Experimental observation of oblique detonation waves around hypersonic free projectiles

Abstract

We studied the oblique detonation waves around hypersonic projectiles. Projectiles (10 mm diameter, conical nose shape) were fired at hypersonic speeds (2.8 ± 0.1 km/s) into stoichiometric hydrogen-oxygen mixtures (pressure between 10 kPa and 50kPa, at room temperature T = 300.2 ± 1.4 K). The flowfields around the projectiles were visualized using a multi-frame schlieren technique. We made a comprehensive study of the oblique detonation wave phenomena around the hypersonic projectiles with variations in two parameters: the projectile nose shape and the initial gas pressure. The projectile nose cone open-angle was varied from 60° to 180°. The initial gas pressure was varied from 10 kPa to 50 kPa. Four types of combustion were observed. At the lowest initial gas pressure 10 kPa or at the smallest open angle 60°. two non-ignition types: a detached bow shock wave type and an attached bow shock wave type were observed. At the upper initial gas pressure and bigger opened-angle, an oblique detonation type was observed. The presence of an intermediate type (which we label a straw hat type, consisting of an oblique detonation + a shock induced bow combustion wave) was observed at 20 – 33 kPa and 90° – 120°. The shock shape of this type is similar to the shape of a typical straw hat. Using multi-frame schlieren pictures and comparing the pictures shot in two observation windows separated by a distance of 260 mm, we confirmed that the oblique detonation waves were steady within 92.5 μs. By detonation polar analysis, the heat release behind the detonation wave front was close to that of the theoretical Chapman-Jouguet detonation, (within 20 %, and in most cases within 10 %). It should be noted that the minimum curvature radius in the detonation wave of the straw hat type was a radius of 40 – 50 times the induction length of the detonation wave. We think this is a significant parameter in the phenomena.