igation of oil mist explosion hazards (Phase 2)

Experimental investigation of oil mist explosion hazards (Phase 2)

Prepared by Cardiff University Gas Turbine Research Centre for the Health and Safety Executive

RR1110 Research Report

© Crown copyright 2017

Prepared 2015 First published 2017

You may reuse this information (not including logos) free of charge in any format or medium, under the terms of the Open Government Licence. To view the licence visit www.nationalarchives.gov.uk/doc/open-government-licence/, write to the Information Policy Team, The National Archives, Kew, London TW9 4DU, or email [email protected].

Some images and illustrations may not be owned by the Crown so cannot be reproduced without permission of the copyright owner. Enquiries should be sent to [email protected].

This report and the work it describes were funded through a Joint Industry Project. Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy or the views of the Joint Industry Project sponsors.

Many types of industrial equipment can potentially produce an explosive oil mist if a fault develops. However, information on the conditions in which a mist can be ignited and continue to burn is limited. To help address this, HSE and 14 industry sponsors co-funded a Joint Industry Project (JIP) on oil mist formation and ignition.

This report, produced for the JIP, describes the second phase of experimental tests to examine the ignition of mists produced by small leaks of pressurised, combustible fluids.

The size, concentration and movement of droplets were examined. The results differed significantly from those predicted by simple mist formation theories. This appeared to be the result of fundamental fluid behaviour. For all the test conditions, the droplets in the core of the spray were larger and had higher velocities than those closer to the edge. With viscous fluids the core included long ligaments, though these were difficult to measure.

These results show that simple spray models are not always appropriate for assessing the ignitability of oil mists from pressurised leaks.

ii

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Kyriakos Mouzakitis and Anthony Giles (Technical Contacts) Gas Turbine Research Centre Energy Research Building Cardiff University Heol Cefn Gwrgan Margam Wales SA13 2EZ


iii

ABSTRACT

Flammable liquids released under pressure, at temperatures below their flash point may form

a mist which can be ignited and explode. The objective of this test programme is to provide

HSE's Buxton laboratory with experimental data that will help facilitate the development

by others of guidance on the likelihood of flammable mist formation.

This extended report describes the methodology used and the results obtained for Phase two

of the test program and an additional study of high pressure releases. The liquids concerned

with Phase two were Jet-A1, hydraulic oil and light fuel oil at ambient conditions (20 ⁰C);

and light fuel oil at an elevated temperature (70 ⁰C), with pressures varying from 5 to20 barg.

For the high pressure work, the spray characterisation and flammability of hydraulic oil at

ambient temperature and elevated pressures ranging from 30-150 barg were investigated. The

mists were generated from a 1mm plain orifice, being representative of a small leak.

The work described was conducted by staff at Cardiff University’s Gas Turbine Research

Centre (GTRC) and funded as part of an HSE Joint Industry Project.

iv

Table of Contents page no

ABSTRACT ...................................................................................................................... iv1.0 INTRODUCTION ................................................................................................. 1

2.0 EXPERIMENTAL FACILITIES........................................................................... 2

2.1 Spray Characterisation techniques ..................................................................... 2

2.2 Phase Two Rig Design ....................................................................................... 4

2.3 High Pressure study Rig design ......................................................................... 8

3.0 EXPERIMENTAL METHODS............................................................................. 11

3.1 Experimental set up ............................................................................................ 11

3.2 Experimental procedure ..................................................................................... 13

4.0 EXPERIMENTAL RESULTS............................................................................... 14

4.1 Introduction ........................................................................................................ 14

4.2 Concentration Calculation Methodology ........................................................... 14

4.3 JET-A1 Experimental Results ............................................................................ 16

4.3.1 Droplet Diameters ....................................................................................... 19

4.3.2 Concentration .............................................................................................. 22

4.4 Hydraulic Oil Experimental Results .................................................................. 25


4.4.2 Concentration .............................................................................................. 28

4.5 LFO Ambient Experimental Results .................................................................. 30


4.5.2 Concentration .............................................................................................. 34

4.6 LFO Heated at 70 oC Experimental Results ....................................................... 37


4.6.2 Concentration .............................................................................................. 41

4.7 Impingement Results .......................................................................................... 44

4.8 High Pressure Hydraulic Oil Tests ..................................................................... 47


4.8.2 Concentration .............................................................................................. 49

5.0 OBSERVATIONS ................................................................................................. 50

REFERENCES ................................................................................................................. 54

APPENDIX A: MEASURING PRINCIPLES OF A PDA SYSTEM .............................. 55

APPENDIX B: RAW PDA DATA FOR JET-A1 ............................................................ 59

APPENDIX C: RAW DATA FOR HYDRAULIC OIL................................................... 79

APPENDIX D: RAW DATA FOR LFO AT AMBIENT CONDITIONS ....................... 91

APPENDIX E: RAW DATA FOR FOR LFO HEATED 70 OC ..................................... 103

APPENDIX F: RAW DATA FOR FOR HIGH PRESSURE HYDRAULIC OIL .......... 118

v

1

1.0 INTRODUCTION

The European ATEX directives (99/92/EC and 94/9/EC) which are implemented in the UK

under the Dangerous Substances and Explosive Atmospheres Regulations (DSEAR, 2002)

require employers to classify areas into zones where explosive hazards may occur. Hazardous

Area Classification (HAC) for explosive gas atmospheres is well established, however, the

same situation is not currently the case for high flashpoint liquid releases that could give rise

to an explosive mist atmosphere. This issue was first highlighted by Bowen and Shirvill

(1994) and extended to demonstrate the relevance of low-pressure impinging releases by

Maragkos and Bowen (2002).

The test program comprised of two phases of work. Phase one consisted of ‘basic’

experiments designed to generate flammability maps for a range liquids spray releases. Phase

two incorporated more detailed analysis of the release using Phase Doppler Anemometry

(PDA) to determine droplet size and velocities to improve the understanding of conditions

leading to mist flammability.

A report describing the Phase one activity “Phase one: Flammability of Pressurized

Liquid Releases” was accepted by HSE in March 2014. An initial Phase two report

describing results and observations for work carried out during Phase two of the test program

was accepted by HSE in January 2015. This extended report describes additional high

pressure experimental study; investigating the flammability and characterising the spray

generated from high pressure releases of hydraulic oil.

2

2.0 EXPERIMENTAL FACILITIES

2.1 Spray Characterisation techniques

Phase two of the experimental program concentrated on explaining and understanding the

spray combustion physics underlying the observations from Phase one, and under-pinning

them with limited additional runs using a suite of (optical) diagnostics. There are several non-

intrusive ethods to characterise a spray such as Interferometic Particle Imaging (IPI), Shadow

sizing, Laser Induced Fluorescence, Particle Image Velocimetry and Phase Doppler

Anemometry.

The IPI technique utilises a laser with light sheet optics, a dual camera system and

appropriate software package. It can measures the size and velocity of spherical, transparent

particles such as spray droplets, glass beads or air bubbles in water.

Shadowgraphy is a technique based on high resolution imaging with pulsed backlight

illumination. The measurement volume is defined by the focal plane and the depth of field of

the imaging system. This technique measures size, shape and velocity of a wide range of

particles.

Planar Laser Induced Fluorescence (PLIF) is a diagnostic tool for the investigation of non-

reacting as well as reacting gas and liquid flows. PLIF is a non-intrusive, instantaneous flow

visualization technique with high spatial and temporal resolution and is applied to determine

different flow-field variables in the plane of a laser light sheet: concentration (mole fraction),

density, temperature and velocity fields can be derived from calibrated LIF images.

Particle Image Velocimetry (PIV) is a non-intrusive laser optical measurement technique

that measures whole velocity fields by taking two images shortly after each other and

calculating the distance individual particles travelled within this time.

Phase Doppler Anemometry (PDA) is a non-intrusive optical diagnostic technique capable

of simultaneously measuring the diameter, and up to three components of velocity of

spherical particles and droplets. The measurements are performed on single particles and are

applicable to both liquid droplets in a gas medium (e.g. a spray) and gas bubbles in a liquid

medium (e.g. gas bubbles in two-phase flows). PDA is also capable of measuring particle

3

concentrations and mass flux via interpolation (Aisa et al, 2002).It is a technique based on

absolute physical effects (e.g. light scattering, phase Doppler shift) that requires no in-situ

calibration. Also, no prerequisite assumptions on size and velocity distributions are required.

It is a technique widely used in industry for liquid spray applications, capable of dense spray

measurements. With an appropriate choice of hardware, particle sizes from 0.1 μm to over 1

mm, and velocities up to supersonic can be measured.

All the aforementioned spray measurement techniques are non- intrusive. There are also

intrusive techniques for spray analysis such as isokinetic sampling for the determination of

mass flux over a defined cross sectional area. Isokinetic sampling can increase the accuracy

and reliability of results by collecting particles in a moving stream which moves at the same

velocity in the sampling nozzle as elsewhere in the stream in order to provide a uniform,

unbiased sample.

Taking into consideration the aforementioned techniques, PDA was selected as the preferred

method for the spray characterisation as it is an absolute non- intrusive technique that

requires no calibration, no pre-requisite assumptions on droplet size distributions or velocities

and can be applied in dense sprays. Additionally, it is a point source method allowing the

examination of specific points of interest (ignition – non ignition points). Moreover, it is a

technique that can be used in all three liquids (with set- up changes) providing a “common-

ground” comparison of results; as some other optical diagnostics cannot cope with opaque

liquids and another technique would have to be implemented. In some techniques, PIV, LIF,

a seeding particle is required, which would somewhat affect the viscosity of the liquid

altering its atomisation characteristics.

Isokinetic sampling was also considered for providing more accurate results on volume flux

measurements but through a literature search was found that:

In general, the agreement between the mist fluxes measured with the PDA and iso-

kinetic sampling was within 7% near the spray centreline (Ditch and Yu, 2004).

The implementation of isokinetic sampling in a spray consisting of various droplet

sizes moving with various axial and radial velocity trajectories would be rather

challenging in determining the sampling inlet position and flow conditions.

4

In addition to the utilisation of the PDA technique for quantitatively characterising the spray

the use of high speed imaging would also be beneficial in providing a qualitative analysis of

the spray.

2.2 Phase Two Rig Design

The experimental program of Phase one was conducted in the Atmospheric Spray Rig (ASR)

(Figure 2.1) at the Gas Turbine Research Centre (GTRC), Cardiff University, Port Talbot.

The experimental apparatus consisted of three main constituents:

The pressurised fuel vessel and fuel delivery lines.

The spray chamber.

The ignition system.

Figure 2.1: The Atmospheric Spray Rig (Phase one)

Figure 2.2 provides a schematic representation of the experimental apparatus utilised to

complete Phase one.

Spray boothTraverse

Traverse Controller Fuel vessel

Heated fuel line

Flow Straightener

5

Figure 2.2: Schematic representation of the Atmospheric Spray Rig (Phase one)

Following the findings from Phase one, the rig was significantly modified in order to

undertake Phase two of the experimental program. The main constituents of the experimental

apparatus remained nominally the same, with use of a Phase Doppler Anemometry system

instead of the ignition system:

Pressurised fuel vessel delivery system.

Spray Chamber.

The Phase Doppler Anemometry (PDA) system.

Figure 2.3 shows a picture of the ASR as modified for Phase two and Figure 2.4 a schematic

representation of the rig.

The fuel vessel and Coriolis flow meter are encased in a purposed built enclosure (see

Figure 2.3) to contain any secondary releases. The fuel vessel was pressurised using nitrogen

as the pressurising medium up to 20bar and can be heated via an external electric heating

element. The two fuel delivery lines fitted in Phase one (a ½” hydraulic hose for the delivery

of high viscosity liquids, and a ¼” solid heated line) were replaced by a ½” - one piece solid

stainless steel line capable of pressures up to 200bar that can be heated via an external

electric heating element.

