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

1. 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 1st UK-Japan Bilateral Workshop on Turbulent 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

3. Motivation • What we have learned from the initial study • Flames in fractal-grid generated turbulence showdifferent 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

4. Motivation Aim: Change u’ and investigate the effect on the flame • Bar thickness ratio • Also: blockage ratio • Downstream development of centreline turbulence intensity Physics of Fluids 22(7), 075101 (2010) N. Mazellier, J.C. Vassilicos

5. Motivation Aim: Change u’ and investigate the effect on the flame • Downstream development of centreline velocity fluctuations • Design parameters

6. Experiment Experimental setup and measurement technique • Square duct burner • duct width 62 mm • ubulk = 4.1 m/s • f = 0.7, 0.8, 0.9 • Conditioned Particle Image Velocimetry (CPIV) • chemically inert Al2O3 seeding particles

7. Experiment Idea: Conditioned PIV • 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 burnt region unburnt region • Optics Express 15, 15444 (2007) • S. Pfadler, F. Beyrau and A. Leipertz

8. Experiment Reaction progress variable • Dimensionless temperature • Averaging over instantaneous images yields mean reaction progress variable c = 1 unburnt regions burnt regions c = 0 = Probability to find burnt gas • Optics Express 15, 15444 (2007) • S. Pfadler, F. Beyrau and A. Leipertz

9. Experiment Turbulent burning velocity st (reminder) • stis effective propagation velocity of premixed flames in turbulent flow field • Usually estimated as a function of laminar burning velocity sland velocity fluctuations • High values of st yield more compact flames

10. Results and Discussion PIV raw images • Corrugation and wrinkling of the flame increases considerably for the fractal grids RG FG1 FG2 FG3 u’

11. Results and Discussion Mean reaction progress variable fields (f = 0.7) • Flame angles for fractal grids considerably larger compared to regular grids (stincreases with increasing velocity fluctuations) • 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

12. Results and Discussion Turbulent burning velocity • 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 • 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

13. Results and Discussion Turbulent burning velocity • 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 The turbulent burning velocity does not only depend on the velocity fluctuations of the flow !

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

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

16. Summary Questions?

17. Results and Discussion Turbulent burning velocity (detailed results) • Liu • Guelder • Zimont sl values taken from: Proc. Combust. Inst. 29 (2002) G. Rozenchan, D.L. Zhu, C.K. Law, S.D. Tse

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

19. Experimental setup and measurement technique Conditioned PIV • 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) burnt region Reaction progress unburnt region • Optics Express 15, 15444 (2007) • S. Pfadler, F. Beyrau and A. Leipertz