregular precessional

motion, which translates to large irregular motion of the

flame brush.

4. Vortex breakdown, modelling of the PVC and

related phenomena, comparison with experiment

The occurrence of the PVC is normally linked to

the phenomena of vortex breakdown and the

occurrence of CRZ. There is considerable evidence

from analytical and experimental studies that

precessional motion can exist at low swirl numbers

when CRZs are not present, although there do appear

to be significant differences to the PVC occurring

after vortex breakdown; this is discussed later in this

section. Sarpkaya [68] provided the first very

detailed experimental study of the vortex breakdown

phenomena and showed that the form, type and

occurrence were very much a function of Swirl

number and Re, Fig. 4.1. In his rig, vortex core

precession only started after the formation of the

initial breakdown bubble. Two main types of vortex

breakdown were identified, being a function of Swirl

number and Re. An extensive review of vortex

breakdown has been made by Lucca-Negro and

O’Doherty [69]. The paper reviews experimental,

numerical and analytical studies, as well as descrip-

tions, types and forms of the phenomenon. Although

a clearer picture of the flow structures produced has

emerged, a complete description of the phenomena

has not emerged. As the vortex breakdown phenom-

ena is normally regarded to be a pre-cursor to PVC

Table 1

Summary of flow and flame characteristics and the precessing vortex core

Flame/flow type Combustion intensity

other effects

PVC intensity and frequency (f) Strouhal no. fDelub

Correlation

Pressure drop Remarks

Isothermal Confinement doubles

PVC frequency for SO1.5

Iis!5 w/cm2 for SZ1.8 Good at high Re with/

without furnace.

Strong function of

Swirl no.

Audible low frequency

noise. Large PVC

(a) Premixed fuel and

air. Combustion

extends back

High comb. iIntensity;

Lfw1–5 De

Iw20Iis fpvcw3fisothermal Fair By factorw3 of

isothermal

Wide blow off limits,

violent flame

oscillations large

PCVs present

(b) Diffusion flames

axial fuel entry

Medium combustion

intensity: Lfw3–5 De

Iw0.01–0.1Iis fwfisothermal Poor at low Reynolds

no.

w90% of isothermal Exceptionally wide

blow off limits: PVC

suppressed

(c) Diffusion flames

axial fuel entry very

low f

Very weak combustion

flame burns on PVC

boundary

Iw0.8Iis fw0.85fisothermal Good at high Reynolds

numbers

w85% of isothermal Flame burns on PVC

boundary. Large PVC

present

(d) Tangential fuel

entry, diffusion flame

Medium combustion;

Lfw2–3 De

Iw0.01–0.1Iis fwfisothermal Poor at low Reynolds

numbers

Up to twice isothermal Narrow blow off limits:

flame quiet. PVC

suppressed

(e) Partially premixed

flashback prevented

back to inlets by

extension of exit see

Fig. 2.10

Medium combustion

intensity: Lfw1–2 De

Iw0.5–0.8Iisothermal f function of mixture

ratio and confinement,

Fig. 3.9

Fair, function

equivalence ratio

Similar to isothermal Axial fuel entry for

10–50% of fuel, rest

premixed. Large PVC

present

(f) Premixed confined

LPP configuration

Fwup to 0.75

High comb. intensity PVC suppressed Not applicable Not applicable PVC suppressed due to

system configuration

N.

Syred

/P

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ressin

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ergy

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93

–1

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Fig. 3.15. Intensity variations from a free natural gas, swirling flame, burner of Fig. 2.1, non-dimentionalised by maximum intensity value.

Simultaneous horizontal and vertical views. Intensity variations obtained by subtracting successive images obtained from camera with 1 ms interval

(exposurew0.1 ms) [46].


formation there is clearly a fruitful area for further

research here.

Experimental studies have indicated that for many

different systems (cyclone combustors, dust separators,

hydrocyclones, burners, fluidic vortex devices, vortex

whistles, Ranque–Hilsch tube, turbine runners [1,33,

71]) and different fluids the PVC is a phenomena which

only occurs when vortex breakdown has occurred

beyond critical levels of Swirl number, Re, and for

certain configurations [1,2,25,32,33,37,38,70,71].

Other work [39,40,41,44] has shown that preces-

sional motion may persist to very low values of swirl

number if a swirling jet is fired into a large expansion or

a centre body of significant size is present in the burner

exhaust. For zero swirl the flow past a centre body will

naturally induce a CRZ due to flow separation. As the

level of swirl is increased there is some form of vortex

breakdown occurring/change in recirculation zone/

CRZ structure leading to PVC formation [39],

providing any central fuel jet is of limited velocity as

otherwise the mechanism can differ with precession of

the central fuel jet.

Hallet and Gunther [78] while studying the flow within

a dump combustor, with expansion ratios Do/De ranging

from 1.25 to 3.0, visually observed jet precession within

the combustor chamber. They further concluded that

precession in a dump combustor was not beneficial for

mixing and did not pursue the matter further.

Dellenback et al. [79] conducted a series of

experiments with upstream swirl in a long pipe flow

to further observe the precession phenomenon. They

used an expansion ratio of Do/De of 1.94 and varied

the upstream swirl number from 0.05!S!0.4 for

ReZ30,000 and 100,000. Jet precession direction was

found to be related to Swirl number. At low Swirl

numbers the precession of the jet is opposite to that of

the upstream swirl. When the swirl increases past a

critical swirl number (Sw0.15), the flow precesses in

the same direction as the upstream swirl. However, the

air bubble visualisation technique was not able to

resolve jet precession direction for values for S!0.05.

The results were extrapolated to conclude that at a swirl

number of zero, no precession occurs. The other region

where precession direction was difficult to resolve

was at the critical swirl number, Scr. The authors

interpolated the data before and after the crossover

point and reasoned that no precession exists at the

critical swirl number (Scrw0.15).

There have been many attempts to model the PVC

phenomena using a number of tools ranging from

Fig. 3.16. Top four images show instantaneous successive images from sodium, seeded flame, contours show 5 and 95% intensity levels (images

move sequentially from left to right, top to bottom). Note irregularity both in the axial radial and tangential radial planes. Bottom left figure shows

normalised mean rms intensity from 1000 images [46].


analytical models to CFD and LES. One of the earliest

studies was that of Sozou and Swithenbank [72]. They used

an inviscid model of a vortex core embedded in an axial

flow and a perturbation technique by presuming small,

wave-like disturbances of variables about the asymmetric

flow. The intention was to model high frequency travelling

tangential waves, but the numerical solution converged to a

slow wave or PVC solution. Reasonable agreement with

the data of Chanaud [33] and Vonnegut [71] was found.

Avramenko et al. [73] extended this work by

considering an axisymetric swirling flow with radial

velocities that were an order of magnitude lower than

axial or tangential velocities. Cylindrical polar co-

ordinates were used with the assumption that unper-

turbed velocities and turbulent viscosity are functions

of radius only. The analysis eventually reduces to a

second order differential equation for the perturbed

tangential velocity amplitude. With the assumptions of

a linear form for the unperturbed tangential velocity

and considering only angular perturbations, analytical

solutions are then derived for the perturbed velocity

amplitudes in terms of Bessel functions and an

analytical solution for the Strouhal number in terms

of an effective Reynolds number. The model predicts

Fig. 3.17. Flame boundaries derived from an analysis of three separate successive axial/radial views, burner of Fig. 2.1, exit diameter 80 mm [46].


that the PVC frequency is only a weak function of

viscosity, but linearly dependent upon mean inlet

velocity, in agreement with experiments. The predic-

tion of the PVC frequency was improved by using

values of turbulent viscosity that varied linearly with

radius, adopting complex forms of the solutions,

invoking the variational principle. This produced a

general expression for the Strouhal number which

included the effects of axial perturbations and an

effective turbulent viscosity function [74]. Reasonable

agreement between experimental velocity measure-

ments and the theory was shown, Fig. 4.2.

Bowen et al. [74] extended this work further by

utilising variational techniques based on the principle

that solutions will tend to a state in which energy is

minimised given certain conserved quantities. They

used a stream function vorticity approach for two-

dimensional inviscid incompressible flow over a disc.

They then expanded these equations by Bessel–Fourier

functions, whilst using several variational theorems

which allow critical points of kinetic energy under the

constraints of conserved quadratic entropy and angular

momentum to be derived. Families of relative equilibria

solutions were produced, the first solution representing

the axisymetric case, the second term in higher order

solutions representing sets of vortices rotating about

each other. Prediction of the rotating flowfields

produced by the swirl burner of Fig. 2.1 were

qualitatively in agreement with the experimental

phase locked isothermal data produced near to the

burner exhaust, Fig. 2.7, with the dominant features of

the flow present. The model predicts the existence of a

second peak of tangential velocity opposite to the main

tangential peak, indicative of a second vortex, again

confirmed by experiment, Fig. 2.2(b). The central

region of negative tangential velocity is also well

predicted.