6

Figure 2.3: The Atmospheric Spray Rig (Phase two)

Figure 2.4: Schematic representation of the Atmospheric Spray Rig (Phase two)

Control Room

Outside½”Heated fuel line

Traverse

PDA optics

Waste fuel collector with dual flame arresters

Master PC and Data Logging Computer

Fuel Control

Traverse e-stop

Flame quenching mesh

Spray Chamber

Nitrogen Line

CO

2 Pur

ge s

yste

m

Fuel Vessel relief valve

CO

2 ga

s bo

ttle

N2 g

as b

ottle

ATEX Rated fan Extraction

Traverse controller

PDA ProcessorBSA flow processor

CO2 line

Fuel vessel enclosure

Electrically heated fuel vessel

7

The spray chamber dimensions are 1.2x1.2x2.5m. The nozzle (see Figure 2.5) was mounted

centrally and down-fired so that it created an axisymmetric spray. The nozzle diameter was

1mm with an L/D ratio of 2. The release conditions were monitored via a K-type

thermocouple and a pressure transducer at the nozzle. To protect the sensors from the flame

tracking back to the nozzle, a custom made steel enclosure was constructed. This enclosure

also encased the nozzle and would contain any potential releases from the connection joints.

Moreover steel door protectors were made and fitted, minimising reflection/glare within the

chamber for optimum data collection with the use of a PDA system.

Figure 2.5: The nozzle mounted on the spray chamber

After each release the fuel is collected in a safety can fitted with dual flame arresters via a

drain at the bottom of the spray rig. During Phase one, to prevent fuel “pooling” at the bottom

of the chamber that could potentially lead to pool fires, inclined metal plates (see Figure 2.6)

were fitted to assist and accelerate the drainage of the waste fuel accumulating at the bottom

of the ASR. Moreover nitrogen purge nozzles (see Figure 2.6) were also fitted to extinguish

any pool fires and a flame quenching mesh (see Figure 2.7) was also used to prevent the

flames from propagating to the bottom of the chamber and to the drainage pipework.

Figure 2.6: Inclined plates and nitrogen purge

nozzles (Phase one) Figure 2.7: Flame quenching mesh (Phase one)

8

Figure 2.8: Fuel collecting tray and flame quenching mesh (Phase two)

For Phase two, the fuel collection system was the main area that required a redesign. The

chamber’s support bars were removed along with the inclined plates, and were replaced by a

fuel collecting tray (see Fig 2.8) that was mounted in the base of the chamber with a centrally

located drain leading to the waste fuel collecting can. The flame quenching mesh that was

previously made of two perforated aluminium plates was replaced by a much finer gauge

“frameless” wire quenching mesh to avoid the collection of fuel on the mesh and the

possibility of any flame tracking through the join between the two mesh panels used

previously.

The nitrogen used in the purge system was replaced by carbon dioxide as it is a superior fire

extinguishing medium and as it is less buoyant, it is better suited to create an “inert”

atmosphere at the bottom of the rig. In addition, an ATEX compliant extract fan was installed

to remove any fuel vapour and carbon dioxide at the end of each test run.

2.3 High Pressure study Rig design

In order to accommodate the high pressures (up to 150 barg) of this test program, further

modifications to the ASR’s fuel pressurisation, delivery system and nozzle had to be made;

with all pipework and safety equipment checked and upgraded. Figure 2.9 depicts a

schematic representation of the experimental rig during the high pressure work.

9

Figure 2.9: Schematic representation of the Atmospheric Spray Rig (High Pressure)-PDA set up

The fuel vessel used in the previous phases was replaced by a four litre hydro pneumatic

bladder accumulator (see Figure 2.10) capable of delivering pressures up to 200 barg. The

liquid is stored in a closed cup storage vessel and pumped to the accumulator, which is

subsequently pressurised via nitrogen to the desired delivery pressure.

Figure 2.10: Hydro pneumatic accumulator capable of deliver pressures up to 200 barg

Control Room

Outside½”Heated fuel line

Traverse

PDA optics

Waste fuel collector with dual flame arresters

Master PC and Data Logging Computer

Fuel Control

Traverse e-stop

Flame quenching mesh

Spray Chamber

Nitrogen Line

CO

2 Pur

ge s

yste

m

Accumulator C

O2 ga

s bo

ttle

N2 g

as b

ottle

ATEX Rated fan Extraction

Traverse controller

PDA ProcessorBSA flow processor

CO2 line

Fuel vessel enclosure

10

Moreover, a new nozzle (see Figure 2.11) was constructed to ensure it would endure the

elevated pressures. The nozzle diameter was 1mm with an L/D ratio of 2. The release

conditions were monitored via a K-type thermocouple and a pressure transducer at the nozzle.

All other rig components remained nominally the same.

Figure 2.11: Nozzle assembly for high pressure work.

11

3.0 EXPERIMENTAL METHODS

The PDA system used in Phase two of the experimental program was an off-the-shelf system

provided by DANTEC DYNAMICS. This system is comprised of a transmitter, a BSA P60

Flow and Particle Processor, and a Fiber PDA detector unit (which is shown in Figure 3.1).

The laser used was a Coherent Innova 70-5 Series Argon-Ion Laser. A brief description of the

PDA measuring principles can be found in Appendix A.

Figure 3.1: PDA optics set up

3.1 Experimental set up

In Phase two set up the Coherent Innova 70-5 Series Argon-Ion laser was set at a power

output of 4W. The laser light was split into three pairs of beams providing six in total – two

green, two blue and two violet beams. One beam from each of the three pairs was shifted in

frequency by 40MHz to overcome directional ambiguity inherent in the PDA technique. Four

of the six beams (the two greens and two blues) were sent to the transmitting optics via fibre

optic cables. The laser wavelengths of 514.5 nm (corresponding to green visible light) and

488 nm (corresponding to blue visible light) were used to perform velocity measurements in

12

the nozzle axial and radial directions respectively. The green pair were used for velocity (v

component) and droplet size measurements and the blue pair were used for velocity (u

component) and validation checks. The violet beams (with a wavelength of 476.5 nm) were

not required for 2D PDA systems but were used to help align the green and blue beams

within the control volume to ensure an optimal set-up.

The receiving optics was located in the same plane as the transmitting optics but off-set at an

angle which is dependent on the liquids refractive index and scattering method used. For

PDA testing of fuels with dominant first order refraction (JET-A1 and Hydraulic oil) an angle

of 69 degrees was used (calculated from their respective refractive indices). For the LFO

Ambient and LFO Heated to 70 oC test cases, the preferred scattering mode was reflection

with an angle of 105 degrees, due to LFO’s high refractive index.

A 112 mm FiberPDA transmitting optic with a beam spacing of 74 mm and a nominal beam

diameter of 1.5mm was used. The transmitting and receiving optics were equipped with

lenses with a focal length of 600 mm to cope with the large dimensions of the spray chamber.

Both the transmitting and receiving optics were mounted on a computer-controlled traverse

allowing remote fine adjustment of the optics location. The traverse employed allowed

controlled motion in 3-axes, with over 1m of travel and an accuracy of 0.1 mm in each axis.

The measurable droplet size ranges are affected by beam separation, focal length and aperture

plates selected. The first two parameters cannot be altered in the given set-up (beam

separation is determined by the transmitting optics used and focal length cannot be less than

600 mm to accommodate the dimensions of the spray chamber). However, different aperture

plates can be used to modify the measurable droplet size ranges and therefore optimise PDA

arrangement for the sprays investigated. For the JET-A1 tests aperture plate B was used

which allowed measurements of droplets up to 265 μm, since the expected SMD was in the

order 100-190 μm. For the hydraulic oil and light fuel oil testing, aperture plate C was used

which allowed for measurements of droplets up to 967.4 μm. However, for the high pressure

hydraulic oil test cases a combination of aperture plate C and B was used, as the anticipated

droplet size would reduce. These size ranges were found to capture droplet size data at all

investigated operating conditions. A spherical validation band of 20% was selected for all

tests performed. DANTEC DYNAMICS BSA Flow Software v. 4.50 was used for all PDA

tests in this investigation.

13

3.2 Experimental procedure

Prior to initiating testing of the current phase the PDA hardware and software had to be

configured. This involved the following:

Aligning the transmitting optics to ensure the green pair and blue pair of beams were

in the same vertical and horizontal planes.

Aligning all four fibre optic cables at the laser so as much of the laser beams as

possible was captured and sent to the transmitting optics.

Maximising and regulating beam power so all beams possessed the same power

ratings.

Aligning both pairs of beams within the control volume using a photo-sensitive cell.

Focussing the receiving optics on the control volume using a controlled spray from a

nebuliser.

Setting up appropriate data acquisition files in the BSA flow software and fine tuning

the PDA receiving optics to ensure optimal set up at each radial and axial location.

Once these steps were completed the PDA system was traversed to the desired location. The

data acquisition system and PDA were initiated before the fuel being released. The PDA

system was set to acquire a maximum of 50000 samples or to sample continuously for a 40

second window spanning the entire duration of the 30second release, allowing an extra ten

seconds for capturing any quiescent droplets. Once the data had been collected it was then

saved in an appropriate file format for further processing.

For the high pressure work the experimental procedure was modified due to the high

flowrates:

For the flammability studies the fuel was released for six seconds, with up to five

repetitions to prove a non-ignition event.

For the spray characterisation the liquid was released for ten seconds in each

characterisation run.

All other steps of the experimental procedure remained the same as in each respective test

phase.

14

4.0 EXPERIMENTAL RESULTS

4.1 Introduction

As previously mentioned, Phase two of this project involved carrying out limited additional

testing in order to gather data to aid in explaining and understanding the spray combustion

physics underlying the observations from Phase one. The main parameters of interest in the

characterisation of a liquid release / spray are the type of break-up, the break-up length, the

droplet size distribution produced and the velocity profiles of the droplets / jet. In addition,

the calculation of the local droplet concentration is useful for determining the minimum

amount of fuel required for flame propagation through the release.

Comparisons were made between the correlations from literature and the results of

measurements taken during Phase two; with the aim being to develop understanding in order

to progress towards practical guidance.

4.2 Concentration Calculation Methodology

This section describes the methodology used to calculate the local concentration of droplets

within the spray for the liquid spray releases.

For all the nozzle pressures analysed the spray produced by the liquid releases consisted of a

high velocity central dense liquid core surrounded by a slower moving, less dense spray.

Different problems exist in measuring the fuel concentration within the central core of the

spray to that near the flammable limit, as the droplet concentration within the high velocity

central jet is many times higher than that in the surrounding finer spray. In determining the

local concentration, the methodology by which PDA measures individual droplets needs to be

examined.

The measurement of droplet sizes using PDA takes place in the intersection between two

incident laser beams, and the measuring volume is defined as the volume within which the

modulation depth is higher than e-2

times the peak core value. Due to the Gaussian intensity

15

distribution in the laser beams the measuring volume is an ellipsoid as indicated in Figure

4.1.

Figure 4.1: PDA measuring volume

The size of the measuring volume can be calculated from the beam waist diameter df of the

focused laser beams and the angle θ between them:

𝑑𝑥 =𝑑𝑓

𝑐𝑜𝑠(𝜃2⁄ )

𝑑𝑦 = 𝑑𝑓 𝑑𝑧 =𝑑𝑓

𝑠𝑖𝑛(𝜃2⁄ )

(4.1)

In order to increase the resolution of the measuring volume, the PDA receiving optics view

the control volume through a slit aperture. The effective length of the cross-section of the

control volume visible is given by:

𝑑𝐿 = 𝐿𝑠𝑠𝑖𝑛𝜑⁄ (4.2)

where Ls is the slit aperture size (0.2mm) and φ is the scattering angle.

