Turbulence Production in Premixed Turbulent Flames

Abstract

—A second order closure theory developed earlier is used to study the processes influencing the turbulent velocity field in a premixed turbulent flame with degrees of heat release of practical interest. The flow field is chosen so that the time-averaged flame structure is one-dimensional and statistically stationary. Earlier work suggests that in the absence of turbulence production due to Reynolds stresses as is the case in a flame orthogonal to the oncoming reactants, the case we consider, dilatation resulting from heat release reduces the level of turbulence. In contrast it is shown here that with sufficient heat release turbulence increases on passage through the flame because of a buoyancy production mechanism arising from the self-induced, mean pressure gradient. This mechanism overwhelms the effects of dilatation at temperature ratios characteristic of combustion. The same buoyancy mechanism also causes counter-gradient diffusion as predicted in an earlier paper and as observed in recent experiments. Here adjustments are made to some aspects of the modelling required in the theory in order to achieve reasonable agreement with these new experimental results. Calculations are then carried out to establish the significance of the production of turbulence by this buoyancy mechanism at heat release rates characteristic of combusting flows. The physical interpretation of the present results and their importance in prediction methods for turbulent combustion involving premixed reactants are discussed.