Turbulent Nonpremixed Combustion

1

TurbulentNonpremixed -1

School of Aerospace Engineering

Copyright © 2004-2005, 2020-2021 by Jerry M. Seitzman.

All rights reserved.

AE/ME 6766 Combustion

Introduction to Turbulent Combustion:

Turbulent Nonpremixed Flames

Jerry Seitzman

Methane Flame

0

0.05

0.1

0.15

0.2

0 0.1 0.2 0.3

Distance (cm)

Mo

le F

racti

on

0

500

1000

1500

2000

2500

Te

mp

era

ture

(K

)

CH4

H2O

HCO x 1000

Temperature






Overview

• The specific goals of this section:

1. Describe turbulent non-premixed jet flames based on experimental results

– flame length

– flame structure

– mixture fraction relations and finite-rate chemistry effects, including local extinction

2. Introduce non-premixed flamelet modeling

3. Present flame index parameter for differentiating non-premixed and premixed combustion in non-premixed combustors

2






Nonpremixed Combustion

• Most common form employed in high power and industrial (especially liquid fueled) combustors

– reactants entering combustor initially nonpremixed

– simpler (no requirement to prevaporize/mix reactants)

– easier to control flame stability

Process Heating (flare) generon.com

Yuan et al., Intl J Spray & Comb Dynamics 2018

(diesel)

energy.gov






Nonpremixed Combustion

• Engineering issues

– flame shape, size, location (combustor length,…)

– flame stability (liftoff, blowout, …)

– heat transfer (radiation-soot, distance to walls,…)

– pollutant emissions (NOx, UCH/CO,…)

Process Heating


(flare) generon.com

(diesel)

energy.gov

3






Nonpremixed Combustion• Combustion science issues

– local mixing (fuel, oxidizer, hot products,…)

• scalar dissipation, turbulent transport, partial premixing, differential diffusion

– highly nonlinear dependence of reaction rates on T, Yi,…

– density/buoyancy, flow instabilities

Process Heating


(flare) generon.com

(diesel)

energy.gov






Common (Jet) Burner Configurations

• Typical to use fuel jets flowing into

– quiescent oxidizer (air)

“jet flame”

– coaxial oxidizer jet

– swirled coaxial oxidizer

• Many other practical configurations exist

– piloted jets, impinging jets, jets-in-crossflow,…

fuel

fuelair air

fuelair air

4






Turbulent Nonpremixed Jet Flames

from Hottel and Hawthorne, 1949

• Based on visual and photographic observations

– transition

from

laminar

to turb.

– change

in Lf

growth






• Measurements of maximum (average) height of

flame for different tube diameters and exit

velocities

Turbulent Jet Flame Length

Lf

Qe=ueRe2

Lf

de/2=Re Re=2

Re=1

(in.)

(cm3/s)

de (in.)

ue

Lf, ded1

~2d1

~3d1

5






Jet Flame Length Relations

• Empirical relations available for flame lengths of

turbulent jet flames (Kalghati)

– flame Froude number

21

*

e

stoich

ef

f

dLL

stoich

eef

fD

duL

2

flameefn ,

523

507.01

5.13512

52

*

f

f

f

f

Fr

FrFr

Fr

L

2141

23

TTTgd

fuFr

fee

stoichef

momentum dominated

buoyancy effected

compare to laminar case(X.13)

(X.13a)

(X.13b)

no longer dependent on convection vs. diffusion velocities(ue vs. D/de )






Nonpremixed Jet Flame Structure• Instantaneous flame structure

- laser sheet imaging

• Reaction zone relatively thin and highly wrinkled

• Large vortical structures increase interfacialarea between fuel and oxidizer

– bring reactantscloser together faster

diffusion

– structures grow (larger downstream)

from Ö. Gülder

FuelAir

6






Mixture Fraction Relations• Recall for laminar nonpremixed jet flames we could

relate local scalar field (e.g., T, Yi) to local f

• H2-airexampleassuming fast chem.and Lei=1

– can alsoget resultsif no reactions

Mixing Lines

(no reaction)

