Visualization of ignition modes in methane-based mixture induced by shock wave focusing

Abstract

In this study, different ignition modes and detonation initiation due to shock wave focusing are observed by combining a transient overpressure recording technique and a high-speed schlieren photography system. In plane reflection, only weak and strong ignition modes are observed; while there are three ignition modes in the 90-degree and 60-degree wedges by introducing shock wave focusing: peak local ignition mode (PLIM), boundary ignition mode (BIM), and strong ignition mode (SIM). In PLIM, the flame is generated at the apex of the wedge-shaped reflector, but the propagation speed is slow, the flame area concentrates on the apex of the reflector and maintains for a long time. In BIM, the flame is firstly generated at the apex of the wedge and quickly propagates along the tube wall; later the flame propagates slowly from the tube wall to the central unburned area. In SIM, oblique shock waves are generated when the shock waves are reflected from the wedged walls, the interaction of oblique shocks inducing the initiation of detonation. In addition, the incident shock velocity (Vi) range corresponding to different ignition modes for different shock wave focusing types is given. The lower velocity limit of SIM for planar, 90-degree, and 60-degree wedge reflectors is 980 m/s, 915 m/s, and 880 m/s, respectively. Therefore, it is more conducive to initiate the detonation wave in the 60-degree wedge with a lower speed shock wave. The instantaneous maximum pressure (Pmax) and maximum pressure after ignition (Pign) of the three reflectors are investigated corresponding to each ignition mode. It is found that Pign and Pmax can be used to determine SIM. This study provides new insight into different ignition modes in wedge reflectors due to shock focusing; the results are beneficial to understanding the physics of shock wave focusing and its mechanism of shock focusing-induced detonation.