Laminar to turbulent transition in premixed and nonpremixed jet flames

Abstract

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. 1 Doctoral student 2 Professor, AIAA Senior Member 3 Assistant Professor, AIAA Senior Member Copyright © 1998 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. Contribution of these two effects in enhancing the stability of premixed flames is discussed. In the premixed flame analysis, the planar laminar flame speed and Markstein length scale are identified to be the additional parameters affecting transition. These parameters influence both the flame length and the radial position of the flame, and hence the location of flame tip where significant thermal expansion occurs. A detailed study on the effect of planar laminar flame speed and Markstein number on transition length is presented. It is shown that premixed flame jet develops an instability at low flame propagation speeds triggered by the interaction between the flow field and the nonlinear flame propagation. The stabilizing effects of Markstein diffusivity are presented. It is noted that therrnodiffusive instabilities represented by negative Markstein numbers are not considered, but can be significant in practical flames.