T






• Let’s examine what happens if

we take many measurements at a

single point in the flame

• Each time local flame conditions

change

• Next examples based on

– laser Raman scattering for f

– laser Rayleigh scattering for T

Scalar Measurements in Jet Flame

7






• Nonpremixed hydrogen jet flames

– look at two ue cases low, high urms

– turb.large f fluctuations (near flame)

H2-Air Jet Flame Measurements

Magre and Dibble, Comb.

and Flame 73, 195 (1988)

high Da

(ue=U)

lower Da

(ue=3U)

results near equil.

relationship

finite rate (slow)

chemistry effects

each point represents instantaneous measurement at same point in the flowfield

chemoDa

rmseoud






CH4-Air Jet Flame Measurements

Masri et al., Combustion and Flame 73, 261 (1988)

x/D=10

• Compare to equil. & stag. flame calc. (depends on strain)

usually nearfast chem.

low straincalcs.

slow chemistry, often get

incomplete combustion,

even localextinction

x/D=20

change ofx scale

a=1 s-1

320 s-1

ue=48m/s, D=7.2 mm

CH4

chemistry

slower

than H2

same conditions, new location

mixing line

8






Local Flame Extinction

• Can observe localized breaks in flame front

– localized extinction

– can grow: larger region of extinction

– can shrink: ignition of unburned (mixed) reactants and propagation to close “holes”






Nonpremixed Flamelet Approach• Repeating general

turbulent combustion regime diagram

• For Ka ~< 1, all turbulent scales slower than chemistry

– region near reaction zone retains laminar-like structure

Da

oRe

0.1

1

10

102

103

102 103 104101 105

Ka=1

Ka=100

Ka=0.01

flamelets

broken flamelets

distributed reaction zones

21Reo

DaKa 1 kDaKa

chemoDa

• Can characterize turbulent flame as ensemble of laminar flamelets

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Transport Equation for f• Can write conservation equation for mixture fraction

• Common engineering approach to solve for time-averaged conditions

0~~

fDfufu

t

dttxft

f0

t,

1lim average time

ff average Favre~

density weighted

ffff ~

n fluctuatio Favre

(X.15)

0

fDfuf

t

(X.14)






Transport Equation for f

• Need models for last two correlated terms (“closure”)

– for turbulent transport, one simple approach isgradient transport model

– often assume molecular diffusion term negligible compared to turbulent transport

fufu

~~

ffuT

~

T = eddy diffusivitye.g., from k- model

0~~

fDfufu

turb. transport diffusion

desired solutions

from mass/mom.(coupled)

10






Nonpremixed Flamelet Modeling

• Need approach to relate f to T, Yi

– generally assume no external heat losses

– could use chemical equil., but chem. often not that fast

• Flamelet models

– use local scalar dissipation rate, , e.g.

– rely on strained non-premixed laminar flameletcalculations, = (a); a = strain rate

2

2~ fD

fYY F

ii

)(

ff

YY

F

i

i

iiii

mYDYuYt

i

F

i

F

i mf

YfDfDfuf

tf

Y

2

22

0






Nonpremixed Flamelet Modeling

• Flamelet equation

– to get average chemical species formation/destruction

need average flamelet equation

– one approach: probability distributions (PDFs) of f

i

F

i mf

YfD

2

22

dfdfPf

Ym

F

i

i ,

~

2

12

21

0 0

from laminar experiments or

calculations(on-line or library)

Favre-avg. Joint PDF

shape=?

11






Non-Premixed Combustors

• Can get premixed and non-premixed combustion in same system/combustor

• Lifted (gaseous) jet flames– leads to partial premixing

• helps stabilize flame

• Reignition of local extinction regions• Liquid-fueled combustors

– fast vaporization can produce regions of premixed combustion if vaporized fuel mixes with oxidizer before reacting

• How to differentiate?– alignment of fuel and

oxidizer gradients atflame

airair fuel

f=fstoich

premixed

non-premixed

FO F

O

premixed non-premixedmax,max,

max,max,

OF

OF

YY

YY

Takeno Flame Index

= 1 = 1

Turbulent Nonpremixed Combustion

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