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Low-Swirl Flame Stabilization Methodfor Lean Premixed Turbulent Flames

and Its Adaptation toHeating and Power Equipment

Robert K. Cheng

Senior Scientist

Leader, Combustion Technologies Group

Environmental Energy Technologies Division

Lawrence Berkeley National Laboratory

Presentation at ABMA Mid-Winter Meeting Jan. 18, 2004


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Acknowledgement

Sponsors

DOE-Basic Energy Sciences, Chemical Sciences

DOE-Basic Energy Sciences, Laboratory Technology Research

California Institute of Energy Efficiency/SoCalGas

DOE-EERE, Office of Industrial Technology

DOE-EERE, Distributed Energy Resources

Collaborators

D. Yegian, D. Littlejohn, G. Hubbard, K. Hom & I. Shepherd (LBNL)

J. Rafter & C. Taylor (Maxon), K. O. Smith (Solar Turbines)

C. Castildini (CMC Eng.), C. Benson (TIAX),M. Miyasato, V. McDonell, R. Hack & G. S. Samuelsen (UC Irvine),

H. Rieher (Industrial Combustion)


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Modes of Gaseous Combustion

Excess fuel + Air

AirAir

NOx

Partially Premixed Flame

two reaction zones

Fuel +

Excess air

NOx

Lean Premixed Flamewave-like flame front

Fuel

Air Air

NOx

Diffusion Flame

controlled by mixing


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Turbulent Combustion as a

Fundamental Research Problem

Non-premixed (dif fusion) flames

Turbulent and molecular mixing control combustion rates,efficiency and pollutant formation

Reactions occur at stoichiometric contours passive toturbulence

Combustion products diffuse into fuel and oxidizer streams

Reaction rate models expressed in terms of speciesconcentrations

Premixed flames Self propagating flame front separate reactants from products

Flame front exhibits wave behavior and generates significantfeedback to turbulent field through the pressure field

Reaction rate models expressed in terms of flame speed

No unified theory due to differences in the predominantphysical processes of non-premixed and premixed flames


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LBNLs Basic Research Focuses on

Gaseous Premixed Turbulent Flames

Theoretical Interest

Turbulence intensity and sizes of eddies

control burning rate, power density and

flame stability Technological Interest

Reduction of NOx emissions through lean

combustion Programmatic objectives

Elucidate turbulence/flame interactions

processes

Build an experimental foundation to

advance combustion theories & models

Transfer scientific knowledge to practical

use


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LBNLs Basic Research Emphasizes

Combustion Fluid Mechanics

Approach: Laboratory investigations

and theoretical development toquantify flame turbulence

interactions

clean experiments to reveal andisolate various processes

systematic variation of combustion

and turbulence parameters

Goal: Support the development ofcomputational tools suitable for the

design of advanced combustion

systems


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Addressing Problems Relevant to

Lean Premixed Combustion SystemsIncomplete knowledge on flame behavior

fast burning, compact & intense flames turbulence effects on emissions

flame interaction with combustion chambers

Flame holders dictates performance restrict operating range (5:1 turn-down vs. 10:1 for

non-premixed systems)

impact fuel flexibility, costs, & durabilityFlame generated flow dynamics

noise and vibrations

flash-back and blow-out hazards


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Conventional Flame Holders

Pilot Flame

premixture premixture

products

Fla

me

fuel

V-gutter

premixture premixture

products

Fla

me

highly swirled

premixture

Bluff Body + Swirl

recirculating

products

Flame

Based on the theoretical principle of continuous ignition source providedby a pilot flame or in the hot recirculation zone behind the stabilizer

L Bl ff d Fl I t biliti


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Lean Blow-off and Flame Instabilities

Associated With Different Flame Holders

Are Barriers to Reaching Low-Emissions

NOx

Flame Temperature, air/fuel ratio

Pollutant

concentratio

ns

CO

Onset of flame

instability

Lean

blow-off

Flammability

limit


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Low-swirl Combustion Exploits

Aerodynamics to Overcome the

Barriers to Attaining Low-Emissions

Lean

blow-offof

conventionalburne

rs

Onsetofflameinstability

inconventionalburners

NOx

Flame Temperature,

Pollutantc

oncentration

s

CO

No flame oscillations prior to lean blow-off

almost at the theoretical flammabili ty limit

Flammability

limit

L i l C b i E l i


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Low-swirl Combustion Exploits

