Variable density effects in premixed turbulent flames

Earlier calculations of planar, oblique and normal combustion waves based on the Bray-Moss model of premixed turbulent flows have been improved and the theory exploited further. Variable density effects on the Prandtl-Kolmogoroff model for the eddy transport coefficient and on the scalar dissipation are shown to bring the predictions of the theory in more satisfactory agreement with experimental results for the orientation of the combustion wave in the case of strong interaction, the case most amenable to comparison. The theory is used to compare the distributions through the reaction zone of several statistical quantities as given by conventional and Favre averaging; larger differences are found in some quantities with the implication that different modelling to achieve closure may be required for the two means of averaging when the heat release and therefore variable density effects are significant. In particular it is found that the triple correlation terms, which enter the momentum and scalar fluxes according to conventional averaging, are the principal source of the differences in the fluxes as given by the two means of averaging.