REDUCED KINETIC MECHANISM OF IGNITION FOR NONPREMIXED HYDROGEN-AIR INA SUPERSONIC MIXING LAYER

Transient ignition processes in a two-dimensional spatially evolving s upersonic mixing layer consisting of a parallel nonpremixed airstream and a hydrogen stream both with temperatures higher than 1000 K were i nvestigated numerically by using the full chemistry and its reduced ch emistry. A phenomenon different from that examined in previous studies , in which ignition of hydrogen/oxygen mixtures was considered, was fo und in the nonpremixed case examined hare. It was shown that the conce ntration of O was greater than that of OH before ignition, but became smaller with the development of ignition process. Fourteen important r eactions for ignition were obtained and verified using sensitivity ana lyses of ignition delay time and radical concentrations. Several diffe rent four-step and three-step reduced kinetic mechanisms were then ded uced by introducing the steady-state approximation to different specie s. Comparison of these reduced kinetic mechanisms with the full chemis try showed that the steady-state approximation of O used in previous s tudies caused serious errors in the prediction of ignition delay time in supersonic flow, in which nonpremixed character is predominant and the transport phenomenon is important. Ignition locations predicted wi th the proper four-step and three-step reduced kinetic mechanisms were within 5% and 20% of those predicted with the full chemistry. Finally , these two reduced mechanisms were used to evaluate the effect of vis cous dissipation on ignition in the supersonic shear layer. Good agree ments between the results of the present reduced kinetic mechanisms an d those of the full chemistry were obtained.