The first attempt to use CFD to characterise and

describe the PVC was by Sato [75,76]. Fluent with a

three-dimensional axisymetrical grid was used to model

the swirl burner furnace combination of Fig. 2.10(c).

Although a non-time dependent analysis was used he

showed for the isothermal state that the flow would

easily perturb and stick to a sidewall producing

structures similar to those experimentally recorded

and shown in Figs. 2.7 and 2.8. Bowen et al. [74] and

Lucca-Negro [77] extended the work of Sato using

Fluent and the RNG and RSM turbulence models

operating in a time dependent mode. Good qualitative

agreement between the CFD predictions and the

measured PVC characteristics, Figs. 2.7 and 2.8 were

found, although there was a tendency for the CFD

predictions to revert to axisymetry over time.

Guo et al. [80] used the CFX code and a VLES kK3

turbulence model approach for time dependent

analysis of turbulent swirl flow passing into a sudden

expansion, Do/DeZ5, ReZ105. The flow was unstable

over the whole swirl number range from 0 to 0.48, with

a large PVC type structure normally being present. The

analysis shows that with zero swirl the limit cycle is a

mixture of precession and flapping oscillation: the

flapping motion is significant up to SZ0.5. Increase of

Fig. 3.18. (a) Isothermal tangential velocity profiles in the combustion chamber. O LDA:LES [45]; (b) tangential velocity profiles in the combustion

chamber-combustion conditions. O LDA:LES [45].


Fig. 3.19. (a) Axial velocity profiles in the combustion chamber-combustion conditions. O LDA:LES [45]; (b) temperature profiles in the

combustion chamber. O thermocouples:LES [45].


Fig. 4.1. Occurrence of the vortex breakdown phenomena [68].


swirl number beyond 0.08 makes the precessional

motion dominant with more regular limit cycles. At the

same time the precessional frequency drops until it

reaches a critical swirl number, Scrw0.23, and then

increases again with Swirl number, but in the reversed

direction. Dellenback et al. [79] found similar results

experimentally, but with Scr at 0.15. Two instantaneous

visualisations of the precessional flow for swirl

numbers of 0.13 and 0.25 are shown in Figs. 4.3(a),

(b) and 4.4. The change in pressional direction should

be noted. Beyond values of SZ0.48 vortex breakdown

occurs with the formation of a CRZ and ERZ. The

Strouhal number, Fig. 4.5, varies linearly with swirl

number as found elsewhere, Fig. 2.5, although there is a

Fig. 4.2. Comparison of predicted and measured axial velocities at

exit of swirl burner [73].

sharp change at Scr, 0.23. The definition of Strouhal

number used here is as follows:

2ffiffiffiffip

p

� �Do

De

� �2 f De

ub

� �

Compared to conventional definitions of Strouhal

number the inclusion of the terms ð2=ffiffiffiffip

pÞðDo=DeÞ2

increases the value by more than 28, and thus the values

in Fig. 4.5 must be divided by this value to compare

with Fig. 2.5, i.e. giving values ranging from 0 to

0.0075. The modelled processes are thus similar to

those occurring with precessing jets [51–57], where

values of (a comparable) Strouhal number between

0.001 and 0.005 were found. This is hardly surprising

considering the extent of the jet expansion (Do/DeZ5)

and the low level of swirl. Thus, this form of precession

is quite different to that associated with the CRZ and

which normally occurs for higher values of S, where

values of Strouhal number are from 0.2 upwards.

Guo et al. [41] used RANS kK3 time dependent

calculations and extensive measurements to character-

ise swirl flow instabilities in the Swirl burner developed

at the University of Sydney, Fig. 2.6 [41–44], primarily

for non reacting flows. Data from the earlier work of

Al-Abdeli et al. [44] was used for comparison. For

isothermal conditions they showed that for ujZ66 m/s

and ubZ16.3 m/s increases in Swirl number, Sg,

eventually lead to the detection of distinct frequency

peaks indicative of precession. This was initiated at

SgZ0.34 with a 20 Hz irregular oscillation, leading to

stable strong precession at SgZ0.4, again at 20 Hz.

Further increases of Sg to 0.57 produced a further peak

at 28 Hz, followed at SgZ0.68 and 0.91 peaks at w28

and 26 Hz, respectively. Increase of Sg to 1.59 showed

no distinct frequencies, but considerable noise. A lower

jet velocity of 50 m/s for SgZ0.4 gave a frequency

peak of 17 Hz, whilst a jet velocity of 90 m/s for SgZ0.57 gave a 35 Hz frequency. As discussed earlier there

are obviously interactions between the swirl, bluff body

and central jet which are difficult to separate. High

velocity central jets are well known to cause substantial

changes in flow patterns both for bluff body [81] and

swirl flows [1,2] and further work is needed to separate

effects.

RANS prediction of the Strouhal number variation

with Sg are shown in Fig. 4.6(a) and (b). Fig. 4.6(a)

(ubZ16.3 m/s) shows good agreement with measured

and predicted vales of Strouhal number: Fig. 4.6(b)

(ubZ29.7 m/s) shows poorer agreement for a higher

bulk fluid velocity. Here again the Strouhal number

(SSN) is defined unconventionally, see the

Fig. 4.3. Instantaneous visualisation of swirl flow showing its spiral nature, SZ0.13 [80]: (a) isosurface of axial velocity; (b) image showing

corresponding vortex core location.


nomenclature. The values of Strouhal number may be

converted to the conventional definition (fDe/ub) by the

parameter 1.2Sg. If this is carried out the values of

Strouhal number reduce to a range of w0.04 (SgZ2.1)

to 0.1 (SgZ1) maximum. For high swirl numbers, SgO0.8 these values of the SSN and conventional Strouhal

number are very low w0.1–0.04, indicating again that

this is different to conventional swirl burner systems as

described by Fig. 2.5. This is confirmed by Fig. 4.7

which shows snapshots the isothermal flow evolution

and structure change with variation in Sg. Regions of

reverse flow are displayed by the contour line u!0. For

low values of Sg, 0.35, the central jet deflects little and

the flow is virtually symmetric. A downstream

recirculation bubble appears at about x/DeZ1.67

from the burner face at SgZ0.6. This bubble appears

to restrict the central jet movement as the precessional

frequency is reduced by subsequent growth of bubble

size. A recirculation ring exists in a stagnant region

behind the burner face. The extent of the downstream

bubble increases with Sg, eventually merging with the

upstream recirculation zone. Subsequent increases of

Fig. 4.4. Instantaneous visualisation of swirl flow, SZ0.25, note

change of direction of precession from Fig. 4.3 [80].

Sg cause the bubble to intersect the central jet so the jet

precesses within the confined space created by the

recirculating bubble. As has been commented earlier

these very complex structures differ very significantly

from the conventional CRZs discussed in [1,2]. Limited

reacting flow studies were undertaken where it was

found that increasing heat of reaction of the fuel

suppressed precession. The Strouhal number results

from the Sydney Swirl Burner show that the precession

generated is very similar to that of the precessing jets of

references [51–58].

Wegner et al. [82] used time dependent RANS, LES

and experiments to characterise isothermal swirl flow

instability in an Ijmuiden type of movable block swirl

generator [1,2], that has been extensively studied in

the EU funded TECFLAM programme. The device is

shown in Fig. 4.8, together with the computation grid

used. As can be seen the computational grid extended

back into the device and to the sets of inlets used to vary

the swirl level. The RANS method employing a full

Reynolds stress model was able to capture the PVC

phenomena both qualitatively and quantitatively in

Fig. 4.5. Variation of Strouhal number with Swirl number, without

CRZ presence [80].


parts. Good accuracy was achieved for predictions of

PVC frequency, however the energy contained in the

coherent motion of the PVC was significantly under-

predicted by the unsteady RANS. Measured and

predicted values of Strouhal number for SZ0.75 are

in agreement with the results of [39], which are already

plotted in Fig. 2.5.