Aerodynamics to Overcome the

Barriers to Attaining Low-Emissions

NOx

Flame Temperature,

Pollutantc

oncentration

s

CO

No flame oscillations prior to lean blow-off

almost at the theoretical flammabili ty limit

Flammability

limit


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Premixed Flames Stabilized

by Low SwirlNovel concept discovered in 1991 at LBNL

Defies recirculation theory on f lame stabilization

Scientific Interest

Scientific background lacking for low-swirl flows

Challenging modeling problem

Excellent laboratory research tool

Technological Interest

Capability to support ultra-lean flames

Simple design

Patent awarded 1998


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Flame Holders Were Considered

Essential to Anchor

Lean Premixed Flames

Premixed flames requires a physical stabilizer so that it can anchor. This

flame is stabilized by a bluff body of about 1 cm diameter

Reactants


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Low-Swirl Eliminates the Need for a

Flame Holder

By introducing a very small amount of swirl air (swirl number S 0.6), this

video shows that the flame can self propagate without the bluff body


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Low-Swirl Flame Stabilization Exploits

Propagating Nature of Premixed Flames

Fuel/Air

mixture

Propagating against the divergent

flow, the flame settles where the

local velocity equals the flame speed

Small air jets swirl the perimeter of

the fuel/air mixture but leave the

center core flow undisturbed

Flow divergence (generated by low-swirl) above the burner tube is the

key element for flame stabilization

L Di ti Ch t i d


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Laser Diagnostics Characterized

Flame Stabilization Mechanism

Flow divergence provides a

much more stable mechanismfor lean flames than high swirl

flows or flame holders

Flame brush propagates atturbulent flame speed that

increases linearly with

turbulence intensity

flashback conditions predictable

Swirl intensity controls flame lift

off position r / d

x/d

-1 -0.5 0 0.5 1

0

0.5

1

1.5

2

2.5

1.0

0.8

0.6

0.4

0.2

0.1

0.0

-0.1

U / Uexit

_ _

c


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Scientific Studies

Using Jet-LSBs Exploiting LSBs capability to

supports premixed turbulent

flames under a wide range ofturbulence and mixture

conditions has helped to resolve

key scientific problems Investigate evolving turbulent

flame structures from low to

intense turbulence

Verify new theory on

classification of premixed

turbulent flames

Relate turbulent f lame speed to

combustion intensity

SWF29

SWF28

SWF26

Increas

eturbulence

These images of OH fluorescence

obtained by laser diagnostics are

the data for studying combustion

intensities and flame structures


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First Technology

Transfer ProjectSupported by

DOE-LTR (1994-1997)

CRADA with Teledyne-Laars

Adaptation to pool heaters

Design and test LSBs that meetsthe operational requirements of15 KW to 100 KW units

Sizes similar to laboratory LSBs

Non-modulated systems, noturndown requirement

Issues

need simpler design requiringonly one flow supply (no jets)

firing sideways or downwards toattain > 85% efficient

stable inside chamber

cannot compromise on energyefficiency

cost must be lower than Alzetaburner ($100/per unit)


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Vane-Swirler Developed for LSB

New design fundamentally different than that of conventional vane-swirler Open center channel allows a portion of flow to bypass swirl vanes

Angled guide vanes induce swirling motion in annulus

Screen balances pressure drops between swirl and center channel

Patent awarded in 1999

Vane-swirler

Rh

R

Screen

Premixture

Exit tube

Top view of patentedvane-swirler


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Development of Vane-Swirler

Relied on Laser Diagnostics

-40 -30 -20 -10 0 10 20 30 40-3

-2

-1

0

1

2

3

4

5

r (mm, Rh = 20.5)

Velocity

(m/s)

Jet Swirler

Vane SwirlerU (m/s) W (m/s)

Varied screens

blockage, vane

angle, and

inner tube

diameter

Measured

mean velocity

profiles and

compare withprofiles of jet-

swirler


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Vane-Swirler for LSB Can Be Made

From Simple and Low-Cost MaterialsVane-LSB produces

the same shape as Jet-

LSB

Lifted flame does not

transfer heat to burner

throat

Estimated fabricationcost for pool heaters

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