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
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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
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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
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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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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