DNS of partially premixed flame propagating in a turbulent rotating flow

Abstract

The propagation of non-premixed and premixed flames in a turbulent rotating flow is studied. A synthetic Direct Numerical Simulation (DNS) model problem is proposed in which a turbulent columnar vortex interacts with an initially planar and stoichiometric premixed flame. Various cases are obtained by varying the characteristic circumferential velocity of the vortex and the incoming fuel and air distribution. Turbulent fully premixed stoichiometric or lean cases and non-premixed cases are considered. Flash-back of the reaction zone at a velocity larger than the laminar burning velocity is attributed to turbulence and to the generation of additional vorticity by the flame density jump, a mechanism previously reported in the literature for a tubular vortex interacting with a laminar premixed flame. The impact of recirculating burnt gases is isolated from DNS and the volume of burnt gases embedded in the turbulent rotating flow is found to play a non-negligeable role in flash-back, specifically for the non-premixed fuel and air distribution. Turbulent non-premixed mixture is observed to flash-back at smaller velocities than the fully premixed stoichiometric and lean ones. This is explained by the confined flow deviation upstream of the flame that is reduced with non-premixed injection, due to the non-uniform character of the density field in burnt gases of non-premixed flames. The presence of stoichiometric surfaces does not always favor flash-back, which appears to be more controlled by the penetration of recirculating low density gases into the rotating flow. The flash-back velocity of the turbulent non-premixed mixture is found to scale with the maximum circumferential flow velocity. The leading reaction zone progressing inside the rotating turbulent flow is composed of an upstream rich partially premixed flame, followed by a spinning partially premixed ring that surrounds a weakly burning diffusion flame, which separates fuel, left behind the rich flame, of oxidizer diluted by burnt products.