Deflagration and detonation induced by shock wave focusing at different Mach numbers

Shock wave focusing is an effective way to create a hot spot or a high-pressure and hightemperature region at a certain place, showing its unique usage in detonation initiation, which is beneficial for the development of detonation-based engines. The flame propagation behavior after the autoignition induced by shock wave focusing is crucial to the formation and self-sustaining of the detonation wave. In this study, wedge reflectors with two different angles (60 degrees and 90 degrees) and a planar reflector are employed, and the Mach number of incident shock waves ranging from 2.0 to 2.8 is utilized to trigger different flame propagation modes. Dynamic pressure transducers and the high-speed schlieren imaging system are both employed to investigate the shock-shock collision and ignition procedure. The results reveal a total of four flame propagation modes: deflagration, DDT (Deflagration-to-Detonation Transition), unsteady detonation, and direct detonation. The detonation wave formed in the DDT and unsteady detonation mode is only approximately 75%-85% of the Chapman-Jouguet (C-J) speed; meanwhile, the directly induced detonation wave speed is close to the C-J speed. Transverse waves, which are strong evidence for the existence of detonation waves, are discovered in experiments. The usage of wedge reflectors significantly reduces the initial pressure difference ratio needed for direct detonation ignition. We also provide a practical method for differentiating between detonation and deflagration modes, which involves contrasting the speed of the reflected shock wave with the speed of the theoretically nonreactive reflected shock wave. These findings should serve as a reference for the detonation initiation technique in advanced detonation propulsion engines. (c) 2023 Production and hosting by Elsevier Ltd. on behalf of Chinese Society of Aeronautics and Astronautics. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).