Turbulent Counterflow Diffusion Flame Structure and Dilution Effects

Abstract

Laminar counterflow combustion configurations have been studied extensively in the investigation of fundamental combustion phenomena, such as fuel dilution, strain rate, and pressure dependence on flame extinction limits. However very few studies have been performed on turbulent counterflow diffusion flames. The turbulent opposed flow configuration chosen here consists of a fuel jet pipe of d = 6.4 mm in diameter directed downward toward an air jet 50 mm in diameter, shielded wilh a concentric air jet of 13 mm diameter. The vertical distance between the fuel jet exit and the air flow can be adjusted to investigate the influence of the overall strain rate along the centerline of this configuration. A hydrogen-helium mixture was chosen to investigate the effects of dilution on the structure of the diffusion flame. To investigate the combustion effects on the turbulent mixing, reacting and non reacting conditions were studied at the same Reynolds number. In order to study the reaction zone structure, high speed tomography based on Mie scattering was employed using a copper vapor laser and a Fastax high speed camera. Different seeding techniques were used to visualize both the air and fuel jets. The tomographic records were digitized and recorded in a digital computer for statistical treatment. Significant differences in the wrinkle scales between the reacting and the non reacting flows were found. A fractal statistical analysis of the tomography records was done to quantify these differences. Seeding of both fuel and air jets provided a means for the estimation of the reaction zone thickness. From the lime resolved tomographic records an evaluation of the stretch of the reaction zone boundaries was made.