As has been discussed earlier, Roux et al. [45]

carried out an LES study of the atmospheric flow from a

gas turbine combustor into a large furnace chamber,

Figs. 3.11(b) and 3.15(a). For the isothermal state a

major PVC 540 Hz oscillation was found in the main

chamber as well as a strong second acoustic mode at

360 Hz. The PVC frequency did not coincide with any

major acoustic modes of the system and was the

dominant mode of oscillation. A visualisation of the

540 Hz PVC at the exit of the swirl burner in

the combustion chamber is shown in Fig. 4.9, and the

helical nature is clear. The Strouhal number is

estimated at around 0.7–0.8 from available data and

Fig. 4.6. Variation of Strouhal number with swirl number, Sydney

swirl burner [41] for bulk fluid velocities ubZ16.3 and 29.7 m/s.

this is agreement with the data for the swirl burner of

Figs. 2.1 and 2.5 for a geometric swirl number of about

1. Fig. 4.10 shows the measured and modelled pressure

fluctuations for two positions in the system. The PVC

can be seen to be only influential in the combustion

chamber, whilst an acoustic resonance dominates in the

inlet plenum. Combustion results and the suppression

of the PVC are discussed in the next section.

Selle et al. [64] studied a Siemens industrial gas

turbine burner firing into a combustion chamber using

LES and detailed experimental results. The burner is of

complex geometry with both a central axial and an outer

diagonal swirler. An instantaneous visualisation of the

isothermal predicted PVC is shown in Fig. 4.11 for this

configuration in the form of a snapshot of a pressure

isosurface, the rotational frequency is 275 Hz. The flow

inside of the spiral structure is recirculating in a CRZ,

with the entire structure, PVC and CRZ rotating about the

central axis causing large pressure perturbations. The

sense of the rotation of the whole spiral, as a structure, is

that of the surrounding swirling flow, but the sense of the

winding of the spiral is opposite to that of the swirl. Not

enough information is provided to calculate Strouhal and

Swirl numbers. Again suppression of the PVC with

combustion is discussed in the next section.

5. Oscillations in swirl burner furnace systems,

related systems and associated driving mechanisms

In order to explain in part the driving mechanisms

for instability in swirl stabilised combustion systems,

it is useful to characterise the complex mechanisms

occurring under oscillation conditions and flow

conditions where the PVC is suppressed. There are

few articles in the literature which quantify the

processes occurring under regular, stable, oscillatory

conditions whilst analysing the underlying processes,

apart from Fick [30], Froud, [19,85], Dawson, [83,86],

Syred et al. [18,61,84], Rodriquez-Martinez [28,29],

Roux et al. [45], Ancacleto et al. [50], Schildmacher et

al. [87–90]. In each of these references, analysis has

been made of the flow and other structures in swirl

combustion systems oscillating under representative

but very different conditions using a variety of

techniques including phase locked LDA, LDA, phase

locked temperatures via fine wire thermocouples and

LES. A variety of different oscillations have been

investigated with a range of different driving mechan-

isms, these are discussed below. This is complemented

by two studies [45,64] where the PVC is suppressed by

combustion.

Fig. 4.7. Sydney swirl burner, RANS visualisation of instantaneous flow-field showing evolution with Sg for ubZ16.3 m/s. Contour lines show

reverse flow zones where axial velocity is zero [41].


5.1. Driven PVC oscillations in the 2 MW swirl

burner/furnace system, 100% axial fuel entry

This occurred with the large 2 MW swirl burner/

furnace system shown in Fig. 3.10(a) and (b). This

involved a 24 Hz oscillation [30,84], identified as of

large, high amplitude, PVC form. It occurred with

100% axial fuel injection (no premixing), an equival-

ence ratio of 0.092. This gave a value of Strouhal

number of 4.8, some four times that expected with the

isothermal state or 5–8 times that with partial

premixing, Fig. 3.9. Indeed under these conditions a

suppressed PVC would have been expected. This

oscillation only occurred at low values of equivalence

ratio as indicated, but over a significant range of

flowrates, and there are similarities to the combustion

state reported in [61] and previously described in

Section 3. Acoustic analysis of the swirl burner/furnace

Fig. 4.8. Ijmuiden movable block swirl generator and computational

grid [82]: (a) schematic of device; (b) computational grid.

Fig. 4.9. Isosurface of low pressure to visualise the isothermal PVC

formed at the exit of the swirl burner [45].


system showed that there were a number of different

acoustic modes corresponding to this resonance,

including basic Helmholtz and inlet travelling waves.

It thus appears that the large PVC is being driven

by coupling with acoustic resonances of this system.

The oscillation was analysed by measurement of phase

averaged temperature (via compensated fine wire

thermocouples), axial and tangential velocities, as

shown in Figs. 5.1–5.3. Fig. 5.1 shows the rotating

axial and tangential velocities x/DeZ0.5 below the

burner exit in the furnace. In comparison with the flow

field found with the PVC freely exhausting in open air,

(Figs. 2.7 and 2.8) there is less circumferential non-

uniformity, especially with the axial flow, Fig. 5.1(a).

The tangential flow shows an elongated, elliptical

shaped higher velocity region extending over phase

angles of 200–2508, whilst due to the confinement the

flow has not spread radially as much as the unconfined

system, Fig. 5.1(b). There is evidence of a small CRZ in

the centre of the flow, Fig. 5.1(a), and this matches the

characteristics of the open flame found with axial fuel

entry in the vicinity of the burner exhaust, Fig. 3.1(b).

Here, almost all the combustion takes place past the

burner exhaust, there being a thin narrow tulip shaped

CRZ which extends back into the burner and the back

plate. The phase averaged tangential velocity,

Fig. 5.1(b) shows a small central region of negative

tangential velocity, characteristic of the PVC. The

rotating temperature field, Fig. 5.1(c), shows that

combustion is occurring in a region surrounding the

small CRZ of diameter about 0.56De, just within the

annular high velocity regions shown in Fig. 5.1(a) and

(b). The irregular nature of the outer periphery of the

flame appears to be due to shear effects from this high

velocity region, with the flame moving into lower

velocity regions opposite to the high velocity PVC

region. Combustion occurs in regions with velocities up

to about 7 m/s. Thus here, at x/DeZ0.5 a fairly stable

combustion region is surrounded by the rotating PVC

and associated flows. Correlation of Fig. 5.1(c) and (b)

shows there is a small trailing arm of hot 1200 8C

combustion gases which have expanded into a low

tangential velocity region of flow for phase angles 120–

1808, thus increasing the diameter of the flame in this

region by 40% or more: there are similarities to

Fig. 3.11(b). This periodic variation in heat release

can be one of the feedback mechanisms for the

Rayleigh criteria. There is also clear evidence of a

large external recirculation zone near to the outer walls

as shown by the negative axial velocities here.

Further downstream at x/DeZ1.5, Fig. 5.2a–c, phase

averaged axial and tangential velocities have become

much more uniform circumferentially; the CRZ has

Fig. 4.10. Pressure fluctuation spectra for isothermal flow at two locations [45]. Solid line experiment, dashed line LES.


expanded somewhat in size, Fig. 5.2(a), but is still off

centred and of kidney shape. There again is evidence of

an external recirculation zone close to the outer walls as

shown by reversed axial velocities, Fig. 5.2(a).

Residual vortex core precession is also present,

Fig. 4.11. Pressure iso-surface visualisation of the isothermal pv

Fig. 5.2(b), in the centre of the flow as shown by

negative tangential velocities. The phase averaged

temperature contours, Fig. 5.2(c), show that here the

flame has expanded radially beyond the regions of

highest velocity, being about 1.3De in overall diameter.

c generated by an industrial gas turbine swirl burner [64].

Fig. 5.1. Phase averaged characterisation of oscillating flow in the

2 MW swirl burner furnace at x/DeZ0.5 below burner, 100% axial

fuel injection, fZ24 Hz [30]: (a) axial velocity; (b) tangential

velocity; (c) temperature 8C.

Fig. 5.2. Phase averaged characterisation of oscillating flow in the

2 MW swirl burner furnace at x/DeZ1.5 below burner, 100% axial

fuel injection, fZ24 Hz [30]: (a) axial velocity; (b) tangential

velocity; (c) temperature 8C.


The axial velocity contours in a single axial radial plane

at a phase angle of 1058, Fig. 5.3, are typical of those

found. The CRZ is not symmetrical, being of annular

form and slightly titled to one side; only one view is

presented as the differences between successive phase

angles is small. The driving mechanism for the

Rayleigh criteria appears to arise from small variations

in the diameter of the flame, Figs. 5.1(c) and 5.2(c);

examination of the original data indicates that

throughout the cycle at x/DeZ1.5, the flame diameter

(as characterised by the 1035 8C contour) contracts by

up to 15% or more, especially between phase angles of

190 and 2608 on the exterior boundary and 270–908

internally. This variation in heat release appears to be

sufficient to provide the requisite driving mechanism

Fig. 5.3. Axial velocity contours at a phase angle of 1058 for the

2 MW burner, fZ24 Hz, 100% axial fuel injection [30].


for large amplitude oscillations. Examination of the

overall flow field suggests that as the flame is not

impinging on the wall in the system it is free to wobble

radially in response to external perturbations (such as

arise from acoustic resonances), like the flames shown

in Fig. 3.14(a) and (b), and observed by other workers

[42,45]. This has resulted in a large PVC type of

resonance for a situation where PVC suppression would

normally occur. Clearly, stabilisation methods for such

flames require methodologies to reduce the wobble at

the base and to better stabilise the flame downstream by

avoiding the weak, doubtless intermittent, flow regimes

between the flame and the outer walls.