During the gathering of data, the PDA system measures the Doppler bursts for droplets as

they pass through the measuring volume. Each Doppler burst detected is counted, but to

optimise data gathering droplet velocities and sizes cannot be determined for every detected

burst. However, it is accepted that the measured velocity and size profiles of the validated

droplet samples is representative of the overall distributions.

To calculate from first principles a basic concentration value for the low density mist near the

“flammable limit”, first the total number of detected droplets (N), and the Volume Mean

Diameter (D30) can be used to provide a value for the total fuel volume of fuel that travelled

through the control volume during the sample period (T).

16

𝑇𝑜𝑡𝑎𝑙 𝐹𝑢𝑒𝑙 𝑣𝑜𝑙𝑢𝑚𝑒 =4

3𝑁𝜋 (

𝐷30

2)

3

(4.3)

The mean residence time of a droplet within the measuring volume is dx/U, where U is the

mean axial velocity of the droplets. Therefore the instantaneous fuel volume of fuel within

the control volume is:

𝐼𝑛𝑠𝑡𝑎𝑛𝑡𝑎𝑛𝑒𝑜𝑢𝑠 𝐹𝑢𝑒𝑙 𝑉𝑜𝑙𝑢𝑚𝑒 = 4

3𝑁𝜋 (

𝐷30

2)

3

.𝑑𝑥

𝑈𝑇(4.4)

Knowing the size of the measuring volume and the density of the fuel it is therefore

straightforward to provide a mean mass concentration for each measurement location.

In addition to this basic methodology for calculating the droplet concentration, for some

release points the Dantec PDA software was able to provide a concentration value based on

an inbuilt algorithm.

4.3 JET-A1 Experimental Results (Flashpoint = 38⁰C)

JET- A1 liquid spray characterisation results together with a limited analysis of the data are

presented in this section.

Figure 4.2 shows the positions at which PDA measurements of the JET-A1 releases were

undertaken together with the results of the flammability studies carried out in Phase one. For

each of the three axial locations at which PDA measurements could be taken, results were

obtained for the central axial position as well as radial positions either side of the “flammable

limit”. Measurements of any droplet distributions for all of the points within the releases are

to be used to ascertain the likely droplet distribution for the overall release, which can then be

compared to that predicted within the literature.

Figure 4.3 displays a sample of the PDA measurements obtained (at the 600mm axial,

000mm radial position for a 15barg release of JET-A1); with histograms showing the

measured axial and radial velocity and droplet size distributions. The axial velocity profile is

represented by LDA1, with the negative velocity representing the downwards spray direction.

The radial velocity profile is represented in LDA4, with negative velocities being outwards

from the spray centre (towards the detector).

17

The overall shape profile of the JET-A1 releases for all the pressures at which measurements

were taken (5 to 20barg) consisted of a high velocity central dense liquid core surrounded by

a slower moving, less dense spray. At the centreline (000mm radial) location at which the

results shown in Figure 4.3 were taken, the measured droplets within the central jet are seen

to be moving over a range of axial velocities between 11 and 52m/s; with a mean axial jet

velocity of 33.8m/s. The profile of the radial velocities of the droplets measured is centred at

approximately 0m/s, signifying that at the centreline positions there is very little overall radial

motion of the droplets seen in the release; which is to be expected with an axisymmetric

spray.

The measured droplet sizes shown in Figure 4.3 for the 600mm axial, 000mm radial position

illustrate that for liquids such as JET-A1, a pressurised release produces a wide range of

droplet sizes (further analysis of the droplet measurements taken for the full JET-A1 dataset

are described in the section 4.3.1).

The results of the PDA droplet characterisation for the JET-A1 dataset are shown in Table

4.1. It can be seen that at each of the axial locations that when the release pressure is

increased, the mean axial centreline velocity of the measured droplets also increases. Also it

can be seen that as the axial distance is increased (i.e. the measurement position is moved

further away from the nozzle), there is also a decrease in the mean axial velocity of the

droplets measured. At all pressures and axial distances the mean radial velocity of the

droplets measured is negligible when compared to the downwards velocity of the central

liquid core.

Comparing the mean axial velocity measured for the droplets near to the flammable limit to

that observed within the central core of the spray, it can be seen that the velocities are

significantly lower away from the central core.

18

Figure 4.2: JET- A1 flammability study results and PDA measurement positions

19

Figure 4.3: Sample PDA results (15barg release, 600mm axial, 000mm radial)

4.3.1 Droplet Diameters

For each of the locations at which droplet measurements were taken, histograms have been

obtained for the axial and radial velocities as well as the droplet sizes (the histograms for

each measurement location are included in Appendix B).

From the compiled droplet size data several different mean diameters can be calculated, some

of which are presented in Table 4.1. Each of the mean diameters shown in Table 4.1 has

applications in different types of spray analysis; such as that used in calculating volumetric

and mass fluxes as well as in reaction rate, evaporation and combustion calculations. For the

purposes of the analysis carried out here the most important of these mean diameters are the

Arithmetic Mean Diameter (D10), the Volume Mean Diameter (D30) and the Sauter Mean

Diameter (D32).

20

Table 4.1: JET-A1 droplet measurement summary

Pressure

(barg)

Position

(mm)

Number

of

Droplets

Mean Droplet

Velocity (m/s) Mean Droplet Diameter (μm)

Axial Radial Axial Radial D10 D20 D21 D30 D31 D32 D43

5

300

000 148117 -22.1 -0.11 43.7 61.9 87.8 79.5 107.2 131.0 159.5

045 44 -0.49 -0.15 69.3 84.9 104.1 93.5 108.6 113.2 116.4

055 30 -0.21 -0.18 27.4 31.4 36.1 34.3 38.4 40.9 43.9

600

000 165304 -20.9 -0.10 61.3 81.6 108.6 99.2 126.1 146.4 170.3

045 4510 -1.45 0.09 59.1 76.9 100.0 92.2 115.1 132.6 156.8

055 1435 -1.05 0.23 56.0 73.3 96.0 88.5 111.2 128.8 154.2

900

000 168940 -17.7 -0.14 69.2 88.4 112.9 104.6 128.6 146.5 168.6

050 10759 -2.81 0.23 51.2 69.1 93.2 85.2 109.8 129.4 154.9

080 1755 -1.15 0.07 42.2 57.0 77.0 70.0 90.2 105.6 125.3

10

300

000 132889 -29.6 -0.13 37.8 51.1 69.1 65.3 85.8 106.5 138.3

045 523 -0.47 -0.19 30.7 44.5 64.5 58.0 79.6 98.4 119.0

055 162 -0.30 -0.12 30.9 45.4 66.9 58.8 81.0 98.3 115.8

600

000 217291 -28.4 -0.10 52.2 67.3 86.8 81.9 102.5 121.1 148.1

075 2425 -0.71 -0.03 44.7 59.0 77.8 71.8 90.9 106.2 131.0

085 1386 -0.39 -0.06 36.9 47.5 61.1 56.1 69.1 78.1 89.7

900

000 173767 -23.3 -0.12 57.7 73.6 93.9 88.2 109.1 126.8 151.6

095 5969 -0.82 0.03 45.7 59.7 78.0 71.2 88.9 101.3 118.0

105 2856 -0.75 0.08 37.6 51.6 70.7 63.4 82.2 95.6 111.1

15

300

000 242638 -36.5 -0.21 38.0 48.0 60.8 59.3 74.1 90.4 121.8

060 475 -0.49 -0.17 20.0 28.1 39.3 36.4 49.0 61.1 74.5

075 714 -0.17 -0.19 16.7 22.7 30.8 29.5 39.3 50.2 64.6

600

000 252098 -33.8 -0.14 48.8 61.2 76.7 73.7 90.5 106.8 133.7

110 5122 -0.26 -0.07 27.0 39.4 57.5 51.4 71.0 87.6 107.0

130 557 -0.10 -0.13 17.7 22.5 28.7 26.9 33.2 38.4 44.4

900

000 215973 -27.4 -0.16 54.5 68.0 84.9 81.0 98.8 115.1 140.4

125 6136 -0.44 -0.09 26.6 38.9 56.8 50.2 69.0 83.8 99.3

150 2165 -0.20 -0.15 24.8 34.4 47.6 43.8 58.1 70.9 87.8

20

300

000 309297 -43.3 -0.23 38.6 48.2 60.2 59.2 73.3 89.3 122.2

060 1506 -0.17 -0.29 15.2 18.8 23.1 22.1 26.7 30.9 36.6

075 473 -0.30 -0.24 17.6 20.6 24.1 23.2 26.6 29.3 33.0

600

000 441127 -36.2 -0.40 61.0 72.3 85.7 83.4 97.6 111.1 135.2

110 5364 -0.17 -0.10 17.8 22.2 27.7 26.8 32.9 39.0 48.7

155 1212 -0.15 -0.12 19.7 25.4 32.6 30.6 38.1 44.6 52.4

900

000 280262 -30.8 -0.41 50.1 62.8 78.7 75.6 92.8 109.3 137.0

150 1865 -0.21 -0.09 15.8 22.0 30.7 29.0 39.3 50.3 65.3

170 1999 -0.10 -0.20 16.1 21.1 27.7 26.6 34.1 42.0 53.9

21

The Arithmetic Mean Diameter (D10) is the ensemble or (arithmetic) number mean diameter

of the measured droplet samples and is useful for analysing the general composition of the

spray. Looking at the calculated values for D10 there is a general trend in which the

Arithmetic Mean Diameter of the measured droplets increases as the axial distance from the

orifice is increased; possibly due to larger droplets having greater momentum and being less

buoyant than the smaller droplets, and are therefore less able to move away from the central

core.

The Volume Mean Diameter (D30) is calculated from the mean of the measured droplet

volumes and is most useful for the analysis of a sprays mass flux. The measured values for

D30 obtained for the JET- A1 releases show that away from the central core of the spray, the

values for D30 are consistent. Within the central core of the spray the D30 are larger than the

surrounding spray, and tend to increase as the spray moves away from the release; suggesting

that the smaller droplets are either coalescing or moving away from the central core.

The Sauter Mean Diameter (D32 or SMD) is probably the most widely used mean diameter

for assessing the quality of a spray and is defined as the diameter of a sphere that has the

same volume/surface area ratio as that of the entire spray. From Table 4.1 It can be seen that

the SMD calculated for the JET-A1 releases is larger in the central core of the spray and

again increases as the distance from the orifice is increased.

Figure 4.4: Comparison of measured SMD to literature correlations

22

Figure 4.4 displays the measured SMD values for all location points and compares them to

the global SMD predicted by correlations in literature (using the measured nozzle pressure

differential and release flow rates). It can be seen in Figure 4.3 that the Miesse (1955)

correlation predict SMD values for the spray releases much larger than that measured in this

study. The TNO Yellow book (2005) and Faeth (1991) correlations predict larger SMD

values at the lower release pressures than that measured. The measured SMD values compare

favourably to that predicted by the Maragkos (2002) correlation, with the largest measured

SMD at each release pressure being near to the predicted value. Whilst providing reasonably

good predictions for the well-atomised kerosene releases, it is expected to perform less well

in predicting mean droplet size for poorly atomised jet releases.

From Table 4.1 it can be seen that the average measured SMD for the positions at which

ignitions occurred and did not occur during Phase one are 86.15 μm and 64.93 μm.

Therefore, it can be seen that SMD cannot be used on its own to assess the flammability of a

mist.

4.3.2 Concentration

This section presents the results obtained for the local droplet concentration calculations for

JET-A1. The calculated JET-A1 concentrations for all the test locations are listed in Table 4.2

and the mass per unit volume concentration values are shown in Figure 4.5.

The Dantec PDA software is capable of calculating mass flux and concentrations, however

the software requires a statistically large enough sample before this calculation is carried out.

Near the edge of the “ignition envelope” (i.e. away from the central core of the spray) the

droplet numbers are too low for the Dantec software to provide a concentration / mass flux

value. The concentrations calculated for the centreline locations are presented in Table 4.2 for

comparison.

