Oblique detonation wave stability around a spherical projectile by a high time resolution optical observation

Abstract

Spherical projectiles were launched into detonable mixtures over a wide range of projectile velocities from near to about 1.8 times the Chapman-Jouguet (C-J) velocity. Oblique detonation waves (ODWs) and shock-induced combustions (SICs) stabilized around the projectiles were visualized with high time and high spatial resolutions using the Schlieren technique and a high-speed camera with a 1-s frame speed. Unsteady wave structures called Straw Hat type structures consisting of a SIC region followed by a C-J ODW were observed near stabilizing criticalities of a C-J ODW, and they were divided into two propagation types, depending on whether the C-J ODW could be stabilized (Kasahara et al. 2001, 2002, Maeda et al. 2011). In the present study, we suggested wave structures of the Straw Hat types based on our examination of dozens of continuous images. Triple points were observed at the intersection of a bow shock, a C-J ODW and a transverse detonation or shock wave when projectile velocities were slightly higher than C-J velocities. Onsets of local explosions in the SIC region for stabilizing the ODW in the Straw Hat type structures have been reported (Maeda et al. 2011). We observed this stabilizing mechanism by visualizing onsets of periodical local explosions and their transition to spherical detonation waves when the projectile velocity was much higher than the C-J velocity. We also determined stabilizing criticalities using a stoichiometric acetylene-oxygen mixture diluted with argon or krypton in 50% or 75% volumetric fractions, respectively. We found that the stabilizing criticalities did not depend only on the ratio of the projectile diameter and the cell size of the mixture.

Initiation and sustaining mechanisms of stabilized Oblique Detonation Waves around projectiles

“…The experimental setup was nearly identical to that used in our previous studies [8,[14][15][16], therefore, only the essentials are given here. The two-stage light-gas gun launched the projectile with velocities 2000m/s to 2500m/s into the detonable mixture.…”

“…Each mixture was diluted with argon in a 50% volumetric fraction to make the C-J velocity lower than the projectile velocity. For the acetylene/oxygen mixture, a 75% dilution was also projectile diameter, : cell size) as used in previous studies [5,7,8,[14][15][16]. The detonabilities were controlled by varying the filling pressures, and are represented by the cell sizes accessed from the Detonation Database [17].…”

“…The stabilized ODWs are inherently three-dimensional conical geometries, and have finite curvature radiuses. Our previous studies [15,16] showed that the detonation wave near the projectile was attenuated below the C-J velocity by the curvature effect.…”

“…For the conditions slightly below the stabilization criticality, some visualization studies showed the possibilities of unsteady propagation regimes [8,9,13] in which the Deflagration to Detonation Transition (DDT) occurred behind the bow shock around the sphere projectile. Our previous studies [14,15] revealed the propagation mechanisms and wave structures of unsteady regimes via high time-resolution Schlieren visualizations using a high-speed camera. Although these unsteady regimes were dominated by the complex and unstable DDT process, the stabilized ODWs were dominated by the continuous direct initiation of the overdriven detonation by the projectile and the attenuation of it by the expansion Proceedings of the Combustion Institute 34 (2013) 1973-1980 S. Maeda, S. Sumiya, J. Kasahara, A. Matsuo waves [13].…”

Numerical Simulation of Incompletely Premixed Oblique Detonation Stabilized on a Solid Surface

“…A lot of works have been done on fundamental physics of oblique detonation, [13][14][15][16] but almost all of them assume perfectly premixed conditions. Understanding of oblique detonation in a concentration gradient is lacking.…”