5.2. Helmholtz and other resonances and vortex wobble

/precession in a 100 kW swirl burner/furnace system,

partial premixing

The next type of resonance generated in a swirl

burner/furnace system is quite different, as discussed by

Froud [31], Froud et al. [19,85]. Here, the swirl burner/

furnace system of Fig. 2.10(c) was operated in a wide

range of different modes with the deliberate aim of

stimulating regular oscillations so the system could be

appropriately described and driving mechanisms ident-

ified [31]. One such case for a swirl number of 1.5 is

shown in Figs. 5.4 and 5.5 [31,85], where a 900 mm long

extension has been added to the exhaust of the furnace

(same diameter as the swirl burner exhaust). This caused

a high intensity regular oscillation in the system as

characterised by Figs. 5.4 and 5.5 where first and second

harmonic frequencies and amplitudes as a function of

equivalence ratio are shown. For a constant fuel flow rate

(110 l/min), the air flow rate is varied such that a range of

equivalence ratios from 0.4 to 1.2 is covered. Other

variables include the isothermal PVC frequency and the

Helmholtz frequency of the system calculated by

assuming that the gases in the furnace act as the

capacitance and the flow oscillates in the extended

furnace exhaust pipe as the neck. An average combus-

tion temperature is assumed to give the result shown in

Fig. 5.4. Other acoustic resonances were investigated,

but did not fit the data. Many interesting features are

shown:

– A high-amplitude, low-frequency resonance occurs

for equivalence ratios 0.57–0.83, the frequency of

which is close to that of the predicted Helmholtz

oscillation.

– A second resonance of much reduced amplitude

occurs over the equivalence ratio range 0.57–0.83

with frequencies in the range 130/140 Hz, some three

times higher than the first harmonic. The frequency

varies quazi linearly with equivalence ratios between

0.57 and 0.83, hence with flow rate as the natural gas

flow rate is held constant at 110 l/min.

A PVC structure could also be seen to be forming in

the exhaust of the furnace as discussed by Fick [30],

see Fig. 2.10(a) and (b) plus associated text.

Similar results were achieved with axial fuel entry

alone, different fuel flow rates and variable furnace

exhaust extension pieces. Longer extension pipes gave

sharper resonant peaks and much higher amplitudes of

oscillation.

Fig. 5.4. Oscillation frequencies as a function of equivalence ratio for swirl burner furnace, Fig. 2.10(c), 900 mm exhaust extension, 50% axial fuel

injection, partially premixed [31,85].

Fig. 5.5. Corresponding first and second harmonic oscillation

amplitudes as a function of equivalence ration (arbitrary units) [31,85].


For all cases combustion was not complete until the

gases had entered the final extension pipe section, as

shown by surface temperatures.

As the resonance was essentially driven by a

Helmholtz oscillation the frequency was constant over

a wide range of equivalence ratios and flow rates and

hence, it is difficult to immediately associate the PVC

or related flow instabilities with this resonance.

Phase averaged tangential, axial velocities and

temperatures (again using compensated thermocouples)

over the oscillation cycle, just past the swirl burner exit

and in the furnace, were used to characterise the

mechanisms of oscillation in the system for an

equivalence ratio of 0.671 and an oscillation frequency

of 41 Hz, Fig. 5.6 (phase angle 08), 5.7 (phase angle

908), 5.8 (phase angle 1808), 5.9 (phase angle 2708)

[83].

Examination of the axial velocity levels through the

system, Figs. 5.6–5.9, shows the device is acting like a

pulsating combustors as the axial flow is virtually

stopped between phase angles of 90 and 1808. It is only

at a phase angle of 08 that a conventional type of swirl

burner flow exists with a continuous flow stream

leaving the burner exit, hitting the furnace wall at about

x/Dew0.5De and then staying attached as it moves

through the furnace. A large CRZ exists in the centre of

the flow, with some evidence that it extends down to the

end of the furnace. Highest levels of tangential velocity

are not reached until x/Dew1 downstream of the swirl

burner exit for all phase angle shown, 0, 90, 180, 2708,

Figs. 5.6–5.9 The fluctuating temperature measure-

ments, Fig. 5.6, show that at a phase angle of 08,

combustion is confined to a central rod shaped region in

and around the CRZ and this appears to act as a pilot

flame through the oscillation. At a phase angle of 908,

Fig. 5.7, the rising pressure of the oscillation has

virtually stopped flow entering the furnace from the

swirl burner, leaving a weak, annular CRZ close to the

swirl burner exit and a weak ERZ. The flame has

weakened in the central region, but a flame front can be

seen to be propagating backwards down the wall of the

combustor, Fig. 5.7 in the low velocity region. Flow

continues to swirl with high tangential velocities in the

main section of the furnace, Fig. 5.7. Both Figs. 5.6 and

5.7 show very significant levels of negative tangential

velocity in and around the centre line along the whole

length of the furnace (phase angles 0 and 908). This

must be associated with vortex core precession or some

form of vortex wobble. Fig. 5.8 (phase angle of 1808)

shows the flame front has now moved completely down

the outer wall and become joined to the central region

of combustion.

The corresponding axial velocity contours, Fig. 5.8,

show that apart from small regions towards the far end

of the furnace, axial velocities are low, creating

conditions favourable for flame stabilisation in the

main section of the furnace. Quite high levels of


tangential velocity still persist in the downstream

section of the furnace, Fig. 5.8. Finally, Fig. 5.9

shows results for a phase angle of 2708. The axial

velocity contours show that flow is reissuing from the

swirl burner exhaust as the pressure in the furnace

reduces, this can also be seen for the tangential

velocities. The outer annular flame front has retreated

up the furnace, just leaving the central rod shaped pilot

flame. Again Fig. 5.9 shows significant levels of

negative tangential velocity in and around the centre

line of the system, indicative vortex precession or

wobble. Both Figs. 5.8 and 5.9 show areas of negative

tangential velocity in the region of the external

recirculation zone, close to the burner exit for r/roO0.5, again indicative of unstable swirling flow. The

negative tangential velocities in and around the centre

line of the system thus indicate that as in the previous

case, with the 2 MW swirl burner/furnace system, the

flow is wobbling radially as it leaves the swirl burner

exit (where there is little or no combustion, only some

recirculation of hot gases via the CRZ back into the

burner exit). Wether this wobble is irregular or regular

in nature is difficult to resolve as some data smearing

does occur with phase locked LDA and temperature

measurements and there may be undetected generation

Fig. 5.6. Phase averaged (phase angle 08) temperat

of axial radial eddies from any PVCs that are present.

Simple calculations based on the maximum tangential

velocity and its radius as it leaves the swirl burner exit

(phase angles 270 and 08) indicate that a double PVC

may be present with a frequency of about 80 Hz for part

of the cycle. However, flame wobble probably produces

circumferential variation of heat release, triggering the

formation of axial radial eddies, generating alternating

patterns of rich and lean combustion sufficient to

reinforce combustion oscillations via the Rayleigh

criteria, especially as the flame propagates back along

the furnace wall towards the swirl burner exhaust at a

phase angle of 1808, Fig. 5.8, as with the 2 MW system

previously described.

More recently, Rodriquez-Martinez [27,29], Daw-

son et al. [86] have extended the work on the 100 kW

swirl burner/furnace system, Fig. 2.10(c), producing a

number of stability maps similar to that shown in Figs.

5.4 and 5.5. Of relevance here is the detailed phase

averaged velocity characterisation of a low frequency

(41 Hz, SZ2.18, equivalence ratio 0.9) system

oscillation, this time excited by travelling waves in

the inlet pipe. This lead to instantaneous flow reversal

in the pipes over part of the limit cycle oscillation.

The configuration of the furnace was changed slightly

ures, 8C, axial and tangential velocities [85].