From an examination of the calculated concentration values shown in Table 4.2 and Figure

4.5 it can be seen that the minimum calculated concentration from the PDA measurements for

an “Ignition” point can be as low as 2.67 g/m3. However the average concentration of the

ignition positions (which represent the outer ignitable limit found during Phase one) was

23

68.53 g/m3. In addition, the average value of the “Non-Ignition” points from the PDA

measurements was 21.13 g/m3.

Figure 4.5: Calculated mass per unit volume concentrations for all JET-A1 measurement points

At the centreline positions, for all release pressures and axial locations the average number of

detected droplets per second was in excess of 12,000. This illustrates how dense the JET-A1

spray is in the central core compared to near the edge of the “ignition envelope”, where the

maximum mean droplet detection rate was 280 droplets per second. Obtaining measurements

of dense sprays, such as that seen in the central core of the JET-A1 releases studied is very

challenging; as such the uncertainty in the derived concentration values for the centreline

positions is much greater.

The very low droplet numbers in the region surrounding the “ignition envelope” for the JET-

A1 releases means that even though there is a high degree in confidence of the calculated

mean concentrations, the instantaneous concentration during the release is likely to have a

much higher degree of variation due to the transient nature of the spray formed in the releases

studied.

24

Table 4.2: Concentration Calculations

Pressure (barg)

Position (mm) Mean Droplet

Axial Velocity

(m/s)

Sample Period

(s)

Number of

Droplets

Total Measured

Droplet Volume

(m3)

Droplet Concentration

Axial Radial Vol/Vol Mass/Vol

(g/m3)

Dantec Derived

Mass/Vol (g/m

3)

5

300

000 -22.11 10.06 148117 3.90E-08 9.55E-04 764.1 886.7

045 -0.49 40 44 1.88E-11 5.22E-06 4.180 NA

055 -0.21 40 30 6.30E-13 4.09E-07 0.327 NA

600

000 -20.87 11.83 165304 8.45E-08 1.87E-03 1493.3 1160.0

045 -1.45 40 4510 1.85E-09 1.74E-04 139.3 NA

055 -1.05 40 1435 5.21E-10 6.76E-05 54.11 NA

900

000 -17.73 10.16 168940 1.01E-07 3.07E-03 2452.1 1803.9

050 -2.81 40 10759 3.48E-09 1.69E-04 135.3 215.5

080 -1.15 40 1755 3.15E-10 3.74E-05 29.90 NA

10

300

000 -29.57 10.10 132889 1.94E-08 3.54E-04 283.0 313.9

045 -0.47 40 523 5.34E-11 1.55E-05 12.39 NA

055 -0.30 40 162 1.73E-11 7.86E-06 6.287 NA

600

000 -28.35 14.66 217291 6.25E-08 8.21E-04 656.5 518.9

075 -0.71 40 2425 4.70E-10 9.03E-05 72.23 NA

085 -0.39 40 1386 1.28E-10 4.48E-05 35.84 NA

900

000 -23.27 10.0 173767 6.24E-08 1.46E-03 1170.6 696.4

095 -0.82 40 5969 1.13E-09 1.88E-04 150.1 225.73

105 -0.75 40 2856 3.81E-10 6.93E-05 55.44 NA

15

300

000 -36.51 16.34 242638 2.65E-08 2.42E-04 193.8 177.9

060 -0.49 40 475 1.20E-11 3.34E-06 2.671 NA

075 -0.17 40 714 9.59E-12 7.70E-06 6.157 NA

600

000 -33.8 16.18 252098 5.28E-08 5.27E-04 421.6 316.4

110 -0.26 40 5122 3.64E-10 1.91E-04 152.9 NA

130 -0.10 40 557 5.67E-12 7.74E-06 6.189 NA

900

000 -27.36 13.80 215973 6.01E-08 8.68E-04 694.6 677.6

125 -0.44 40 6136 4.06E-10 1.26E-04 100.8 NA

150 -0.20 40 2165 9.53E-11 6.50E-05 51.97 NA

20

300

000 -43.31 20.95 309297 3.36E-08 2.02E-04 161.6 134.3

060 -0.17 40 1506 8.51E-12 6.83E-06 5.462 NA

075 -0.30 40 473 3.09E-12 1.41E-06 1.125 NA

600

000 -36.15 40 441127 1.34E-07 5.05E-04 404.4 213.3

110 -0.17 40 5364 5.41E-11 4.34E-05 34.70 44.29

155 -0.15 40 1212 1.82E-11 1.65E-05 13.23 NA

900

000 -30.79 17.63 280262 6.34E-08 6.37E-04 509.8 520.3

150 -0.21 40 1865 2.38E-11 1.55E-05 12.37 NA

170 -0.10 40 1999 1.97E-11 2.69E-05 21.49 NA

25

4.4 Hydraulic Oil Experimental Results (Flashpoint = 223⁰C)

Hydraulic oil liquid spray characterisation results together with a limited analysis of the data

are presented in this section.

Figure 4.6 shows the positions at which PDA measurements of the hydraulic oil releases were

carried out together with the results of the flammability studies carried out in Phase one. For

each of the three axial locations at which PDA measurements could be taken, results were

obtained for the central axial position and the radial position at which ignition was attempted

during Phase one. Analysis of the droplet measurements and concentration calculations for

the hydraulic oil dataset are described in sections 4.4.1 and 4.4.2 respectively.


The results of the PDA droplet characterisation for the hydraulic oil dataset are shown in

Table 4.3. For each of the locations at which droplet measurements were taken, histograms

have been obtained for the axial and radial velocities as well as the droplet sizes (the

histograms for each measurement location are included in Appendix C).

The overall release profile of the hydraulic oil for all the pressures at which measurements

were taken (5 to 20barg) consisted of a central high velocity liquid core, which was

surrounded by a very small number of slower moving droplets in the form of a low density

spray.

From Table 4.3 it can be observed that at all pressure cases and axial distances, the mean

radial velocity of the droplets is significantly lower compared to the axial velocity

component. Also, as the pressure increases the mean axial velocity increases. Unlike the JET-

A1 liquid releases no other velocity trends can be identified due to low droplet count in most

test points. On the other hand, the low droplet count highlights the fact that the hydraulic oil

consists of a solid jet which possibly only shows signs of breaking-up at the 900mm

downstream position at the higher pressures tested. Even, at the 20bar case the droplet count

is merely about one thousand droplets at the 300 mm and 600mm axial locations and only at

26

the 900mm central position do we see several thousand droplets recorded, indicating that the

breakup of the liquid core begins to occur in that region.

Figure 4.6: Hydraulic oil flammability study results and PDA measurement positions

27

Moreover, Table 4.3 shows that at each axial location as the release pressure is increased the

SMD decreases (when a sample of at least several hundred droplets is considered). Also it

can be observed that as the axial distance is increased the SMD decreases. Figure 4.7

compares the experimentally measured SMD to SMD predictions from the literature (using

the measured nozzle pressure differential and release flow rates). For the hydraulic oil

releases there is a much greater range of measured SMD values; however it can be seen in

Figure 4.7 that the Miesse (1955) correlation again predicts SMD values for the spray

releases much larger than those measured in this study. The TNO Yellow book (2005) and

Faeth (1991) correlations predict larger SMD values at the lowest release pressures than that

measured. If only the positions at which more than 1000 droplets were seen are considered,

the Maragkos (2002) correlation consistently predicts a SMD value close to that measured

over the range of release pressures.

Table 4.3: Hydraulic Oil droplet measurement summary

Pressure

(barg)

Position

(mm)

Number

of

Droplets

Mean Droplet



5

300 0 1084 -18.75 -0.34 367.3 405.3 447.2 425.4 457.8 468.8 482.4

12 0 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

600 0 400 -9.98 0.00 161.8 198.9 244.5 220.3 257.1 270.5 276.5

18 1 -9.04 3.82 88.2 88.2 88.2 88.2 88.2 88.2 88.2

900 0 4 -2.68 -1.22 5.90 5.90 5.90 5.90 5.90 5.90 5.90

20 0 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

10

300 0 21 -23.18 -1.56 274.2 316.1 364.3 345.6 388.0 413.2 443.0

15 132 -0.02 -0.02 6.67 7.00 1.58 7.34 7.70 8.06 8.65

600 0 296 -37.66 -4.15 341.2 391.3 448.8 412.7 454.0 459.2 464,0

20 235 -0.18 0.08 20.6 21.9 23.3 23.2 24.7 26.1 29.0

900 0 269 -5.71 -0.30 52.3 81.5 126.9 119.2 179.9 254.9 342.7

25 18 -0.60 -0.20 9.8 11.3 12.9 12.5 14.2 15.5 16.8

15

300 0 58 -0.09 -0.06 10.6 12.1 13.7 13.2 14.8 16.0 17.0

17 0 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

600 0 54 -8.65 -0.47 63.4 139.7 307.9 214.9 395.5 508.2 549.0

20 108 -0.70 0.12 17.1 27.3 43.4 39.1 59.1 80.3 103.6

900 0 1873 -6.97 0.13 46.9 83.2 147.5 125.4 205 284.9 356.6

30 270 -0.67 0.07 39 50.8 66.2 59.3 73.2 81.0 88.3

20

300 0 2599 -13.15 -0.18 34.6 86.3 215.1 143.2 291.4 394.7 442.4

20 102 -0.10 -0.05 14.0 16.0 18.3 17.6 19.8 21.4 23.5

600 0 1307 -9.52 -0.30 44.4 94.5 200.9 149.2 273.5 372.3 441.3

25 694 -0.29 -0.04 12.1 19.6 31.7 27.9 42.3 56.5 66.9

900 0 13936 -8.98 -0.22 44.8 80.2 143.5 128.9 218.6 333.1 441.2

35 518 -0.53 0.04 24.6 54.3 120.2 106.9 222.9 413.3 547

28

Figure 4.7: Comparison of measured SMD to literature correlations for Hydraulic oil

4.4.2 Concentration

The calculated concentrations for all the test locations are listed in Table 4.4 and the mass per

unit volume concentration values are shown in Figure 4.8. The droplet numbers are too low

for the Dantec software to provide a concentration / mass flux value. To note that no ignition

was observed for hydraulic oil at any test condition.

Concentration values shown in Table 4.4 and Figure 4.8 show that in most cases the

calculated concentration is below the widely accepted LEL of 50 g/m3. However, in three test

points the concentration was higher than 50 g/m3 but due to the low volatility of the hydraulic

oil, the low number of droplets present in the region and their large SMD value ignition was

not observed during Phase one.

