Direct numerical simulation of cellular structures in jet diffusion flames

Abstract

Three-dimensional, direct numerical simulations of cellular jet flames were performed to elucidate the nature of cellular structures in jet diffusion flames close to extinction. Such cellular structures were reported for flames near the extinction limit if the reactant Lewis numbers are below unity. The first part of this work consisted of extending an existing direct numerical simulation code for low Mach number reactive °ows. Detailed, multi-step chemical description and and detailed evaluation of the transport properties was added by coupling the code with the Chemkin libraries and a new scalable solver for the fully implicit integration of the sti® system of energy and species conservation equations. The extended numerical code was validated against numerical opposed-jet results and applied to the experimental investigation of an opposed-jet diffusion/edge flame. The detailed results obtained by direct numerical simulation enhanced the understanding of the experimentally observed phenomenon of a hysteretic by-stability of the di®usion and edge flames. The simulation of cellular structures in jet di®usion °ames was based on a recent experimental investigation of the formation of different cellular structures in weakly burning flames with varied jet and co-flow compositions and velocities. The fuel and oxidizer were hydrogen and oxygen, respectively, both diluted with carbon dioxide. Despite many di±culties inherent with the simulation of weakly burning °ames, the experimental ¯ndings could be reproduced numerically. The analysis of the cellular structures gave insight to the involved physical processes associated with the formation of the cellular flames in a non-premixed jet configuration. An additional investigation on the dependency of the resulting cellular structure of the jet diffusion flame on the initial velocity profile of the jet enhanced the general understanding of the investigated phenomenon. Furthermore, it revealed the proportional dependency of the number of cellular structures on the initial vorticity thickness of the jet.