Fig. 5.7. Phase averaged (phase angle 908) temperatures, 8C, axial and tangential velocities [85].


from that of Froud [31,85], with a reduced furnace exit

diameter, Dfe/DeZ0.7, and small changes to the

length of the furnace taper and length. Inside the swirl

burner furnace system there were some differences in

the flow patterns generated, but there still clearly

existed a fundamental pattern of the flow into the

furnace from the swirl burner being periodically shut

off over the limit cycle of oscillation. As angular

momentum and swirl velocities were largely con-

served in the swirl burner and furnace system over the

limit cycle oscillation, the considerable variation in

axial velocity caused large variation in swirl number

and hence, size and shape of the CRZ. No negative

tangential velocities were found with the measured

tangential velocities inside the system, although

directional intermittency measurements clearly

showed that the centre line of the vortex in the

furnace was wobbling off centre more than 30% of the

time. Similar effects were found close to the outer

wall, but the internal measurements were not as

detailed as those of Froud [31,85]. Detailed phase

locked axial and tangential velocities were taken just

above the top of the furnace exit. Fig. 5.10(a)–(e)

shows the phase averaged pressure trace used for

triggering purposes and illustrates there are inputs

from several harmonics in the system; similarly the

geometry and geometrical ratios used are illustrated.

The phase averaged axial velocity, Fig. 5.10(a) shows

that although flow is issuing from the furnace exit for

all phase angles, the velocity and hence, flow rate

doubles over most of the section for phase angles 240–

758. The associated axial directional intermittency

plot, Fig. 5.10(c), shows that there is some irregular

wobble for all phase angles, the most intense effect

being between 0 and 908. The tangential velocities,

Fig. 5.10(b), show a very different pattern with the

most intense swirling flow being confined to phase

angles between 300 and 908. The corresponding

tangential directional intermittency plot, Fig. 5.10(d)

shows that this flow is very unstable over the whole

limit cycle of oscillation, both close to the outer wall

and in the central region of flow. Instantaneous flow

reversal is occurring up to 40% of the time for phase

angles 250–458 close to the outer wall and again this

infers a high level of vortex wobble and/or precession,

probably originating from excitation of the swirling

flow leaving the swirl burner and entering the furnace

as discussed in the data from Figs. 5.6 to 5.9.



5.3. Characterisation of high frequency oscillations in

a 100 kW swirl burner furnace system, partial

premixing

Rodriquez-Martinez [27,29] and Dawson [28,83,86]

have extended the work with the generic 100 kW swirl

burner/furnace system of Fig. 2.10(c) with 60% of the

fuel being premixed with the air, the rest being

introduced axially to investigate not only low frequency

oscillations but those in the range w240/260 Hz, again

produced by subtle changes in furnace geometry,

Fig. 5.11. The two geometries are designed to contrast

the effects of a sudden expansion and a quarl. Fig. 5.12

contrasts pressure and frequency spectra data from the

two different configurations, a and b refer to geometry

2a and c and d to geometry 2b with a quarl inserted. The

quarl substantially reduces the amplitude of the

oscillation. The spectral analysis for both cases,

Fig. 5.12(b) and (d) shows the predominant peak of

the Helmholtz resonance at w240 Hz, although there is

also a low frequency peak present for both cases at

around 40 Hz, corresponding to the bulk mode low

frequency oscillation previously described. Fig. 5.13

shows the corresponding rms pressure and frequency as

a function of equivalence ratio. The high frequency

oscillation persists for an equivalence ratio range

w0.55–0.85, dependent on the case, reverting to the

low frequency bulk mode oscillation beyond these

limits, w40/50 Hz, although at much lower amplitude

levels. The effect of the quarl is seen to substantially

reduce the amplitude of oscillation, case 2b compared

to the case without it, case 2a, over virtually the whole

of the equivalence ratio range where this high

frequency oscillation is found. The quarl has little

effect on the amplitude of the low frequency

oscillations. Simultaneous measurement of light emis-

sion and pressure from the system enables a Rayleigh

index to be constructed, which showed, as to be

expected that maximum excitation occurred in the flow

region immediately downstream of the swirl burner exit

in the furnace [27,29,85]. The high frequency oscil-

lations are attributed to near in phase coupling of a

natural Helmholtz resonance of the swirl burner and

furnace with the combustion process and swirl

dynamics. The exhaust of the swirl burner acts as the

neck of the resonator, and periodic heat release occurs

via the mechanisms discussed above [27,83], including

wobbling and precessional motion of the swirling flow



as it leaves the burner exit, as well as shed partially

burning radial axial eddies.

Detailed phase locked velocity measurements for

both this high frequency oscillation case are shown

in Figs. 5.14 and 5.15, case 2a, and 5.16, case 2b

[27,29,86]. The phase locked velocity levels for case

2a, Fig. 5.14 show considerable differences from those

found with the low frequency oscillation, Figs. 5.6–5.9,

in that the flow through the swirl burner is not

periodically arrested, but slows and accelerates in

tune with the near sinusoidal pressure wave shown on

Fig. 5.14(e). Fig. 5.14 compares and contrasts axial and

tangential velocities as well as their directional

intermittencies for six phase angles. Most variation

occurs with the axial velocity which shows the CRZ

expanding and contracting with the sinusoidal pressure

wave, becoming detached and quite weak at a phase

angle of 2408, whilst extending well down into the swirl

burner and the burner back plate at a phase angle of 08.

This arises because the variations in axial velocity and

hence flow rate into the furnace cause a variation in

swirl number (as in previous cases) estimated from 0.8

at phase angle 2408, rising to nearly four at phase angle

08. Changes in size and shape of recirculation zones are

well known to produce substantial pressure pertur-

bations and this also probably adds to the mechanisms

contributing to instability. An external recirculation

zone is also evident as the swirling jet fires into the

furnace. Axial directional intermittency levels show

that much of the swirling jet entering the furnace is

quite stable, but with layers of significant intermittency

on the sides as it interacts with the CRZ and ERZ. The

phase locked tangential velocities have very small

regions of negative tangential velocity on the centre

axis (indicative of vortex wobble), but significant

regions close to the outer wall, whose size and location

vary considerably over the pressure cycle. The

tangential directional intermittencies are possibly the

most revealing showing very considerable intermit-

tency approaching 80% close to the wall for some phase

angles.

Again, as with the 2 MW swirl burner system and

the 24 Hz PVC type oscillation, the flow and hence,

flame is wobbling and precessing in the furnace,

possibly several PVCs are spiralling in the system

over part of the limit cycle oscillation at a much

higher frequency. Reference to Fig. 5.12 shows that

there is some modulation on the pressure signal for


case 2a, with a superimposed frequency probably

originating from the low frequency 40/50 Hz peak

evident on the power spectrum. This effect is also

illustrated by Fig. 5.15(a) and (b), which show the

phase averaged axial and tangential velocities and

associated directional intermittencies, Fig. 5.15(c) and

(d) just above the furnace exit for case 2a. The results

are quite different to the low frequency oscillation

results above the furnace exit, Fig. 5.10. The

tangential velocity field is virtually uniform above

Fig. 5.10. Phase locked velocity levels just above furnace exit [28,29,83] at X

velocities, axial directional intermittency, tangential directional intermitten

(a) Phase averaged axial velocities; (b) phase averaged tangential veloc

intermittency; (e) pressure trace; (f) schematic diagram of swirl burner furn

the furnace exhaust, whilst there is still some

variation in the axial velocity through the pressure

cycle for phase angles 90–2858. However, the

directional intermittency levels for both axial and

tangential velocities are both very high, although anti-

phased. Again this is indicative of wobble in the main

flow leaving the burner which is amplified by the

combustion process and Helmholtz resonance.

The corresponding data from case 2b with the quarl,

is shown in Fig. 5.16, although with a restricted set of

/De Z 0.52. Phase averaged axial velocities, phase averaged tangential

cy, pressure trace, schematic diagram of swirl burner furnace

ities; (c) axial directional intermittency; (d) tangential directional

ace.



data owing to the quarl interfering with laser access.

The presence of the quarl has made the axial velocity

contours much more uniform over the oscillation cycle

with a more uniform and consistent CRZ, albeit of

reduced size. Apart from a phase angle of 08, the ERZ

has disappeared giving much more stable flows in this

region and much lower values of directional inter-

mittency. Again the tangential velocity profiles are

quite uniform over the oscillation cycle, whilst the

levels of directional intermittency are substantially

reduced, both at the wall and towards the central axis.

Above the furnace exit the flows were much more

stable with substantially reduced levels of directional

intermittency [27,29]. Doubtless, better shaping of the

quarl section could have improved these results and

reduced the pressure amplitude even further.