29

Table 4.4: Hydraulic Oil Concentration Calculation

Pressure

(barg)

Position (mm) Mean

Droplet

Axial

Velocity

(m/s)

Sample

Period

(s)

Number

of

Droplets

Total

Measured

Droplet

Volume

(m3)



(g/m3)

Dantec

Derived

Mass/Vol

(g/m3)

5

300 0 -18.75 40 1084 4.37E-08 3.18E-04 276.5 NA

12 0.00 40 0 0.00E+00 0.00E+00 0.0 NA

600 0 -9.98 40 400 2.24E-09 3.06E-05 26.62 NA

18 -9.04 40 1 3.59E-13 5.42E-09 4.72E-03 NA

900 0 -2.68 40 4 4.30E-16 2.19E-11 1.90E-05 NA

20 0.00 40 0 0.00E+00 0.00E+00 0.0 NA

10

300 0 -23.18 40 21 4.54E-10 2.67E-06 2.323 NA

15 -0.02 40 132 2.74E-14 1.87E-07 0.162 NA

600 0 -37.66 40 296 1.09E-08 3.95E-05 34.36 NA

20 -0.18 40 235 1.54E-12 1.16E-06 1.014 NA

900 0 -5.71 40 269 2.38E-10 5.69E-06 4.950 NA

25 -0.60 40 18 1.84E-14 4.18E-09 0.004 NA

15

300 0 -0.09 40 58 6.93E-14 1.05E-07 0.091 NA

17 0.00 40 0 0.00E+00 0.00E+00 0.0 NA

600 0 -8.65 40 54 2.81E-10 4.43E-06 3.855 NA

20 -0.70 40 108 3.38E-12 6.58E-07 0.573 NA

900 0 -6.97 40 1873 1.93E-09 3.78E-05 32.93 NA

30 -0.67 40 270 2.95E-11 6.01E-06 5.226 NA

20

300 0 -13.15 40 2599 4.00E-09 4.15E-05 36.06 NA

20 -0.10 40 102 2.90E-13 3.95E-07 0.344 NA

600 0 -9.52 40 1307 2.27E-09 3.25E-05 28.32 NA

25 -0.29 40 694 7.90E-12 3.71E-06 3.231 NA

900 0 -8.98 40 13936 1.56E-08 2.37E-04 206.5 NA

35 -0.53 40 518 3.31E-10 8.52E-05 74.16 NA

30

Figure 4.8: Calculated mass per unit volume concentrations for all Hydraulic oil measurement points

4.5 LFO Ambient Experimental Results (Flashpoint = 81⁰C)

Light Fuel Oil at Ambient conditions spray characterisation results together with a limited

analysis of the data are discussed in this section.

Figure 4.9 shows the positions at which PDA measurements of the LFO Ambient releases

were carried out together with their relation to the results of the flammability studies carried

out in Phase one. For each of the three axial locations at which PDA measurements could be

taken, results were obtained for the central axial position and the radial position at which

ignition was attempted during Phase one. Analysis of the droplet measurements and

concentration calculations for the LFO at ambient conditions dataset are described in section

4.5.1 and 4.5.2.

31

Figure 4.9: LFO Ambient flammability study results and PDA measurement positions

32


The results of the PDA droplet characterisation for the LFO Ambient dataset are shown in

Table 4.5. For each of the locations at which droplet measurements were taken, distributions


histograms for each measurement location are included in Appendix D).

The overall release profile of the LFO ambient is similar to the hydraulic oil. For all the

pressures at which measurements were taken (5 to 20barg) consisted of a central high

velocity liquid core surrounded by slower moving droplets in the form of a much less dense

spray.

It is noted from Table 4.5 that for all pressure cases and axial distances, the mean radial

velocity of the droplets is significantly lower when compared to the axial velocity

component. Moreover, as the pressure increases the mean axial velocity increases. In

common with the hydraulic oil liquid releases no other velocity trends can be identified due

to low droplet count at most test points. The low droplet count highlights the fact that the

LFO ambient releases consisted of a liquid core which only possibly starts to break-up at the

900mm downstream position at higher release pressures. Even, at the 20bar case the droplet

count is only just over one thousand droplets at the 600mm axial location and at the 900mm

axial position we get to see several thousand droplets recorded, indicating that the break-up

of the liquid core begins to occur in that region.

Moreover, from Table 4.5 shows that at each axial location as the release pressure is

increased the SMD decreases1. Also it can be observed that as the axial distance is increased

the SMD decreases1. Figure 4.10 compares the experimentally measured SMDs for the LFO

ambient releases to the SMDs predicted in the literature (using the measured nozzle pressure

differential and release flow rates). Similar to that shown for the hydraulic oil releases, there

is large range in measured SMD values at each of the release pressures studied; however it

can be seen in Figure 4.10 that the Miesse (1955) correlation is again predicting SMD values

for the spray releases larger than that measured in this study for the lower release pressures.

The TNO Yellow book (2005), Faeth (1991) and Maragkos (2002) correlations predict SMD

values that sit within the range of that measured. If only the positions at which more than

1 Mostly evident in the 900mm axial position where the sample is in the order of a few hundred to thousands

droplets

33

1000 droplets were recorded are considered, the Maragkos (2002) correlation under-predicts

the SMD over the range of release pressures by a similar margin to the over-prediction by

Miesse (1955); with the measured SMD sitting in between the values predicted by these

correlations. As it is more conservative to use a correlation that under predicts the droplet

SMD produced during a release, of the correlations applied that of Maragkos (2002) is

suggested for modelling work.

Table 4.5: LFO Ambient droplet measurement summary

Pressure

(barg)

Position

(mm)

Number

of

Droplets

Mean Droplet



5

300 000 0 0.0 0.0 0.00 0.0 0.0 0.00 0.0 0.0 0.00

010 0 0.0 0.0 0.00 0.0 0.0 0.00 0.0 0.0 0.00

600 000 0 0.0 0.0 0.00 0.0 0.0 0.00 0.0 0.0 0.00

012 0 0.0 0.0 0.00 0.0 0.0 0.00 0.0 0.0 0.00

900 000 400 -18.8 -0.31 603.5 663.7 730.0 704.1 760.6 792.4 826.5

015 0 0.0 0.0 0.00 0.0 0.0 0.00 0.0 0.0 0.00

10

300 000 14 -0.5 0.0 24.7 27.2 29.8 29.8 32.8 36.0 41.7

010 34 -4.1 0.16 74.1 123.0 204.2 186.1 295.0 426.1 505.2

600 000 11 -12.2 -0.8 77.6 84.4 91.9 91.4 99.2 107.1 119.8

012 1 -20.4 1.35 104.0 104.0 104.0 104.0 104.0 104.0 104.0

900 000 5864 -25.6 -0.45 282.3 374.3 496.3 447.7 563.8 640.5 723.9

022 40 -7.4 0.94 95.8 118.3 146.1 138.8 167.1 191.3 219.5

15

300 000 4 -30.8 0.0 454.6 471.7 489.4 488.1 505.8 522.7 551.2

015 0 0.0 0.0 0.00 0.0 0.0 0.00 0.0 0.0 0.00

600 000 237 -26.3 -0.57 235.6 315.9 423.5 384.0 490.2 567.5 669.0

017 15 -6.57 -0.12 81.8 97.6 116.5 113.1 132.9 151.7 176.4

900 000 14825 -29.5 -0.50 199.2 287.8 415.7 368.0 500.2 601.9 708.1

027 184 -5.69 0.90 86.6 129.8 194.5 204.8 314.9 509.7 790.6

20

300 000 6 -15.6 0.0 51.1 51.8 52.6 52.5 53.3 54.0 55.2

017 21 -30.6 0.0 519.6 548.4 578.9 577.5 608.8 640.3 695.9

600 000 1866 -27.6 -0.76 174.2 274.3 431.7 360.2 517.9 621.3 713.1

020 424 -6.05 0.33 68.2 95.6 134.1 152.4 227.9 387.1 670.7

900 000 20806 -32.8 -0.67 178.8 263.3 387.6 344.1 477.4 587.9 702.5

030 1700 -6.42 0.63 77.0 98.0 124.7 134.8 178.4 255.3 530.8

34

Figure 4.10: Comparison of measured SMD to literature correlations for LFO Ambient

4.5.2 Concentration


unit volume concentration values are presented in Figure 4.11. The droplet numbers are

insufficient for the Dantec software to provide a concentration / mass flux value. During

Phase one, no ignitions were observed for LFO ambient under any free spray test conditions.

Examination of the concentration shown in Table 4.6 and Figure 4.11 it can be seen that in

most cases the calculated concentration is below the widely accepted LEL of 50 g/m3.

However, in three test locations from the Phase one ignition experiments the concentration

was found to be between 10 and 50 g/m3 (which could potentially be flammable), but

possibly due to the large measured SMD values and low number of droplets present in the

region no ignitions was observed.

35

Figure 4.11: Calculated mass per unit volume concentrations for LFO Ambient measurement points

36

Table 4.6: LFO Ambient Concentration Calculation

Pressure

(barg)

Position (mm) Mean

Droplet

Axial

Velocity

(m/s)

Sample

Period

(s)

Number

of

Droplets

Total

Measured

Droplet

Volume

(m3)



(g/m3)

Dantec

Derived

Mass/Vol

(g/m3)

5

300 000 0.0 40 0 0.00E+00 0.00E+00 0.0 NA

010 0.0 40 0 0.00E+00 0.00E+00 0.0 NA

600 000 0.0 40 0 0.00E+00 0.00E+00 0.0 NA

012 0.0 40 0 0.00E+00 0.00E+00 0.0 NA

900 000 -18.8 40 400 7.32E-08 5.12E-04 476.3 NA

015 0.0 40 0 0.00E+00 0.00E+00 0.0 NA

10

300 000 -0.5 40 14 1.94E-13 5.12E-08 0.048 NA

010 -4.1 40 34 1.16E-10 3.73E-06 3.471 NA

600 000 -12.2 40 11 4.36E-12 4.70E-08 0.044 NA

012 -20.4 40 1 5.89E-13 3.80E-09 0.004 NA

900 000 -25.6 40 5864 2.76E-07 1.42E-03 1318.3 NA

022 -7.4 40 40 5.66E-11 1.01E-06 0.940 NA

15

300 000 -30.8 40 4 2.74E-10 1.17E-06 1.092 NA

015 0.0 40 0 0.00E+00 0.00E+00 0.0 NA

600 000 -26.26 40 237 7.02E-09 3.52E-05 32.77 NA

017 -6.57 40 15 1.15E-11 2.30E-07 0.214 NA

900 000 -29.5 40 14825 3.87E-07 1.73E-03 1609.1 NA

027 -5.69 40 184 8.28E-10 1.92E-05 17.84 NA

20

300 000 -15.6 40 6 4.56E-13 3.85E-09 0.004 NA

017 -30.6 40 21 2.07E-09 8.94E-06 8.314 NA

600 000 -27.6 40 1866 4.57E-08 2.18E-04 202.8 NA

020 -6.05 40 424 7.87E-10 1.71E-05 15.95 NA

900 000 -32.8 40 20806 4.44E-07 1.78E-03 1657.0 NA

030 -6.42 40 1700 2.18E-09 4.48E-05 41.65 NA

37

4.6 LFO Heated at 70 oC Experimental Results (Flashpoint = 81⁰C)

Light Fuel Oil Heated at 70 oC spray characterisation results together with a limited analysis

of the data are presented in this section.

Figure 4.12 shows the positions at which PDA measurements of the LFO Heated at 70 oC

releases were carried out together with the results of the flammability studies carried out in

Phase one. For each of the three axial locations at which PDA measurements could be taken,

results were obtained for the central axial position as well as radial positions either side of the

“ignition envelope” where ignition was observed during Phase one. Analysis of the droplet

measurements and concentration calculations for LFO Heated at 70 oC dataset are described

in the sections 4.6.1 and 4.7.1 respectively.


The results of the PDA droplet characterisation for the LFO Heated at 70 oC dataset are

shown in Table 4.7. For each of the locations at which droplet measurements were taken,

distributions have been obtained for the axial and radial velocities as well as the droplet sizes

(the histograms for each measurement location are included in Appendix E).

Heating the LFO to 70 oC significantly reduces its viscosity from 170 mm

2/s

to 18 mm

2/s,

promoting atomisation when compared to the results at ambient conditions. The overall

release profile of the LFO Ambient for all the pressures at which measurements were taken (5

to 20barg) consisted of a central high velocity liquid core surrounded by a slower moving,

much less dense spray.

From Table 4.7 can be observed that at all pressure cases and axial distances, the droplet

number recorded is significantly larger than when tested at ambient conditions. Moreover, it

can be seen that the mean radial velocity of the droplets is significantly lower when compared

to the axial velocity component. In addition, as the pressure increases the mean axial velocity

increases.

