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A parametric study of the effect of fractal-grid generated turbulence on the structure of premixed flames Thomas Sponfeldner, S. Henkel, N. Soulopoulos, F. Beyrau, Y. Hardalupas, A.M.K.P. Taylor, J.C. Vassilicos 1 st UK-Japan Bilateral Workshop on Turbulent flows generated in fractal ways London, 29 th March 2011

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2 Outline Motivation Experiment Results and Discussion Mean reaction progress variable Turbulent burning velocity Future work Summary

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3 Motivation What we have learned from the initial study Flames in fractal-grid generated turbulence show different burning velocities to flames in regular-grid generated 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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4 Motivation Aim: Change u’ and investigate the effect on the flame Bar thickness ratio Also: blockage ratio Physics of Fluids 22(7), 075101 (2010) N. Mazellier, J.C. Vassilicos Downstream development of centreline turbulence intensity

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5 Motivation Aim: Change u’ and investigate the effect on the flame Downstream development of centreline velocity fluctuations Design parameters FG1FG2FG3 (%) 34 37 R t (-)0.560.43

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6 Experiment Conditioned Particle Image Velocimetry (CPIV) chemically inert Al 2 O 3 seeding particles Experimental setup and measurement technique Square duct burner duct width 62 mm u bulk = 4.1 m/s = 0.7, 0.8, 0.9

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7 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 unburnt region burnt region

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8 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 = Probability to find burnt gas c = 0 c = 1

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9 s t is effective propagation velocity of premixed flames in turbulent flow field Usually estimated as a function of laminar burning velocity s l and velocity fluctuations High values of s t yield more compact flames Experiment Turbulent burning velocity s t (reminder)

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10 Corrugation and wrinkling of the flame increases considerably for the fractal grids Results and Discussion PIV raw images u’u’ RG FG1 FG2FG3

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11 Flame angles for fractal grids considerably larger compared to regular grids (s t increases with increasing velocity fluctuations) Results and Discussion Mean reaction progress variable fields ( = 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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12 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 Discussion Turbulent burning velocity 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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13 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 Discussion Turbulent burning velocity The turbulent burning velocity does not only depend on the velocity fluctuations of the flow !

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14 Future work Idea: Design a regular square grid which produces the same velocity fluctuations as a fractal square grid

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15 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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16 Summary Questions?

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17 Results and Discussion Turbulent burning velocity (detailed results) FG1FG2FG3 u’ (m/s)0.460.470.50 = 0.7 s t (m/s)1.001.211.27 = 0.8 s t (m/s)1.621.851.91 = 0.9 s t (m/s)1.732.162.22 Liu Guelder Zimont s l values taken from: Proc. Combust. Inst. 29 (2002) G. Rozenchan, D.L. Zhu, C.K. Law, S.D. Tse

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18 Flat velocity profile for regular square grid Higher central velocities for fractal square grids Results and Discussion Transverse velocity profiles (200 mm downstream)

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19 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 burnt region unburnt region Reaction progress

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