Numerical study of detonation initiation by a supersonic sphere

Conditions of detonation initiation induced by a supersonic sphere in a stoichiometric hydrogen/oxygen mixture with 70% argon dilution, are investigated numerically. The physical model includes a detailed full chemistry and is solved using an LU-SGS TVD scheme. Transition of the two extremes, shock induced combustion and detonation, are examined over pressures ranging between 0.2 bar and 10 bar. For shock induced combustion regime, the present study exhibits a clear process of coupling and decoupling between shock wave and combustion wave. For detonation regime, numerical results yield a propagation velocity of shock wave within 2% of the theoretical CJ velocity. It is shown that detonation initiation velocity of the projectile decreases with the increase in pressure of the mixture. Moreover, it is found that there is a minimum initiation velocity on the low pressure side, corresponding to the second explosion limit of hydrogen/oxygen mixture. Furthermore, autoignition times calculated by two different chemical kinetics also show the existence of the minimum ignition time. The present analysis exhibits effects of finite rate chemistry and unsteady process on detonation initiation, which is not included in an existing theory. Numerical predictions show a qualitative agreement with experiments, in particular, at low pressures.