38

Figure 4.12: LFO Heated at 70 oC flammability study results and PDA measurement positions

39

Comparing the measured spray droplet characteristics shown for the LFO heated to 70 oC in

Table 4.7 to that of the unheated light fuel oil shown in Table 4.5, it can be seen that the

heated LFO produced significantly more droplets than the ambient temperature light fuel oil;

even at positions close to the release orifice. At the lowest release pressure (5 barg) the

number of droplets measured provides some evidence of primary / mechanical breakup

occurring near to the orifice, with further break-up occurring once the jet is 900mm from the

orifice. As the release pressure increased the results indicate that the breakup distance is

reduced such that it is less than 600mm at 15 barg.

Moreover, from Table 4.7 it can be seen that at each axial location as the release pressure is

increased the SMD decreases. Also, it can be observed that as the axial distance is increased

the SMD decreases. At the test points where ignitions were observed the SMD ranged from

130μm- 350μm, however the D10 ranged from 50 μm -76 μm, indicating that the spray in the

region mainly consisted of many smaller droplets. Figure 4.13 compares the experimentally

measured SMDs to the SMDs predicted in the literature.

Figure 4.13 compares the experimentally measured SMDs for the LFO heated to 70 oC

releases to the SMDs predicted in the literature (using the measured nozzle pressure

differential and release flow rates). The range of SMD values measured for each of the

release pressures studied for the heated LFO is very similar to that of the unheated LFO

shown in Figure 4.10. The Miesse (1955) correlation is again predicting SMD values for the

spray releases to be larger than that measured in this study over the range of release

pressures. The SMD trend predicted by the TNO Yellow book (2005), and Faeth (1991)

correlations does not match that seen in the measured values, as they are predicting a larger

reduction in SMD with increasing pressure than that seen in the measured values. Similar to

the results of the LFO ambient releases, the Maragkos (2002) correlation under predicts the

SMD over the range of release pressures, while the Miesse (1955) correlation over predicts

by a similar margin; the measured SMD is in between the values predicted by these two

correlations. Again, as it is more cautious to use a correlation that under predicts the droplet

SMD produced during a release the Maragkos (2002) correlation is the most suitable to be

used for modelling work.

40

Table 4.7: LFO Heated at 70 oC droplet measurement summary

Pressure

(barg)

Position

(mm)

Number

of

Droplets

Mean Droplet



5

300 0 18711 -18.7 -0.50 469.0 550.1 643.4 600.2 683.4 726.0 773.2

10 400 -13.9 0.07 85.6 120.2 168.6 158.8 216.2 277.3 371.4

600 0 1496 -11.5 0.03 82.3 125.5 191.1 189.5 287.4 432.3 664.3

15 645 -4 0.17 61.6 81.6 107.9 108.2 143.3 190.4 281.6

900 0 16270 -25.1 -0.29 175.0 240.1 329.3 305.6 403.9 495.3 628.1

25 1508 -3.4 0.37 85.8 139.3 226.3 214.0 338.0 504.8 702.6

10

300 0 814 -24.5 -1.41 174.7 288.3 475.6 379.2 558.7 656.3 755.6

10 2632 -15.4 -0.06 66.2 98.1 145.4 154.7 236.4 384.4 661.9

600 0 17361 -23.1 -0.55 85.3 131.8 203.6 199.8 305.9 459.5 662.1

15 6085 -4.42 0.13 65.5 101.0 155.7 157.7 244.9 385.0 605.6

900

0 51037 -31 -0.34 145.6 210.7 304.9 285.5 399.7 524.1 684.9

35 1391 -2.96 0.40 76.5 112.2 164.6 166.2 244.8 364.1 572.8

45 615 -2.46 0.46 103.7 156.5 236.2 235.1 354.0 530.5 727.3

15

300 0 17083 -37.6 -0.88 152.0 252.6 420.0 345.7 521.4 647.5 750.5

10 6563 -14.2 -0.21 54.5 96.0 169.1 165.1 287.3 488.0 695.0

600

0 41130 -30 -0.61 88.1 128.6 187.5 192.2 283.9 429.8 663.9

15 7300 -9.77 0.42 56.5 74.5 98.2 111.9 157.4 252.4 582.9

20 5313 -5.20 0.42 49.4 66.9 90.6 97.5 137.1 207.6 420.3

900

0 46750 -34.9 -0.27 134.7 188.6 264.1 257.5 356.0 479.9 664.6

25 4532 -10.8 0.43 78.4 113.0 162.9 168.9 247.9 377.4 626.1

35 5846 -5.02 0.48 67.4 120.6 215.8 198.3 340.2 536.3 738.3

20

300

0 38887 -45.8 -1.81 107.1 171.7 275.4 251.2 384.7 537.5 704.7

15 5323 -8.50 0.20 50.8 69.7 95.8 110.3 162.6 275.8 550.9

20 4692 -4.22 0.39 46.5 67.7 98.7 114.1 178.7 323.6 631.3

600 0 74081 -36.6 -0.76 89.1 128.5 185.3 190.6 278.7 419.0 648.7

20 10020 -9.1 0.53 53.3 72.1 97.6 109.7 157.3 253.8 531.8

900

0 77798 -42.6 -0.18 148.9 207.3 288.6 278.2 380.3 501.1 674.3

25 2997 -13.6 0.63 75.0 87.5 102.1 101.6 118.2 136.9 181.7

35 1202 -7.18 0.54 66.5 77.4 89.9 89.5 103.8 119.9 154.1

41

Figure 4.13: Comparison of measured SMD to literature correlations for LFO heated to 70 oC

4.6.2 Concentration


unit volume concentration values are presenteded in Figure 4.14. The concentration / mass

flux values calculated from the Dantec software when a statistically large enough sample was

recorded, are also presented in Table 4.8 for comparison.

The calculated concentration values shown in Table 4.8 and Figure 4.14 show that the

minimum calculated concentration from the PDA measurements at an “ignition” point can be

as low as 14.06 g/m3. However the average concentration of the “ignition” positions (which

represent the outer ignitable limit found during Phase 1) was 76.49 g/m3.

There are test points where higher concentrations were recorded than the considered LEL of

50 g/m3

but no ignitions were observed. Although, the concentration calculated provides a

good indication of the mean concentration over the release period, the instantaneous

concentration during the release is likely to have a higher degree of variation due to the

transient nature of the spray formed. Additionally, the number of droplets present in the

42

region, spray quality as expressed by the SMD, the presence of smaller droplets (30-100μm)

would all play a significant role in droplet ignition and flame propagation.

Table 4.8: LFO Heated at 70 oC Concentration Calculation

Pressure (barg)


Axial Velocity

(m/s)

Sample Period

(s)

Number of

Droplets

Total Measured

Droplet Volume

(m3)



(g/m3)

Dantec Derived

Mass/Vol (g/m

3)

5

300 0 -18.7 40 18711 2.12E-06 1.50E-02 13163.5 12433.01

10 -13.9 40 400 8.39E-10 7.95E-06 7.000 NA

600 0 -11.5 40 1496 5.33E-09 6.09E-05 53.56 NA

15 -4 40 645 4.28E-10 1.40E-05 12.321 NA

900 0 -25.1 40 16270 2.43E-07 1.28E-03 1124.6 178.6

25 -3.4 40 1508 7.74E-09 2.97E-04 261.6 NA

10

300 0 -24.5 40 814 2.33E-08 1.25E-04 110.0 NA

10 -15.4 40 2632 5.10E-09 4.38E-05 38.51 72.87

600 0 -23.1 40 17361 7.25E-08 4.14E-04 364.0 148.9

15 -4.42 40 6085 1.25E-08 3.73E-04 327.9 NA

900

0 -31 40 51037 6.22E-07 2.64E-03 2325.0 NA

35 -2.96 40 1391 3.34E-09 1.49E-04 130.8 NA

45 -2.46 40 615 4.19E-09 2.24E-04 197.1 NA

15

300 0 -37.6 40 17083 3.70E-07 1.29E-03 1139.2 637.6

10 -14.2 40 6563 1.55E-08 1.44E-04 126.7 67.84

600

0 -30 40 41130 1.53E-07 6.71E-04 590.8 191.8

15 -9.77 40 7300 5.35E-09 7.23E-05 63.59 27.85

20 -5.20 40 5313 2.58E-09 6.54E-05 57.57 18.45

900

0 -34.9 40 46750 4.18E-07 1.58E-03 1389.0 NA

25 -10.8 40 4532 1.14E-08 1.40E-04 123.0 26.68

35 -5.02 40 5846 2.39E-08 6.27E-04 551.7 NA

20

300

0 -45.8 40 38887 3.23E-07 9.29E-04 817.5 855.7

15 -8.50 40 5323 3.74E-09 5.80E-05 51.06 68.71

20 -4.22 40 4692 3.65E-09 1.14E-04 100.3 185.8

600 0 -36.6 26.93 74081 2.68E-07 1.43E-03 1262.8 205.3

20 -9.1 40 10020 6.92E-09 1.00E-04 88.38 34.35

900

0 -42.6 40 77798 8.77E-07 2.71E-03 2388.9 469.7

25 -13.6 40 2997 1.65E-09 1.60E-05 14.06 17.09

35 -7.18 40 1202 4.52E-10 8.29E-06 7.295 8.244

43

Figure 4.14: Calculated mass per unit volume concentrations for LFO heated at 70 oC measurement

points

44

4.7 Impingement Results

During the ignition studies reported in Phase one of this project, a small number of

impingement studies were carried out to demonstrate whether the impingement of a spray on

a surface affects its ignitibility due to secondary breakup of the jet caused by its interaction /

impact with a surface.

In this phase, a single case of impingement was selected for spray characterisation. The

selected case was 20 barg Light Fuel Oil at Ambient conditions. During the Phase one

flammability studies, the free-spray LFO Ambient releases did not ignite at all pressures and

locations tested. However, in the impingement studies at 20barg a single ignition condition

was observed. Figure 4.15 shows the position at which PDA measurements of the LFO

ambient release were undertaken together with their relation to the results of the flammability

studies carried out in Phase one. For this location droplet sizes and two velocity components

(axial and radial) were recorded.

Figure 4.15: LFO Ambient-impingement flammability study results and PDA measurement positions

45

The impingement set up was the same as that used in Phase one. A flat mild steel plate was

used as an impingement surface and was positioned 400mm downstream of the orifice. The

PDA was positioned 25mm above the impingement surface and 40mm away from the

centreline of the spray, which matches the ignitor location for the release at which an ignition

was observed.

The PDA set-up was nominally the same as when sampling data for the LFO Ambient free

spray case. The only variation was that the release duration was reduced to 10 seconds as it

was anticipated that the LFO would adhere to the transmitting and receiving optics rendering

the system unable to record any data. The PDA system was set to acquire 50000 droplets or

sample for the entire 10 sec release. During the test it was found that the PDA system was

only able to acquire data for less than 2.2 seconds prior to LFO adhering to the optics and

preventing the system recording any further.

Figure 4.15 displays the PDA measurements obtained at the selected position; with

histograms showing the measured axial and radial velocity and droplet size distributions.

Figure 4.15: PDA results-LFO Amb Impingement case (20barg release, 375mm axial, 40mm radial)

46

The results of the PDA droplet characterisation for the LFO ambient impingement case are

shown in Table 4.9. Also, the results from the 20 barg LFO ambient free-spray releases at

300mm and 600mm are included for comparison.

Table 4.9: LFO ambient comparison of impingement and free spray droplet measurements

Test

case




Free

spray

300 000 -15.6 0.0 51.1 51.8 52.6 52.5 53.3 54.0 55.2

017 -30.6 0.0 519.6 548.4 578.9 577.5 608.8 640.3 695.9

600 000 -27.6 -0.76 174.2 274.3 431.7 360.2 517.9 621.3 713.1

020 -6.05 0.33 68.2 95.6 134.1 152.4 227.9 387.1 670.7

Imping-

ment 375 040 1.17 -15.5 71.6 92.3 119.1 120.6 156.6 205.8 290.1

From a comparison of the PDA data obtained for the 20 barg LFO ambient impingement case

to that of the free-spray cases shown in Table 4.9 it can be seen that impingement has caused

a combined increase in the number of droplets observed per second of sample time, an

increase in the calculated droplet concentration and a decrease in the size of the droplets

measured. This combination of effects is consistent with the ignition that was observed

during the Phase one testing.

