IGNITION ANALYSIS OF UNPREMIXED REACTANTS WITH CHAIN MECHANISM IN A SUPERSONIC MIXING LAYER

Asymptotic analysis is performed to investigate the ignition Of a viscous, two-dimensional and supersonic mixing layer of two parallel streams of oxidant and fuel. A three-step schematic kinetic model proposed by Birkan and Law is adopted to grasp the essential properties of full chemistry. Ignition is shown to be characterized by both thermal runaway and chain branching explosion. The lower branch of the characteristic S-shaped curve, corresponding to a nearly frozen regime, is produced by using the critical Damkohler number of dimensionless ignition distance, which consists of Damkohler numbers of chain branching reactions and that of chain termination reaction. The present results show that, in addition to the initial temperature difference of the two streams, the shear parameters and chemical kinetics have strong effects on ignition distance. Even when the velocity difference of two streams is not large, dissipation plays a dominant role as a beat source for ignition. With Mach number increase, ignition moves downstream at first, then reaches a turning point, and finally moves upstream. This analysis also shows that ignition distance will be greatly shortened with an increase of chain-branching reaction rates and will be delayed with an increase of chain termination rate.