Occurrence conditions for unsteady combustions in shock-induced combustions around spherical projectiles

Abstract

Shock-induced combustion around a spherical body was experimentally investigated by launching the projectile at supersonic speed into a combustible mixture. This study focused on occurrence conditions for an unsteady combustion which was characterized as combustion instabilities with an oscillating combustion front. A spherical body of 4.76 mm dimeter was used as the projectile, and its flight Mach numbers were ranged from 3.5 to 7.5. Four types of combustible mixtures, which were stoichiometric hydrogen-oxygen and ethylene-oxygen mixtures diluted with argon or nitrogen (2H₂ + O₂ + 3Ar, 2H₂ + O₂ + N₂, C₂H₄ + 3O₂ + 12Ar, C₂H₄ + 3O₂ + 2.5N₂), were used and their initial pressures were varied between 25 and 100 kPa. The combustion regimes around the projectile were observed by using a schlieren optical system and high-speed camera. The combustion regimes generally varied from the steady combustion with smooth combustion front to the unsteady combustion with oscillating combustion front, when the projectile Mach number or the initial pressure increased. The occurrence conditions for the unsteady combustion were expressed by the two dimensionless parameters; dimensionless heat release rate, q^*t^* and dimensionless induction length, l_ind^*, which were defined by the post-shock state and flow velocity on the stagnation streamline of the projectile and by assuming the chemical reaction as a constant-volume explosion. The q^*t^* included a temperature gradient in a reaction zone, and represented the strength of the pressure wave driven by the heat release reaction. The l_ind^* included an induction time, and represented the distance between the shock wave and the location where the heat release reaction started. The unsteady combustion occurred when these two dimensionless parameters were above the critical values, and the trend of occurrence condition of the two combustion regimes could be explained by introducing the parameters.