Curvature effects on flame kernels in a turbulent environment

Abstract

This paper presents results from the direct numerical simulation (DNS) of flame kernels in various turbulent environments. The flames are fully premixed and propagate through a field of decaying isotropic turbulence. The DNS code solves the fully compressible reacting flow equations using high-order explicit finite differences in space and a third-order explicit Runge-Kutta method in time. Results have been obtained for three-dimensional time evolution of initially laminar flame kernels. The normalized turbulence intensity u ′/S L was varied from moderately low values that produce only minor flame-wrinkling effects to larger values that produce severe wrinkling. When the turbulence intensity is large in comparison with the laminar flame speed, holes begin to appear in the flame surface and evidence of local flame breakaway is observed. Flame kernels have significant mean curvature, and it is the curvature and other geometrical properties of the flame surface that form the main focus of the present work. Simulation results are presented for a number of different turbulence Reynolds numbers corresponding to computational grid sizes with up to 384 3 points, and a detailed analysis provides statistical data on flame curvature and local shape effects. Turbulence intensity is found to have a major influence on the extent and character of the observed flame wrinkling. In particular, effects due to the mean curvature of the kernel are shown to diminish strongly with increasing turbulence intensity.