Scalar dissipation, diffusion and dilatation in turbulent H2-air premixed flames with complex chemistry

Characteristics of the scalar dissipation rate, N, of a progress variable, c, based on temperature in turbulent H-2-air premixed flames are studied via direct numerical simulation with complex chemical kinetics for a range of flow/flame conditions (Baum et al 1994 J. Fluid Mech. 2811). The flames are in the usually designated wrinkled-flamelet and well-stirred reactor regimes. The normalized conditional average, N-zeta(+) is observed to be higher than the corresponding planar laminar value because of strain thinning and the augmentation of laminar transport by turbulence within the flame front. Also, NI:varies strongly across the flame-brush when u ' /S-l is high. N has a log-normal distribution when u ' /S-l is small and has a long negative tail for cases where u'/S-l is large. In the flame with phi = 0.5, (N) over tilde (+)(zeta)/(N) over tilde (+) shows some sensitivity to P-zeta and the sensitivity seems to be weak in a phi = 0.35 flame. The effect of turbulence on (cb, I) is observed to be marginal. The conditional diffusion and the conditional dilatation, < del .u \ zeta >, peak on the unburnt side of the flame-front and are higher than the corresponding laminar flame values in all cases. The inter-relationship among the conditional dissipation, diffusion, dilatation and velocity is discussed. A model for u(zeta) obtained from the conditional dilatation is found not to perform as well as a linear model. The above results are limited, however, because, the flow field is two dimensional, hydrogen is used as the fuel, the range of dynamic length scales is small and the sample size is small.