The Deflagration-to-Detonation Transition (DDT) in a 40 mm × 73 mm rectangular cross-section tube was investigated for three different mixtures: 2H₂ + O₂ + 40%Ar, C₂H₄ + 3O₂ + 40%Ar, and CH₄ + 2O₂, and the corresponding detonation regularity ranged from very regular to irregular. The DDT process was visualized by using a high-speed schlieren system. Ion probes were used to determine the incident flame velocity. A total of four different DDT onset modes were observed in this series of experiments: flame-boundary-layer interaction, flame-shock interaction, shock-wall interaction, and shock-shock interaction, which shows that DDT is a random process. Although the detonation initiation appearances are different, the essential physical characteristic is the same: the local hot spot created by the energy focus. One or more bow shocks created by Mach reflection remain as strong transverse shocks after the detonation front. The corresponding numerical simulations show that the strong transverse shock propagation behavior strongly depends on the location where the hot spot forms. Two propagation modes, namely, the opposite double-wave mode and single-wave mode, were identified from both the experiments and simulations. The cellular structures of the two modes in simulations agree well with the long-exposure images captured from experiments. This work provides some fresh new insights into the DDT process, which may improve the understanding of DDT formation mechanisms.

Shock wave focusing is an effective way to create a hot spot or a high-pressure and high-temperature 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° and 90°) 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.