Combustion-torch ignition: Fluorescence imaging of OH concentration

Abstract

The temporal and spatial development of the OH concentration during the ignition of a lean methaneair mixture (φ = 0.65) by a combustion torch has been studied. In the experiment the combustion torch was formed by a jet of high temperature combustion product gases that exit a thin-plate circular orifice connecting a small cylindrical prechamber with the main combustion chamber. This starting jet was driven by a spark-ignited flame which propagates through the prechamber. By decreasing the diameter of the prechamber orifice the initial gas velocity of the combustion torch was systematically increased. With this variation in velocity the flow field of the combustion torch, determined from high-speed schlieren videography, was altered significantly. At the lowest velocity a laminar vortex-ring structure was formed. As the velocity was increased the combustion-torch flow field develops the features of a highly turbulent jet. The fluid physics of the combustion torch has a significant influence on its chemical structure and the development of the subsequent ignition process in the main combustion chamber. Unique observations of the chemical structure of the combustion-torch ignition process were obtained by quantitative imaging of the OH concentration using laser-induced fluorescence.