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

Asymptotic analysis is performed to investigate the ignition Of a visc ous, two-dimensional and supersonic mixing layer of two parallel strea ms 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 runa way and chain branching explosion. The lower branch of the characteris tic S-shaped curve, corresponding to a nearly frozen regime, is produc ed by using the critical Damkohler number of dimensionless ignition di stance, which consists of Damkohler numbers of chain branching reactio ns and that of chain termination reaction. The present results show th at, in addition to the initial temperature difference of the two strea ms, 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 ign ition. With Mach number increase, ignition moves downstream at first, then reaches a turning point, and finally moves upstream. This analysi s 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.