The Rayleigh Criteria for stratified flows [62] is useful

here. Although temperature measurements are not

available examination of tangential velocity contours

and associated directional intermittencies, Fig. 5.14(c)

and d, Fig. 5.15(b) and (d), configuration 2a, shows

significant levels of negative tangential velocity in the

region of the ERZ and near the swirl burner exit over at

least 60–70% the oscillation cycle. Thus, at the burner exit

in the furnace, moving radially outwards from the

entering annular, highly swirling shear flow, gradients

of angular momentum, rwr, must be negative, thus

confirming the unstable nature of this region. For

configuration 2b and the corresponding Fig. 5.16(c) and

(d) there are no regions of negative tangential velocity and

thus certainly stability in the outer region of flow close to

the walls is much improved.

Even for dilute combustion systems operating beyond

an equivalence ratio of 0.6, thus putting them beyond the

range of the high frequency oscillation with this

configuration, the role of the quarl in stabilising wobbling

or irregular precessing swirling flow is evident, as well as

the improvement of the gradient of rwr. Clearly, the flow

and flame stabilisation methods proposed herein cannot

eliminate the acoustic response of the system.

The mechanism of instability and coupling thus

appears to be irregularities in the flame boundaries

and/or reaction surfaces/areas, primarily associated

with wobble or precession of the main vortex, possibly

distortions of the CRZ, axial/radial eddy shedding from

the shear layer, triggered by PVCs. Associated with this

CRZ distortion is the production of a PVC whose radius

of precession is governed by the motion and distortion

of the CRZ, and the actual instantaneous level of swirl

at a given point in the oscillation cycle.

Other work on Industrial gas turbines using CFD has

shown precessing vortices leaving the combustor can

exhaust, passing through and attaching to the turbine

guide vanes, causing overheating problems [87],

Fig. 5.11. Swirl burner/furnace configurations to produce 240/260 Hz oscillations [27,28,86]. Differences between configuration 2a and 2b involve

the removable insert which forms a conical exit or quarl at the exit of the swirl burner as it enters the furnace.

.


confirming the results shown in Fig. 2.10; this is

discussed in more detail later.

Here the reader is referred to the work of references

[65,67], where methods for detailed acoustic analysis of

gas turbine combustors and systems are described,

together with amelioration techniques, such as small

vortex generators, elliptical burners and enlarged,

better located lances.

5.4. Combustion oscillations in a swirl burner

combustion chamber systems and suppression

of the PVC

As discussed earlier in Sections 3 and 4, Roux et al.

[45] have made a very detailed study of the flow

characteristics of a vaned type swirl burner firing into a

square combustion chamber using both modelling, LES,

acoustic analysis and experimental measurements, pri-

marily LDA. Section 4 described the isothermal

characterisation of the system and the appearance of

a strong PVC signal, both measured and predicted at

540 Hz, located close to the region where the swirling

flow fires into the combustion chamber. A weaker 340 Hz

acoustic mode exists everywhere in the system. Measured

and calculated velocities and temperatures have been

presented earlier in Fig. 3.15(a), (b), 3.16(a) and (b),

whilst an instantaneous LES 1250 K isosurface was

shown in Fig. 3.11 for combustion conditions, fZ0.75.

For this mode of combustion the PVC is suppressed

as discussed earlier in Section 3 whilst two self excited

acoustic modes appear experimentally around 300 and

570 Hz. They correspond to the first two modes of the

combustor, 1/4 and 3/4 wave, respectively, with the 3/4

wave being the most amplified from 360 Hz (iso-

thermal) to 570 Hz (combustion). Both the LES and

Helmholtz acoustic solver gave good correlation with

the experimental data, differences from experiment

being attributed to errors in the acoustic boundary

conditions. Fig. 5.17 shows the field of rms pressure

taken from the LES predictions along the chamber axis

together with the modal structure predicted by the

Helmholtz solver for the 3/4 wave mode. Even though

the LES signal contains the signature of all modes, its

shape matches the structure of the 3/4 wave predicted

by the Helmholtz solver. Unlike the rms pressure

profile for the isothermal flow, the match between the

Helmholtz solver and the LES is good everywhere,

Fig. 5.12. Time series traces of pressure signals at burner wall and associated power Spectral densities for configuration 2a (a and b) and 2b (c and

d), Fig. 5.11 [27,29,86].


even in the combustion chamber, confirming that the

whole flow is locked on the 3/4 wave mode.

Apparently, changes in flow structure due to combus-

tion, especially the distribution of swirl flow (see

Section 3), have altered the characteristic of any

nascent PVC such that its frequency is well displaced

from that of the Helmholtz resonance and amplification

is unable to occur. This is in contrast to the oscillations

reported in the 2 MW swirl burner furnace system of

Fig. 3.10(a) and (b), Section 5.1. Similar results for the

suppression of the PVC with combustion have been

reported by Selle et al. [64] for an Industrial LPP gas

turbine combustor. The conditions causing suppression

of the PVC appear to be very similar to those reported

above [45]. These results on the suppression of the PVC

need to be treated with caution as the swirl combustors

were not operating with a conventional combustor can,

where there is a high level of confinement. For

conditions of high swirl even when PVC does not

develop near to the swirler, the intense Rankine vortex

so formed can give rise to PVC in the exhaust of the

combustor can [87].

The next section discusses the effect of equivalence

ratio on suppression of oscillation, including the PVC,

and important effects are highlighted. Finally, the

effects of vortex core precession in the exhaust of a

combustor can are described.

Fig. 5.13. Stability maps for high frequency oscillations, systems 2a

and 2b, as a function of equivalence ratio [27,29,86]: (a) pressure rms;

(b) frequency Hz.

5.4.1. Instabilities generated in industrial premixed gas

turbine combustor systems

Schildmacher et al. [88,89] have described a series

of experiments undertaken on an industrial gas turbine

combustor to investigate various instability modes, the

test rig burner and combustor liner are illustrated in

Fig. 5.18(a) and (b). Initial investigation of the

isothermal flow indicated a vortex shedding phenomena

whose frequency was linearly proportional to flowrate

[88]. The accompanying large eddy simulation studies

[90] showed that there was PVC which triggered vortex

Fig. 5.14. Phase averaged contour plots at six phase angles for configuration 2a [27,29,86]: (a) axial velocities; (b) axial directional intermittency;

(c) tangential velocities; (d) tangential directional intermittency; (e) phase averaged pressure trace: directional intermittency is the phase averaged

% of negative samples, contour cut off at 3%.


shedding in the shear flow region at the burner mouth.

This is the same phenomena as reported in Dorresten

[47] and the radial axial eddy phenomena discussed in

Section 2.1. In addition, investigations of fuel concen-

tration showed that alternating patterns of rich and lean

fuel concentration is generated by this vortex shedding,

though time averaged fuel concentration were axisy-

metrical and much more homogeneous [88,89,91].

Under combustion conditions with premixing of the air

and fuel, pressure fluctuations were found to strongly

increase with equivalence ratio, Fig. 5.19, starting at

fZ0.66. The pressure amplitude at peak frequency was

twice the turbulent combustion noise level at nominal

operating conditions without oscillations, fZ0.5.

Fig. 5.15. (a) Phase averaged axial (left) and (b) tangential (right) velocity contour plots just above the furnace nozzle exit (x/DeZ4.0) for

configuration 2a, sudden expansion. Velocities normalized by the mean inlet flow velocity uiZ4.3 m/s [27,29,86]; (c) contour plots of the directional

intermittencies of the axial (left) and (d) tangential (right) velocities just above the furnace nozzle exit (x/DeZ4.0) for configuration 2a [27,29,86].


The amplitude of the oscillations grew steadily with f,

in contrast to a sudden excitation which can often

happen in other systems. For fO0.77 the pressure

amplitude was more than 50 times higher than the

turbulent noise at nominal operation. Phase locked

velocity measurements were used to analyse the

variation of local swirl number over the oscillation

cycle for the highest amplitude oscillations, Fig. 5.20,

fZ0.83. This shows the strongest fluctuation is

between 0.1!S!0.8 and is located in the reaction

zone (x/DeZ0.63, 0.56). The periodicity of the signal

indicated the presence of coherent structures, probably

PVC induced or derived. Fig. 5.20 also indicated that

for a short time around 1508 phase angle, swirl

stabilisation of the flame is interrupted, which may

cause strong strain rates in the reaction zone and local

flame quenching. Only minor fluctuations of swirl were

recorded outside of the recirculation zone. This work

compliments that discussed in Sections 5.2 and 5.3

where considerable variation of swirl number (derived

from integrating the measured phase locked velocities

across the flow field) through the oscillation cycle was

shown [27–29,81,84].

