Influence of chemical kinetics in the self-ignition of nonpremixed supersonic hydrogen-air flows

Self-ignition tests of a model scramjet combustor were conducted by using parallel sonic injection of gaseous hydrogen from the base of a blade-like strut into a supersonic vitiated air stream. The range of stagnation pressure and stagnation temperature studied varied from 1.0 to 4.5 MPa and from 1300 to 2200 K, respectively. Experimental results show that the self-ignition limit, in terms of either global or local quantities of pressure and temperature, exhibits a nonmonotonic behavior resembling the classical homogeneous explosion limit of the hydrogen-oxygen system. Specifically, for a given temperature, increasing pressure from a low value can render a non-ignitable mixture to first become ignitable, then non-ignitable again. This correspondence shows that, despite the globally supersonic nonpremixed configuration studied herein, ignition is strongly influenced by the intricate chemical reaction mechanism and assumes a premixed, homogeneous explosion character. Consequently, self-ignition criterion based on a global reaction rate approximating the complex chemistry is inadequate. An auxiliary computational study on counterflow ignition was also conducted to systematically investigate the contamination effects of vitiated air. Results indicate that such contamination effects are expected to be weak for the present experimental data because of the counterbalancing influence of the H₂O-inhibition and NO-promotion reactions in effecting ignition.