Test

case

Position (mm) Sample

Period

(s)

Number

of

Droplets

Total

Measured

Droplet

Volume (m3)



(g/m3)

Dantec Derived

Mass/Vol (g/m3)

Free

spray

300 000 40 6 4.56E-13 3.85E-09 0.004 NA

017 40 21 2.07E-09 8.94E-06 8.314 NA

600 000 40 1866 4.57E-08 2.18E-04 202.7 NA

020 40 424 7.87E-10 1.71E-05 15.95 NA

Imping-

ment 375 040 2.2 147 1.35E-10 2.78E-04 258.4 NA

47

4.8 High Pressure Hydraulic Oil Tests

In this test program limited studies were conducted in order to investigate the flammability of

Hydraulic oil at higher supply pressures ranging from 30-150 barg. Subsequently, droplet

characterisation data were collected. The aim of this study was to underpin the conditions

required for a high flashpoint, high viscosity liquid to ignite. Figure 4.16 presents the

positions at which PDA measurements were carried out in relation to the results of the

flammability studies. As it can be seen no ignition were observed at any pressures.

Figure 4.16: High Pressure Hydraulic oil flammability study results and PDA measurement positions

48


The results of the PDA droplet characterisation for the hydraulic oil dataset are shown in

Table 4.10. For each of the locations at which droplet measurements were taken, distributions


histograms for each measurement location are included in Appendix F).

The overall release profile of the hydraulic oil for all the supply pressures at which

measurements were taken (30 to 150barg) consisted of a central high velocity liquid core,

which was surrounded by slower moving droplets in the form of a much less dense spray.

From Table 4.10 can be observed that at all pressure cases and axial distances, the mean

radial velocity of the droplets is significantly lower when compared to the axial velocity

component. Also, as the pressure increases the mean axial velocity increases. No other

velocity trends can be identified due to low droplet count in most test points. The low droplet

count highlights the fact that the hydraulic oil releases consist of a central dense high velocity

core. Also included in Table 4.10 are the droplet sizes predicted by the correlation of

Maragkos (2002), which has been found to be the most appropriate correlation for Hydraulic

oil.

Table 4.10: High Pressure Hydraulic oil droplet measurement summary

49

4.8.2 Concentration

The calculated concentrations for all the test locations are listed in Table 4.11 The droplet

numbers are insufficient for the Dantec software to provide a concentration / mass flux value.

No ignition was observed for hydraulic oil at any test conditions.

The concentration values shown in Table 4.11 show that at the release pressures used in this

study, in several cases the calculated concentration is above the widely accepted LEL of 50

g/m3. However, the low volatility of the hydraulic oil and its high flashpoint have resulted in

no observed ignitions occurring, even though the droplet sizes measured are similar to that

seen for ignition cases with JET-A1 and heated LFO.

Table 4.11: High Pressure Hydraulic oil Concentration calculation

Pressure

(barg)

Position (mm) Mean

Droplet

Axial

Velocity

(m/s)

Sample

Period

(s)

Number

of

Droplets

Total

Measured

Droplet

Volume

(m3)


Axial Radial Vol/Vol

Mass/V

ol

(g/m3)

Dantec

Derived

Mass/Vo

l (g/m3)

30 600 0 -31.94 20 31 7.40E-10 6.10E-06 5.3 NA

10 -4.96 20 2500 3.46E-09 1.84E-04 159.8 NA

70 600 0 -10.72 20 687 3.81E-10 9.37E-06 8.148 NA

15 -10.6 20 303 2.62E-10 6.50E-06 5.7 NA

110 600 0 -17.52 20 2251 2.02E-10 3.48E-05 30.3 NA

17 -11.14 20 4151 1.15E-09 1.74E-05 15.1 NA

150

600 0 -20.64 20 5772 1.66E-09 3.93E-05 34.2 NA

20 -6.90 20 5040 5.12E-09 6.53E-05 56.8 NA

900 0 -28.04 20 8755 2.03E-09 7.77E-05 67.6 NA

15 -17.5 20 30913 9.15E-09 8.60E-05 74.8 NA

50

5.0 OBSERVATIONS

Phase two of the experimental program attempts to provide insights and a better qualititative

understanding the spray combustion physics underlying the observations from Phase one. For

the spray characterisation PDA was selected as a non-intrusive optical diagnostic method for

the determination of the droplet size and velocities of the spray particles.

This extended report also contains observations / results from the study of high pressure

hydraulic oil releases.

The key observations form the tests conducted can be summarised as follows:

For all fuels the spray release consists of a dense liquid core surrounded by finer

droplets.

For all fuels it can be seen that the droplets within the liquid core are moving at higher

velocities and have a larger SMD than the surrounding droplets. The finer surrounding

droplets have very low axial and radial velocity components, such that they form a low

velocity mist around the central core.

The measured SMD was compared with the calculated global SMD from correlations

found in the literature, and it was determined that the correlation of Maragkos (2002)

provided the best fit for the well-atomised JET-A1 releases. The Maragkos (2002)

correlation was also shown to be suitable for predicting the SMD for the hydraulic oil

releases studied although large non-spherical fluid elements will not have been

accounted for in these poorly atomised releases. For the light fuel oil (both ambient

and heated to 70⁰C), no single correlation could not be found that adequately

corresponds to the measurement data, with the measured SMD being between that

predicted by Maragkos (2002) and Miesse (1955). The Maragkos (2002) correlation

consistently under predicts the SMD for the light fuel oil releases, whereas the Miesse

(1955) over predicts.

For most of the measurement positions the Dantec BSA flow post-processing software

was unable to provide concentrations for the spray surrounding the liquid core as the

droplet numbers recorded were insufficient, so fuel concentrations have to be

calculated from raw droplet data.

51

Concentration values were calculated from first principles for all sample locations and

were found to be in reasonable agreement with the values that the Dantec algorithm

could calculate.

For the Jet-A1 liquid:

o Observations of the spray showed that even at positions very close to the

release orifice there were comparatively large numbers of droplets recorded,

suggesting that the Jet-A1 liquid was breaking up very close to the orifice and

that produces flammable mists/sprays even at low pressures.

o Towards the limits of the ignition envelope at the edge of the spray, typical

mean droplet sizes (SMD) reduce to be between 30-100 μm. This has two

implications concerning spray flammability:

- These droplet sizes on the boundary of the spray are more readily ignitable

than that closer to the centreline, and potentially burn with higher flame speed

(Ballal and Lefebvre 1979; 1981).

- In this size range, flames can propagate at concentrations below the vapour

LFL due the well established droplet-droplet relay mechanism for spray

propagation (Hayashi et al. 1981). This is consistent with measured spray

concentrations.

o In the region between ignition-propagation and no propagation, concentrations

typically reduced such they were near to or lower than the vapour LFL.

o The liquid properties of the Jet-A1 are such that even at very low release

pressures the SMD of the droplets measured are comparatively small, with a

large proportion of the droplets likely to be under that required for them to be

ignitable. Because of this, overall droplet concentration at an ignition location

is the main parameter in determining whether a Jet-A1 release is ignitable.

For the hydraulic oil, including high pressure releases:

o Observations of the jet showed that there was comparatively few droplets

formed at all the conditions tested, with a significant amount of fine droplets

only being observed once pressures approached 150barg. There was little

evidence of any Primary / mechanical breakup occurring at release pressures

less than 50 barg. Even at 150 barg the breakup observed consisted of fine

droplets forming around the central dense core at a distance of approximately

52

200mm from the orifice, with the central core only showing signs of

disintegrating at an axial distance of 600mm.

o The limited droplet measurements that could be obtained due to the very poor

nature of the spray agreed with observations made during ignition testing.

Pressures of 150barg are insufficient to fully atomise the hydraulic oil,

resulting in an inadequate number of ignitable droplets being produced.

For the light fuel oil at ambient temperature:

o Observations of the jet showed that this liquid behaved similar to hydraulic

oil, with comparatively few droplets being formed at all the conditions tested.

Again, there was little evidence of any Primary / mechanical breakup

occurring even at the highest pressures. However, at both 15 and 20 barg there

was evidence that breakup was beginning to occur at a distance of 900mm

from the orifice.

o Again, similar to the hydraulic oil the very poor nature of the spray and the

few droplets measurements taken agreed with observations made during Phase

one of the test program. The release pressures used (≤20barg) are insufficient

to atomise the light fuel oil, resulting in too few ignitable droplets being

produced.

For the light fuel oil heated to 70⁰C:

o Observations of the spray showed that the heated light fuel oil produced

significantly more droplets than the ambient temperature light fuel oil, even at

positions close to the release orifice. At the lowest release pressure (5 barg)

there was some evidence of Primary / mechanical breakup occurring near to

the orifice, with further break-up occurring near to the position 900mm from

the orifice. At higher release pressures there are indications of the liquid core

breakup distance reducing to less than 600mm at 15 barg.

o At the test positions at where ignitions were observed during Phase one of the

test program, a range of SMD values of 130μm- 350μm were measured.

However, the measured D10 values at these positions were 50 μm -76 μm,

indicating that the spray mainly consisted of many smaller droplets and that a

few comparatively large droplets are skewing the SMD value. Because of this,

the Sauter Mean Diameter may not be the most suitable parameter for

assessing the ignitability of sprays / mists such as that seen in this work.

53

o Compared to that seen with the Jet-A1 releases, the SMD of the droplets

measured for the heated LFO releases are comparatively large, suggesting a

much smaller proportion of the droplets are under the size that is required for

them to be ignitable. Therefore, for liquids such as LFO, the droplet sizes

produced as well as the overall droplet concentrations need to be assessed for

a release condition to evaluate the ignition risk of any spray / mist formed.

During the Phase one investigation, it was found that through the introduction of

impingement, releases that could not be ignited as a free spray could potentially

become flammable. Droplet measurements for the 20 barg “non-ignitable” LFO

ambient free-spray releases were compared to that obtained for the 20 barg “ignitable”

LFO impingement case; it was found that impingement resulted in a reduction in the

measured droplet diameter. This combined with the increased droplet concentration at

the ignitor location provides an explanation for the observed ignition.

54

REFERENCES

Aı́SA, L., GARCIA, J. A., CERECEDO, L. M., GARCı́A PALACı́N, I. & CALVO, E. 2002.

Particle concentration and local mass flux measurements in two-phase flows with

PDA. Application to a study on the dispersion of spherical particles in a turbulent air

jet. International Journal of Multiphase Flow, 28, 301-324.

ARTIUM-TECHNOLOGIES. Spray Diagnostics [Online]. Available:

http://www.artium.com/ [Accessed 15/01 2013].

BALLAL, D. R. & LEFEBVRE, A. H. 1979. Ignition and flame quenching of flowing

heterogeneous fuel-air mixtures. Combustion and Flame, 35, 155-168.

BALLAL, D. R. & LEFEBVRE, A. H. 1981. Flame propagation in heterogeneous mixtures

of fuel droplets, fuel vapor and air. Symposium (International) on Combustion, 18,

321-328.

BOWEN P.J. SHIRVILL L.C. 1994. Combustion Hazards Posed by the Pressurized

Atomization of High-Flash Point Liquid, Journal of loss Prevention in the Process

Industries, 7(3), 233-241

DANTEC-DYNAMICS. Spray and Particle Characterization [Online]. Available:

http://www.dantecdynamics.com/spray-and-particle-characterization [Accessed 15/01

2013].

