Local scaler flame properties of freely propagating premixed turbulent flames at various Lewis numbers

Local scalar flame properties of freely propagating turbulent premixed flam es including the flame curvature h, and local displacement speed relative t o the fresh gases S-d(u), have been measured simultaneously for methane, pr opane, and hydrogen/air flames at Lewis numbers varying from 0.33 to 1.4 an d ratios of rms turbulent velocity to unstretched laminar burning velocity u'/S-L, 0(u), varying from 0 to 3.1. Three different mixtures were separate ly spark-ignited in a vertical wind tunnel. The expanding flame freely prop agated in a grid-generated decaying turbulent flow. An advanced field-imagi ng technique based on high-speed laser tomography measured the temporal evo lution of local flame properties. Local flame curvature and local displacem ent speed were calculated from flame-front contours. Curvature probability density functions (pdfs) were negatively skewed, especially for nonunity Le wis numbers, and displacement speed distributions underlined the influence of local stretch and thermodiffusive effects on flame speed variations. The temporal evolution of the mean flamelet radius of curvature converges towa rds half of the integral length scale measured in the cold how. Flame respo nse in terms of displacement speed to curvature is found to be statisticall y dependent, and a linear relationship is observed. For propane/air flames, the displacement speed can be assumed to be independent of local flame cur vature at each stage of flame propagation, whereas a very strong increase o f displacement speed with positive curvatures can be observed along the wri nkled flame contour for hydrogen/air flames. (C) 2000 by The Combustion Ins titute.