Effect of the Equivalence Ratio on Ignition and Flame Propagation in a Channel with a Backward-Facing Step

The unsteady autoignition of a non-premixed hydrogen–air mixture in high-speed flow is considered in order to determine the effect of the equivalence ratio on ignition and flame stabilization in a channel. A series of experiments over a wide range of equivalence ratios (0.35 to 1.2) shows that initial ignition occurs in the boundary layer separation region under the influence of the re-reflected bow shock wave ahead of the fuel jet. This zone is a stable ignition source, from which the flame, under certain conditions, propagates upstream to the channel inlet. Two flame stabilization regimes exist at an injection angle of 45°. At low equivalence ratios, flow with a monotonic pressure increase occurs until reaching a plateau with a moderate pressure rise. At equivalence ratios above 0.8, a two-stage combustion regime is observed. The first stage is the heat transfer process, which coincides with the combustion regime at low equivalence ratios and is characterized by increased pulsations due to enhanced thermoacoustic interaction. A pressure increase in the initial ignition region leads to the rapid propagation of the flame front upstream and downstream and to an increase in pressure to maximum values with high combustion efficiency. A comparative analysis of pressure and heat flux distributions along the channel length was performed.