Laminar to turbulent transition in premixed and nonpremixed jet flames

A computational study of laminar to turbulent transition in premixed and nonpremixed jet flames is reported. The Navier-Stokes equations with the low Mach number approximation are solved using the fractional time stepping method. Both premixed and nonpremixed flames are modeled in the limit of infinite reaction rate. The transport equation for mixture fraction is solved in the nonpremixed flame calculations, while the premixed flame calculations are performed with the G-equation model. Dependence of molecular viscosity and diffusivity on temperature is taken into account. The governing equations are solved using a predictor-corrector approach giving second order accuracy in time. The spatial derivatives in the convective terms are obtained using the fourthorder accurate compact scheme to minimize the effects of numerical dissipation. Results obtained from the transient simulations of a cold jet, a premixed flame and a nonpremixed flame at jet Reynolds numbers of 1000 are compared to identify the differences in the transition characteristics of premixed and nonpremixed flames. It is shown that the heat release has a significant effect on transition. A premixed flame developing in product surroundings has much higher stability than a cold jet or a nonpremixed flame with the same heat release parameter. This is due to two effects: i) reduction in density of the surrounding gases and ii) increase in molecular viscosity of the surrounding gases due to higher temperature.