Flame front dynamics studies at deflagration-to-detonation transition in a cylindrical tube at low-energy initiation mode

The aim of this work was to study the dynamics of deflagration-to-detonation transition (DDT) following flame acceleration in a cylindrical tube. We combined high-speed video recordings of self-luminescence and traditional local flow measurements to study the DDT process in a stoichiometric acetylene-oxygen mixture with argon dilution and nitrogen dilution. Experiments were carried out in a cylindrical tube with an inner diameter of 0.04 m and a low-energy ignition source (the ignition energy 0.8 mJ). The quantitative data on the dynamics of the shock wave and the local flame velocities, the spatial behavior of the flame during its acceleration, the flame extension, and the distance between the shock wave and the flame front were obtained. The locations of hot spots relative to the spatial configuration of the accelerating flame front and the tube walls were found as well. The local flow temperatures required for hot-spot formation were evaluated from the experimental measurements of induction zone length and the comparison of ignition delay times of similar mixtures obtained behind the reflected shock waves in shock tube tests. The following results were obtained from the experiments. At flame velocities near the isobaric sound speed of combustion products, the spatial configuration of the flame was stable, and one or several hot spots were initiated ahead of the flame near the tube wall. The distance between hot spots and the leading tip of the flame did not exceed 0.4 m. Depending on the mixture sensitivity, weak, transient, or strong ignition modes were observed. For hot-spot formation, the actual local temperature in the induction zone between the leading edge of the flame front and the shock wave should be 100-200 K higher than the post-shock temperature deduced from a one-dimensional model. The higher post-shock mixture condition in the induction zone is due to an additional local transverse adiabatic compression of unburnt mixture near the tube wall, which is generated by the inclined flame front.