Scalar gradient and related quantities in turbulent premixed flames

Three-dimensional gradient measurements of a progress variable, c, in laminar and turbulent premixed flames are presented as rates of scalar dissipation, N, and scalar diffusion, D. A two-plane laser imaging technique is employed to obtain spatially resolved images of Rayleigh scattering in two planes instantaneously. Conditionally averaged quantities [alpha del c .del c\c = zeta] and [del .alpha del c\c = zeta] are measured in turbulent flames, and they show departure from the values in laminar flames. Local thickening: of the thermal flame thickness is evident, and the broadening effect is more severe at higher turbulence Reynolds numbers. joint pdfs of N and zeta indicate a global increase of broadened flame fronts as turbulence level increases. Pdfs of zeta show obvious bimodality with spikes at zeta = 0 and 1. The value of P(zeta) at zeta = 0.5 is about 0.1 for all cases studied. The ratio between the conditional and unconditional means of N seems to be similar for the turbulent flames studied. Scalar diffusion shows a more dominant peak in the reaction zone than in the preheat zone. The trend of the magnitude of D qualitatively follows that of IV with respect to the turbulence level. (C) 1997 by The Combustion Institute.