Shock-induced combustion around a supersonic spherical projectile was experimentally investigated by high-time- resolution schlieren imaging using a high-speed camera. A projectile of 4.76 mm diameter was launched by a gas gun into a C2H4+3O2+12Ar mixture with the initial pressure varied between 25 and 150 kPa. The Mach number of the projectile ranged from 4.0 to 5.9, which corresponded to 0.7 to 1.1 times the propagation Mach number of the Chapman-Jouguet (C- J) detonation (C-J detonation Mach number). Various combustion regimes, including combustion instabilities with an oscillating combustion front, were observed, and the trend of these regimes was validated using the parameters of a chemical reaction and propagation of the pressure wave driven by a chemical reaction behind the shock wave on the stagnation streamline. Heat release rate parameter, q ∗ was defined as the maximum temperature gradient in a reaction zone divided by a post-shock temperature assuming a constant-volume explosion, and the time scale, t ∗ for propagation of the pressure wave was defined as the projectile diameter divided by the difference between a sound speed and a flow velocity at a post-shock state. When the Mach number of the projectile exceeded approximately 0.9 times the C-J detonation Mach number, the induction length was considerably shorter than the scale of the projectile. In this case, the observed combustion regimes tended to exhibit oscillating combustion with a larger scale as the dimensionless heat release rate parameter became larger, which was defined as the product of the q ∗ and t ∗. This trend was in accordance with the one-dimensional consideration of the stagnation streamline in previous numerical studies using hydrogen-fueled mixtures. This was also confirmed by directly observing that the bow shock on the stagnation streamline was temporally oscillating by coupling with the rapid reaction. In contrast, when the Mach number of the projectile was approximately 0.7 to 0.8 times the C-J detonation Mach number, the induction length was comparable with the scale of the projectile. In this case, the mode of oscillating combustion did not exhibit a specific trend when plotted against the dimensionless heat release rate parameter. These experimental results revealed that one-dimensional considerations are insufficient for determining the combustion regime. The observed combustion regime also indicated that the evolution of the reaction front was probably affected by the flow field formed by the aft body of the projectile.
supersonic spherical projectile, shock-induced combustion