Combustion and Flame, Vol.126, No.4, 1764-1779, 2001
Relationship between turbulent scalar flux and conditional dilatation in premixed flames with complex chemistry
The behavior of the turbulent scalar flux, <(uc " )over tilde>, in hydrogen- and methane-air flames has been studied by using two-dimensional, direct numerical simulation databases involving a multi-step chemical mechanism. The relationship between local dilatation and the scalar flux has been investigated. As observed previously in simulations with a single irreversible chemical reaction, the predominant effect of turbulence is on the conditional diffusion of the scalar, c. The peak conditional dilatation occurs in fluid mixtures with low values of progress variable at all locations inside, the flame brush. This yielded a negative correlation between dilatation and the scalar fluctuations. This negative correlation dictates the scalar flux to be of gradient type. The inter-relationship between the correlation and the scalar flux was observed to be true in all the data sets considered here. Calculation of unstrained and strained laminar flame indicate that the peak dilatation occurs in mixtures with low values of the progress variable for hydrogen flames with equivalence ratio of similar to0.5 and above. Thus, these flames in statistically stationary and one-dimensional situation may never exhibit counter-gradient flux for the turbulent scalar flux. However, the above flames in statistically multi-dimensional situation may exhibit counter-gradient flux.