Even when barely audible oscillations were gener-

ated at fZ0.71 the amplitude was still five times higher

than the turbulent combustion noise: phase locked

velocity measurements showed velocity fluctuations

were of identical frequency to that of the pressure field.

No frequency harmonics were present, the phase

averaged velocity profiles being sinusoidal in form,

whilst the swirl level only varied between 0.35!S!0.5. No definite frequency peak could be found in the

transition region when oscillation started, fZ0.66. The

work concluded that there was a very strong impact of

the heat release on the generation of coherent

structures. For combustion the oscillation frequency

Fig. 5.16. Phase averaged contour plots at six phase angles for configuration 2b with quarl inserted, Fig. 5.11 [27,29,86]: (a) axial velocities; (b)

axial directional intermittency; (c) tangential velocities; (d) tangential directional intermittency: directional intermittency is the phase averaged %

of negative samples, contour cut-off at 3%.


only changes slightly with normalised flow and the

Strouhal number decreases, leading to the conclusion

that resonant frequencies are linked to the acoustic

eigen frequencies of the system and are not too

dependent on the burner air flowrate.

This work provides an interesting contrast to that

of Roux et al. [45] and Selle et al. [64]. Roux et al.

operated at an equivalence ratio of 0.75, Selle et al.

Fig. 5.17. Field of rms pressure predicted via LES along the chamber axis to

the 3/4 wave mode [45].

at 0.5. Although configurations differ, reference to

Fig. 5.19 shows that the work of Roux et al. at fZ0.5 is

beyond the range of equivalence ratios where excitation

of high amplitude PVC type oscillations can be

expected, whilst that of Selle et al. is only just in the

range where excitation is initiated. Here also the effect

of swirler expansion appears to be important, as the

swirler was fired into a square furnace for these two

gether with the modal structure predicted by the Helmholtz solver for

Fig. 5.18. Industrial gas turbine combustor and combustion chamber

[89]: (a) schematic of test Rig; (b) schematic of swirl burner.

Fig. 5.20. Phase locked analysis of swirl number over oscillation cycle

[89].


cases. In contrast, Schildmacher [8,89] used the

complete swirler and combustor can system, giving

higher levels of confinement and a smoother transition

from the swirler to the can.

A very interesting CFD study of a Siemens high

swirl dry low emissions gas turbine combustor [87]

Fig. 5.19. Effect of equivalence ration on pressure amplitude and

frequency [89].

has been produced, arising from development tests on

a 13.4 MW Cyclone engine. Problems arose from

observed interactions between the exhaust flow from

the combustor can and the first row nozzle guide vanes

of the turbine. A schematic diagram of the combustor

arrangement is shown in Fig. 5.21, the system is

designed for dual fuel operation. Temperature indicat-

ing paint was used to verify operating temperatures in

the first row nozzle guide vanes and high temperatures

were found on the six guide vanes having their leading

edge closest to the axis of the six combustors used on

the development engine. Fig. 5.22 shows an example

of one of these vanes (termed a central NGV) and for

comparison an example of a non-central NGV. The

figure shows high temperatures on the suction side of

the central vane and at the hub platform immediately

downstream of this. A degree of flow visualisation is

shown by the leading edge film cooling whose tracks

can be clearly seen on the temperature indicating

paint.

A three-dimensional time dependent CFD analysis of

the system was carried out using the computational

domain shown in Fig. 5.23. A special version of the

turbulent Reynolds Stress model was necessary to

reproduce measured behaviour [87]. Analysis of the

upstream section was first carried out and used to derive

inlet boundary conditions for the full CFD analysis

which covered the combustor can and nozzle guide

vanes. Fig. 5.24 shows a vector plot of the combustor

front end extracted at an arbitary time step. The transient

nature of the flow is evident with a large radial axial eddy

and the formation of PVC (not shown). Fig. 5.25 shows

Fig. 5.21. Schematic diagram of siemens high swirl dry low emissions gas turbine combustor [87].


the temperature at a plane through the centre of the

combustor and an iso-surface of relatively high vorticity,

and indicates the central core of the Rankine vortex

formed. Moving away from the central core the vorticity

drops, given that the outer potential vortex is irrota-

tional. The vortex core so generated in the centre of the

can passes through the NGVs, Fig. 5.26, where the

vortex core is again visualised by a surface of relatively

high vorticity. The vortex core is directed towards the

leading edge of the central NGV; however it is also

attracted by the low pressure on the suction side of this

vane. A second rotation is thus set up near the hub over

the pressure surface, due to a large variation in incidence

angle with span induced by the vortex core. The vortex

core passes the leading edge of the vanes at about 40%

span and migrates towards the hub through the NGV

passage. This migration appears to be due to the core

being attracted by the locus of lowest static pressure and

an interaction between the vortex cores and secondary

flows set up within the NGV. These results are consistent

with the experimental ones from the development

engine.

6. Discussion

Most swirl combustion systems are designed with a

Swirl number SO0.5 to generate a CRZ for flame

stabilization purposes. When a PVC appears it is linked

and possibly coupled with the CRZ. Typically, it is of

helical form and is wrapped around a distorted

asymmetrical CRZ. This flow combination also excites

secondary flows especially radial axial eddies, and

recent LES work indicates that these eddies, shed from

the edge of an inlet shear flow can propagate down-

stream and help to initiate thermoacoustic instability.

Strouhal numbers are usually in the range 0.2–1.5

unless distorted by acoustic coupling.

There are other forms of precession, associated with

jets as shown by the work at the University of Adelaide.

This shows that such jet precession can occur with zero

to quite high Swirl numbers and a variety of different

configurations up to and beyond vortex breakdown.

Usually this is forced precession of a central initially

non-rotating jet. Below a critical swirl number, Scr,

between 0.15 and 0.23 the precession is a mixture of

flapping motion and precession, beyond this it is

dominated by precession with a change in rotational

sense. Strouhal numbers are one to two orders of

magnitude lower than those generated with conven-

tional swirl combustors with a large PVC and CRZ.

Under isothermal conditions the frequency of the

PVC can be characterised for a range of different swirl

flow systems by a Strouhal and Swirl number. There is

evidence that a central fuel injector or bluff body of

significant size can allow the formation of the PVC to

much lower levels of swirl than previously thought

especially when the central fuel jet is of low velocity.

The effect of high levels of confinement (Do/DeZ2)

upon the isothermal PVC is to increase the value

of Strouhal number by more than 2 for SO1.5. The

occurrence of a further vortex breakdown and

associated PVC of different frequency was noted in

the exhaust of the furnace in a swirl burner/furnace

system, and has also been noted by others in diverse

systems. Offset or other arrangements of furnace

exhaust may be beneficial here in eliminating this

source of the PVC.

Phase locked LDA and PIV data showed that the

PVC in isothermal swirling flow is characterised by the

formation of regions of negative tangential velocity in

the near the central axis coupled with elliptical/banana

shaped regions of high axial and tangential flow

close to the burner wall just above the burner exit.

Fig. 5.22. Temperature indicating paint results from first stage NGVs. View of leading edge of central and non Central NGVs [87].


The associated CRZ is also distorted, displaced radially,

and precesses about the central axis. The flow normally

returns to near axi symmetry by x/Dew1–1.5. PIV

studies showed the formation of axial radial eddies in

and around the CRZ near the swirl burner exit, whilst

water models and other experimental work showed

the shedding of axial radial eddies further downstream

both from the outside of the jet flow and from the end

of the recirculation zone. LES work and experiments

have shown the presence of the PVC in many simulated

gas turbine combustion chambers, especially under

isothermal conditions, being of helical form.

Fig. 5.23. Geometry of the computational domain [87].

Fig. 5.24. Time dependent CFD velocity snapshot at a diametrical plane at the head of the combustor. Velocity vectors are coloured with the

velocity component in the direction of the combustor axis. Note the transient radial axial eddy [87].


Qualitative agreement between the LES predictions and

measurements is steadily improving.

Under combustion conditions the behaviour of the

PVC becomes much more complex. Except at

Fig. 5.25. Temperature contours at a diametrical plane through the

exceptionally weak equivalence ratios, 100% axial

fuel injection suppresses the PVC amplitude by more

than an order of magnitude, although its residual

presence can still be detected in many systems. One

combustor and an iso-surface of relatively high vorticity [87].

Fig. 5.26. View of the vortex core approaching the leading edge of the central NGV. The vortex core is visualised by an iso-surface of relatively high

vorticity [87].


reason for this suppression of the PVC appears to be the

radial location of the flame front and the pushing of the

main region of flame stabilisation and CRZ formation

well downstream. The flow appears to be stabilised in

the burner exhaust by an annular region comprising a

rotating flame surrounding a small column of hot

recirculating flow which extends to the back-plate of

the burner. This is surrounded by another annular

region of high axial and tangential velocity where the

flame cannot stabilise. Analysis using the Rayleigh

criteria for the stability of stratified flows and a

modified Richardson number has shown this is a very

stable condition. The PVC can be excited when the

flame front can move into the outer region of high

velocity flow.

