A parametric study of the effect of fractal-grid generated turbulence on the structure of premixed flamesThomas Sponfeldner, S. Henkel, N. Soulopoulos, F. Beyrau,Y. Hardalupas, A.M.K.P. Taylor, J.C. Vassilicos

1st UK-Japan Bilateral Workshop onTurbulent flows generated in fractal ways

London, 29th March 2011

2

Outline

Motivation

Experiment

Results and Discussion Mean reaction progress variable

Turbulent burning velocity

Future work

Summary

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Motivation

What we have learned from the initial study Flames in fractal-grid generated turbulence show

different burning velocities to flames in regular-gridgenerated turbulence

Parametric study to reveal the influence of different design parameters (blockage ratio, bar thickness ratio, fractal dimension,...) on the turbulent burning velocity

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MotivationAim: Change u’ and investigate the effect on the flame

Bar thickness ratio

Also: blockage ratio

)1/(1

1-N

0t

N

ttR

2TA

Physics of Fluids 22(7), 075101 (2010)N. Mazellier, J.C. Vassilicos

Downstream development of

centreline turbulence intensity

tR

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MotivationAim: Change u’ and investigate the effect on the flame

Downstream development of

centreline velocity fluctuations

Design parametersFG1 FG2 FG3

(%) 34 34 37

Rt (-) 0.56 0.43 0.43

tR

6

Experiment

Conditioned Particle Image

Velocimetry (CPIV)

chemically inert Al2O3

seeding particles

Experimental setup and measurement technique

Square duct burner

duct width 62 mm

ubulk = 4.1 m/s

f = 0.7, 0.8, 0.9

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Experiment

Optics Express 15, 15444 (2007)S. Pfadler, F. Beyrau and A. Leipertz

Heat release of combustion leads to steep density drop at flame front

Particles number density decreases accordingly

Can be utilised to identify position of flame front

Idea: Conditioned PIV

unburntregion

burntregion

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Dimensionless temperature

Averaging over instantaneous images yields mean reaction progress variable

unburnt regions

burnt regions

Experiment

Optics Express 15, 15444 (2007)S. Pfadler, F. Beyrau and A. Leipertz

Reaction progress variable

0c

ub

u

TTTTc

1c

c = Probability to find burnt gasc = 0

c = 1

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st is effective propagation

velocity of premixed flames in

turbulent flow field

Usually estimated as a function

of laminar burning velocity sl and

velocity fluctuations

High values of st yield more

compact flames

ExperimentTurbulent burning velocity st (reminder)

sint us

n

suC

ss

ll

t '1

10

Corrugation and wrinkling of the flame increases considerably for the fractal grids

Results and DiscussionPIV raw images

u’

RG FG1 FG2 FG3

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Flame angles for fractal grids considerably larger compared to regular grids (st increases with increasing velocity fluctuations)

Results and DiscussionMean reaction progress variable fields (f = 0.7)

submitted to: European Combustion Meeting, Cardiff, UK, (2011)T. Sponfeldner, S. Henkel, N. Soulopoulos, F. Beyrau, Y. Hardalupas, A.M.K.P. Taylor, J.C. Vassilicos

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Comparison with correlations for flames in regular-grid generated turbulence for the same amount of velocity fluctuations, u’, as produced by the fractal grids

Correlations do not reproduce

experimental data

Results and DiscussionTurbulent burning velocity

n

suC

ss

ll

t '1

submitted to: European Combustion Meeting, Cardiff, UK, (2011)T. Sponfeldner, S. Henkel, N. Soulopoulos, F. Beyrau, Y. Hardalupas, A.M.K.P. Taylor, J.C. Vassilicos

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For a small increase in velocity

fluctuations, FG2 shows a

significantly larger turbulent

burning velocity than FG1

This is not the case for FG3 !

The three flames seem to have

a different u’ dependence

Results and DiscussionTurbulent burning velocity

The turbulent burning velocity does not only depend on the velocity fluctuations of the flow !

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Future work

Idea:

Design a regular square grid which produces the same velocity fluctuations as a fractal square grid

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Summary

Investigation of three fractal square grids and one regular square grid

Considerable higher wrinkling and corrugation for flames in fractal-grid generated turbulence

Flame angle and turbulent burning velocity increase with increasing velocity fluctuations of the flow

Correlations for the turbulent burning velocity based on the velocity fluctuations of the flow do not reproduce experimental data for the three different fractal grids

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Summary

Questions?

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Results and DiscussionTurbulent burning velocity (detailed results)

FG1 FG2 FG3

u’ (m/s) 0.46 0.47 0.50

f = 0.7 st (m/s) 1.00 1.21 1.27

f = 0.8 st (m/s) 1.62 1.85 1.91

f = 0.9 st (m/s) 1.73 2.16 2.22

Liu

Guelder

Zimont

5.0

l

25.0t

l

t 'Re6.01

su

ss

4.0

l

44.0t

l

t 'Re435.01

su

ss

5.0

l

25.0t

25.0

l

t 'RePr52.0

su

ss

sl values taken from: Proc. Combust. Inst. 29 (2002)G. Rozenchan, D.L. Zhu, C.K. Law, S.D. Tse

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Flat velocity profile for regular square grid Higher central velocities for fractal square grids

Results and DiscussionTransverse velocity profiles (200 mm downstream)

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Experimental setup and measurement technique

Optics Express 15, 15444 (2007)S. Pfadler, F. Beyrau and A. Leipertz

Heat release of combustion leads to steep density drop at flame front

Particles number density decreases accordingly

Can be utilised to identify position of flame front(shown as white line)

Conditioned PIV

burntregion

unburntregion

Reactionprogress