DITCH, B. & YU, H.-Z. 2004. Characterization of Water Mist Sprays Using a Phase-

Doppler-Particle-Analyzer and an Iso-Kinetic Sampling Probe. ASME 2004 Heat

Transfer/Fluids Engineering Summer Conference. Charlotte, North Carolina, USA,.

DSEAR 2002. The Dangerous Substances and Explosive Atmpsheres Regulations 2002.

London Uk: Crown.

EU 1994. Directive 94/9/EC Of the European Parliament and the Council. Official Journal of

the European Communities.

EU 1999. Directive 1999/92/EC of The European Parliament and of the Council. Official

Journal of the European Communities.

FAETH, G. M. 1991. Structure and atomization properties of dense turbulent sprays.

Symposium (International) on Combustion, 23, 1345-1352.

HAYASHI, S., OHTANI, T., IINUMA, K. & KUMAGAI, S. 1981. Limiting factor of flame

propagation in low-volatility fuel clouds. Symposium (International) on Combustion,

18, 361-367.

LA-VISION. Spray Applications [Online]. Available:

http://www.lavision.de/en/applications/sprays.php [Accessed 15/01 2013].

MARAGKOS, A. 2002. Combustion hazard quantification of accidental releases of high-

flashpoint liquid fuels. PhD Cardiff University

MARAGKOS A., BOWEN P.J. 2002‚ Combustion hazards due to impingement of

pressurised releases of high flashpoint liquid fuels, Proceedings of the Combustion

Institute, 29 (1) 305-311

MIESSE, C. C. 1955. Correlation of Experimental Data on the Disintegration of Liquid Jets.

Industrial & Engineering Chemistry, 47, 1690-1701.

TNO 2005. Methods for the calculation of physical effects – due to releases of hazardous

materials (liquids and gases) – ‘Yellow Book'. The Hague.

TYREE, C. & ALLEN, J. 2004. Diffusional Particle Loss Upstream of Isokinetic Sampling

Inlets. Aerosol Science and Technology, 38, 1019-1026.

YOSHIDA, H., YAMASHITA, K., MASUDA, H. & IINOYA, K. 1978. EFFECT OF

PROBE DIAMETER ON ISOKINETIC SAMPLING ERRORS. Journal of Chemical

Engineering of Japan, 11, 48-52.

55

APPENDIX A: MEASURING PRINCIPLES OF A PDA

SYSTEM

The PDA technique is an extension of laser Doppler anemometry and is based upon the phase

Doppler principles. Two or more detectors collect the light scattered by single particles

passing through the measurement volume.

The measurement point (control volume) is defined by the intersection of monochromatic,

coherent, linearly polarised, collimated laser beams. The intersecting laser beams interfere

with each other creating interference fringes. The control volume and interference fringes are

shown schematically in Figure A1.

Figure A1: Schematic of control volume formed by the transmitting optics

When a droplet passes through the control volume, the droplet scatters the light with a

Doppler frequency related to the velocity of the particle. The light can be scattered in a

number of modes. Figure A2 demonstrates the typical locations of the first three light

scattering modes for a water droplet in air.

56

Figure A2: The first three light scattering modes for a water droplet in air

In terms of the Fringe model, the frequency of light intensity pattern received at the photo

detectors is directly proportional to the speed with which the droplet traverses the

interference fringes of the control volume. The droplet velocity can therefore be calculated

from the Doppler frequency. The relationship between droplet velocity and Doppler

frequency is given in Equation A.1 where λ is the wavelength of light.

𝑈 = 𝜆𝑓𝑑

2sin (𝜃

2)

(A.1)

A system such as the one described can measure velocity in only one dimension. The addition

of an extra pair of beams intersecting at the control volume allows a second component of

velocity to be measured.

In order to measure particle diameters the use of two photo detectors is needed. As illustrated

in Figure A.3 a particle traversing the control volume scatters light which is collected by a

pair of photo detectors located in the PDA receiving optics. Both photo detectors receive the

same signal – a burst with the same Doppler frequency but with a slight phase difference due

to the different optical lengths travelled by the light. The phase difference (Φ) between the

signals received by the two photo detectors is directly proportional to the particle diameter

(dp). This is demonstrated in Figure A4.

57

Figure A3: Light scattered via reflection mode by droplet traversing through PDA control volume

(a) (b) (c)

Figure A4: (a) Light intensity signal received at photo detectors; (b) The two photo detectors receive the

same signal but with a slight phase difference; (c) Phase difference can be used to calculate droplet

diameter.

For reflection as the dominant light scattering mode, the phase difference between adjacent

photo detectors is given by Equation A.2. For dominant first order refraction the phase

difference is given by Equation A.3. The relative refractive index (nrel) can be calculated via

Equation A.4.

𝛷 = 2𝜋𝑑𝑝𝑠𝑖𝑛𝜃𝑠𝑖𝑛𝜓

𝜆√2(1−𝑐𝑜𝑠𝜃𝑐𝑜𝑠𝜓𝑐𝑜𝑠𝜑)(A.2)

𝛷 = −2𝜋𝑑𝑝𝑛𝑟𝑒𝑙𝑠𝑖𝑛𝜃𝑠𝑖𝑛𝜓

𝜆√2(1+𝑐𝑜𝑠𝜃𝑐𝑜𝑠𝜓𝑐𝑜𝑠𝜑)(1+𝑛𝑟𝑒𝑙2 −𝑛𝑟𝑒𝑙√2(1+𝑐𝑜𝑠𝜃𝑐𝑜𝑠𝜓𝑐𝑜𝑠𝜑)

(A.3)

58

𝑛𝑟𝑒𝑙 = 𝑛𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒

𝑛𝑚𝑒𝑑𝑖𝑢𝑚(A.4)

A PDA system with only two photo detectors can be used to measure particles with a

diameter corresponding to a phase shift of up to 360 degrees or 2π. A two detector system

suffers from what is termed the “2π ambiguity” – a consequence of the fact that because

phase is a multiple of 2π, a two detector system cannot distinguish between the phase

differences produced by very large and very small droplets.

The solution employed in PDA systems is to use three photo detectors grouped into two pairs

allowing the phase differences between both pairs to be cross-checked. The close pair

measure large particle size ranges at a low resolution while the distant pair measure smaller

size ranges at a greater resolution. The phase differences of both pairs will then align along

the correct droplet diameter as shown. In this three photo detector set-up cross-checking of

the two pairs of phase differences will both extend the measurable particle size ranges and

eliminate the 2π ambiguity.

Another useful outcome of this approach is the ability to perform validation checks on the

phase differences measured. For a perfectly spherical droplet both pairs of phase differences

will indicate exactly the same droplet diameter. The particle sphericity validation check, as it

is called, is one means of assessing the quality of a PDA set-up. Throughout this study a

sphericity error of up to 20% was deemed acceptable. Droplets with a sphericity error larger

than this were rejected by the processor and did not influence results.

59

APPENDIX B: RAW PDA DATA FOR JET-A1

JET-A1 5 Bar(g) Test case

Figure B1: JET-A1-5bar- y=300- x=000mm Figure B2: JET-A1-5bar- y=300- x=045mm

60


61


62


63

Figure B9: JET-A1-5bar- y=900- x=080mm

64



65


66


67


68


69



70


71


72


73


74



75


76


77


78


79

APPENDIX C: RAW PDA DATA FOR HYDRAULIC OIL

HYDRAULIC OIL 5 Bar(g) Test case

Figure C1: Hydraulic Oil-5bar- y=300- x=000mm Figure C2: Hydraulic Oil - y=300- x=012mm

80

Figure C3: Hydraulic Oil -5bar- y=600- x=000mm Figure C4: Hydraulic Oil -5bar- y=600- x=018mm

81

Figure C5: Hydraulic Oil -5bar- y=900- x=000mm Figure C6: Hydraulic Oil 5bar- y=900- x=020mm

82



83


84

Figure C11: Hydraulic Oil 10bar- y=900- x=000mm Figure C12: Hydraulic Oil -10bar- y=900- x=025mm

85



86

Figure C15: Hydraulic Oil -15bar- y=600- x=000mm

Figure C16: Hydraulic Oil -15bar- y=600- x=020mm

87


88



89


90


91

APPENDIX D: RAW PDA DATA FOR LFO AT AMBIENT CONDITIONS

LFO AMBIENT 5 Bar(g) Test case

Figure D1: LFO Ambient-5bar- y=300- x=000mm Figure D2: LFO Ambient - y=300- x=010mm

92

Figure D3: LFO Ambient -5bar- y=600- x=000mm Figure D4: LFO Ambient -5bar- y=600- x=012mm

93

Figure D5: LFO Ambient -5bar- y=900- x=000mm Figure D6: LFO Ambient- 5bar- y=900- x=015mm

94


Figure D7 LFO Ambient -10bar- y=300- x=000mm Figure D8: LFO Ambient -10bar- y=300- x=010mm

95


96

Figure D11: LFO Ambient- 10bar- y=900- x=000mm Figure D12: LFO Ambient -10bar- y=900- x=022mm

97



98


99


100



101


102


103

APPENDIX E: RAW PDA DATA FOR LFO HEATED 70 O

C

LFO HEATED 70 O

C 5 Bar(g) Test case

Figure E1: LFO Heated 70 oC-5bar- y=300- x=000mm Figure E2: LFO Heated 70

oC - y=300- x=010mm

104

Figure E3: LFO Heated 70 oC -5bar- y=600- x=000mm Figure E4: LFO Heated 70

oC -5bar- y=600- x=015mm

105


oC - 5bar- y=900- x=025mm

106

LFO HEATED 70 O



oC -10bar- y=300- x=010mm

107


oC -10bar- y=600- x=015mm

108

Figure E11: LFO Heated 70 oC - 10bar- y=900- x=000mm Figure E12: LFO Heated 70

oC -10bar- y=900- x=035mm

109

Figure E13: LFO Heated 70 oC -10bar- y=900- x=045mm

110

LFO HEATED 70 O


Figure E14: LFO Ambient -15bar- y=300- x=000mm Figure E15: LFO Heated 70 oC -15bar- y=300- x=010mm

111


oC -15bar- y=600- x=015mm

112


oC -15bar- y=900- x=000mm

113


oC -15bar- y=900- x=035mm

114

LFO HEATED 70 O



oC -20bar- y=300- x=015mm

115


oC -20bar- y=600- x=000mm

116


oC -20bar- y=900- x=000mm

117


oC -20bar- y=900- x=035mm

118

APPENDIX F: RAW PDA DATA FOR HIGH PRESSURE HYDRAULIC OIL


Figure F1: Hydraulic Oil-30bar- y=600- x=000mm Figure F2: Hydraulic Oil -30bar- y=600- x=010mm

119


Figure F3: Hydraulic Oil -70bar- y=600- x=000mm Figure F4: Hydraulic Oil -70bar- y=600- x=015mm

120



121



122


123

Figure F11: Hydraulic Oil -150bar- y=900- x=035mm

Published by the Health & Safety Executive 12/17


RR1110

www.hse.gov.uk

Many types of industrial equipment can potentially produce an explosive oil mist if a fault develops. However, information on the conditions in which a mist can be ignited and continue to burn is limited. To help address this, HSE and 14 industry sponsors co-funded a Joint Industry Project (JIP) on oil mist formation and ignition.

This report, produced for the JIP, describes the second phase of experimental tests to examine the ignition of mists produced by small leaks of pressurised, combustible fluids.

The size, concentration and movement of droplets were examined. The results differed significantly from those predicted by simple mist formation theories. This appeared to be the result of fundamental fluid behaviour. For all the test conditions, the droplets in the core of the spray were larger and had higher velocities than those closer to the edge. With viscous fluids the core included long ligaments, though these were difficult to measure.

These results show that simple spray models are not always appropriate for assessing the ignitability of oil mists from pressurised leaks.

This report and the work it describes were funded through a Joint Industry Project. Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy or the views of the Joint Industry Project sponsors.

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