Values of Strouhal number are very much a function

of Swirl number, less so of equivalence ratio, also being

complicated by the occurrence of double PVC for

certain swirl number ranges for 100% axial fuel

injection. Partial premixing can change this pattern

with the excitation of the PVC frequency by up to a

factor of 2 for equivalence ratios w0.7.

The effect of confinement and partial premixing

for weak equivalence ratios, 0.1–0.3, shows the value

of Strouhal number being reduced by up to a factor

of 3 compared to the isothermal state. Although

100% axial fuel injection generally suppresses the

amplitude of the PVC, the swirling flames so

produced are still unstable and susceptible to small

perturbations in the flow especially in the burner

exit. The flames were essentially shown to wobble

with large changes in flame shape between

successive 1 ms separated cine images. Similar

findings arise from consideration of snapshot flame

temperature images from LES studies.

Analysis of the mechanism of oscillation of swirl

burner/furnace systems has been carried out in the

context of the Rayleigh criteria and describing how,

with a number of different excitation conditions, the

system flow and flame characteristics can serve to add

heat in phase with naturally occurring acoustically

generated pressure nodes.

The first case describes how, in a large 2 MW swirl

burner/furnace system with 100% axial fuel injection, a

high amplitude PVC oscillation is generated by

resonance with the systems natural frequencies. High

amplitude PVC would not be normally found in this

condition, or be of very low amplitude. Here, the flame

initially stabilises in a low velocity region around the

forming CRZ and inside an annular region of high axial

and tangential flow velocities. The flame propagates

outwards into low velocity regions, giving a circumfer-

ential variation in heat release. This effect propagates

downstream such that the flame engulfs the PVC

region, but is still irregular circumferentially as the

flame propagates into any available low velocity region

of flow. The flame never touches the furnace walls and

is surrounded by a weak area of low velocity flow

which often reverses direction. This flame is thus

unconstrained, can readily wobble, shed axial/radial

reacting eddies, contributing to instability and the

oscillation.

The second case describes low frequency oscil-

lations in a 100 kW swirl burner/furnace system,


excited by a Helmholtz resonance. Here the oscillation

is akin to that found in pulsating combustors as the flow

is periodically arrested over part of the limit cycle of

operation. A central cylindrical shaped pilot flame

exists over all the oscillation cycles, just downstream of

the burner exit, with an annular flame front moving

axially back into the furnace as the flow is arrested.

Phase locked measurements show negative levels of

tangential velocity on and around the central axis

through the limit cycle, indicative of vortex core

precession, albeit of an irregular nature.

Complimentary work on a similar system with a

similar frequency of oscillation, but excited by

travelling waves in the inlet pipes showed some vortex

wobble in the furnace, but essentially a precessing flow

in the furnace exhaust with significant levels of

especially tangential directional intermittency, adding

to evidence that the whole flame is wobbling, thus

deforming the flame externally near to the furnace walls

and internally in and around the CRZ, the primary

flame stabilisation region. This thus again provides the

variation in heat release rate necessary for the Rayleigh

criteria and excitation of oscillations.

The third case uses the same swirl burner furnace

system with some minor changes to the furnace

geometry and this time generates a high frequency

oscillationw240 Hz, via a Helmholtz excited reson-

ance. Two cases are compared, one with a quarl or

conical section inserted at the swirl burner exit, one

without. Without the quarl the flame does not properly

fill the furnace and has considerable wobble. The quarl

produces a flame which substantially fills the furnace

section and thus gives a substantial reduction in

oscillation amplitude over a wide range of equivalence

ratios. Outside of this range the amplitude of oscillation

falls considerably, the frequency dropping back to that

of the low frequency case (w40 Hz), the quarl having

little effect. Again the effect of flow/flame wobble/irre-

gular precession is brought out via the phase locked

measurements, especially the directional intermitten-

cies. The quarl is shown to especially reduce

intermittency, negative tangential velocities and vari-

ation in CRZ size and shape over the limit cycle of

oscillation. Here, the Rayleigh criteria for the stability

of stratified flows is useful in explaining the instability

of the ERZ formed without the quarl.

A substantive body of work has now been generated

on industrial gas turbines using a variety of techniques,

both experimental and numerical, RANS, LES, phase

locked velocity measurements, PIV and advanced

acoustic analysis. In all these systems the presence of

the PVC is reported under isothermal conditions, but

with combustion suppression often occurs for equival-

ence ratios ranging from 0.5 to 0.75. This is a function

of the system configuration, the type of swirl flow

generated and the absence of swirl and angular

momentum in the central region of flow close to the

burner exit. Effectively there is no vortex core to

precess. However, as the equivalence ratio moves into

the range greater than 0.75, the flame front moves

further into the annular shear flow entering the

combustion chamber with the result that severe

oscillations can develop, dependent on system geome-

try and flame front location often with the presence of

helical coherent structures of PVC form.

Thus, the coupling between swirl combustion and

acoustic oscillations (apart from the case of PVC

excitation) appears to arise from regular variations in

heat release rate arising from the following:

† Swirl flow and hence, flame wobble or irregular

precession, causing circumferential and hence, axial

variations in flame shape, combustion aerodynamics,

CRZ and hence, the initial region of flame formation

and stabilization. The PVC is influential here via flow

coupling triggering the formation of axial radial eddies

from the edge of the shear flow and the CRZ, generating

alternating patterns of rich and lean combustion

sufficient to reinforce combustion oscillations via the

Rayleigh criteria.

† This is reinforced as the limit cycle of oscillation

causes natural variations in the swirl number, primarily

due to variation in axial flow rate into or through the

system, there being less variation in the swirl flow

velocity over the limit cycle. This in term cause natural

variation in the size and shape of the CRZ, in accord

with the Swirl number variation. Again this affects the

initial region of flame stabilisation/formation as the

CRZ moves axially in and out of the burner exit and this

again can reinforce oscillation.

6.1. Interaction between the above effects

Remedial effects which can be used on combustors

include:

– the use of higher swirl levels should produce more

regular and stronger CRZs that are less susceptible

to deformation by pressure fluctuations. This will

generate stronger PVCs, but providing these are well

controlled and regular should not cause problems,

providing there is a fundamental mismatch to major

acoustic modes of oscillation;

– control of the wobble of the central flow and flame

appears to be important in reducing the regular and


irregular precession of the flow and flame, a quarl or

carefully shaped exhaust section can be useful here

to remove the ERZ and ensure the flame properly

fills the furnace;

– an off centred furnace exhaust may well be

beneficial in eliminating the formation of other

PVCs, whilst also altering the fundamental acoustic

modes of oscillation of the combustor;

– the use of minature vortex generators to distort the

generation of PVC and axial radial eddies;

– the use of elliptical burners which again distort the

generation of the PVC, axial radial eddies and other

coherent structures;

– The use of substantive pilot lances to stabilise the

point of vortex breakdown and location of the CRZ.

– Investigation of the acoustic response of the system

and derivation of techniques to give acoustic

mismatch to other resonant frequencies.

Finally it must be noted that vortex cores that form in

the exhaust of a combustor may easily deleteriously

interact with other downstream components

7. Conclusions

This paper has reviewed recent work on instability

and oscillations in swirl burner and combustion

systems, using a range of existing and new data on

open and confined swirl combustors, and related them

to the occurrence of instability in such systems. Based

on this, an analysis of the underlying mechanisms by

which naturally occurring acoustic and other reson-

ances can be reinforced is given. A number of remedial

methods are discussed.

For the future, there is a need for many more

fundamental experimental investigations of these

types of flow both to elucidate the coupling methods

between the PVC and excitation of combustion

oscillations as well as the exact mechanisms by

which suppression of the PVC occurs. Examination

of the occurrence and role of the PVC in the exhaust

of combustor cans is also needed. Complimentary

LES and related work is needed for validation and

extrapolation purposes.

Acknowledgements

Professor N. Syred gratefully acknowledges the Royal

Academy of Engineering award of a Global Research

Award, also the facilities provided by the School of

Mechanical Engineering, Adelaide University during his

sabbatical leave. The financial support of the European

Union via several programmes is acknowledged for much

of the more recent work carried out at Cardiff University.

The assistance of Dr Andy Crayford with the diagrams is

gratefully acknowledged.

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Role of PVC in Swirl Combustion Systems

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