H2-air and CH4-air detonations and combustions behindoblique shock waves

Abstract

Two stoichiometric reactive mixtures (H 2 -air and CH 4 -air) at initial conditions of temperature and pressure, T 0 =293 K and p 0 <1 bar, respectively, very different from the point of view of their detonability, are used in order to study the conditions of the onset of combustion and/or detonation when submitted to the action of an oblique shock wave (OSW) at Mach number M of about 6 and at various angles of inclination θ. Generation of an OSW in the reactive mixture at rest is obtained via lateral expansion of the detonation products of a normal CJ detonation wave propagating in a bounded gaseous detonable mixture separated from the mixture under test by a 6- μ m-thick mylar film. Different steady combustion regimes are observed depending on θ and on the reactive mixture. Based on simple polars analysis, the possibility of observing an oblique detonation wave (ODW) are checked for the two mixtures. In the case of H 2 -air mixtures, ODWs are obtained with small degrees of overdrive ( D/D CJ ) (≊1) ofthe detonation (where D is the normal detonation velocity of the ODW) after a predetonation zone, where the OSW is followed by an oblique flame initiated at the beginning of the interaction between the two media. Minimal conditions for detonative combustion are deduced, which correspond to a postshock temperature T s ≊1000 K while the minimal condition for combustion as an oblique flame behind the OSW is T s ≊800 K. For CH 4 -air mixtures, no detonation appears even when the degree of overdrive of the theoretically possible solutions of stable ODW is increased up to 1.1 and beyond for T s up to 1480 K. Only the OSW and oblique flame configuration are observed. For T s <1000 K, no combustion occurs behind the shock wave. Thus, this mixture appears to be especially difficult to detonate.