Investigation of a spark ignition flame kernel interacting with a laminar vortex toroid

Abstract

Chemistry-turbulence interactions play a critical role in most practical combustion environments. Understanding the interaction between a flame kernel and a vortex is an important fundamental step. This dissertation presents high-speed movies of combustion luminosity during the interaction of a laminar vortex toroid with a spark -generated premixed flame kernel in a quiescent combustion chamber. The resulting time evolution of the perturbed flame kernel shows that laminar vortices of various sizes and vortex strengths can increase the kernel growth rate by at least a factor of 3 and significantly increase combustion reaction rates by involving additional highly curved and stretched flame fronts. This dissertation also describes experiments that were conducted to study the Lewis number effect on the flame kernel-vortex interaction. The influence of a time varying strain rate on kernel growth was investigated by studying both lean methane-air (thermo-diffusively unstable) and lean propane-air (thermo-diffusively stable) flame kernels, using both natural CH/OH emission image sequences acquired by a high-speed intensified camera to show details of the disturbed flame kernel growth, and OH-PLIF images to determine the true two-dimensional nature of the interaction. Significant differences are observed in the highly curved regions on the backside of the invading vortex in the two different mixtures. Lewis number effects on local burning rate variations, flame front wrinkling, and pocket formation are reported, and in general, the results are in agreement with predictions from asymptotic theory assuming low stretch rates. Local mixture enrichment by direct injection in the vicinity of the spark plug at the time of ignition can affect flame kernel development and extend the lean limit of flammability of a fuel/air mixture. In the third set of the experiments, flame kernels were ignited in a lean premixed CH4/air mixture with an equivalence ratio of 0.6, while CH4/air mixtures at six different equivalence ratios ranging from 0 to infinity were used to generate the vortex. Chemiluminescence images of kernel -vortex interactions have been captured using both anICCD camera and a high-speed intensified camera. Details about flame kernel-vortex interactions of the six test cases are presented and discussed by comparing image sequences for different cases.