Typical onset modes of DDT and behavior of strong transverse shocks

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.