On the scale effects of flame stabilization under different combustion modes in an ethylene-fueled scramjet combustor

The combustion characteristics in two geometrically similar ethylene-fueled scramjet combustors with mass flow rates of 0.69 and 1.41 kg s(-1) are experimentally investigated to explore the scale effects of flame stabilization under different combustion modes with a Mach number 2.52 inflow. The static pressure distribution, schlieren images, and CH* chemiluminescence images are used to illustrate the combustion process. As the combustion heat release increases, the combustion in scramjet combustors of different scales generally undergoes the mode transition process of "Scram-Dual-Ram." The transition between the Scram and Dual modes is intermittent, with the "pressure step" phenomenon and significant combustion oscillations in the initial state of the Dual mode. The scale effects of flame stabilization include two aspects. On the one hand, the transition between the Scram and Dual modes occurs at different equivalence ratios-0.15 for the small-scale combustor and 0.19 for the large-scale combustor. On the other hand, the scale effects of flame stabilization are inconsistent under different combustion modes. The main reason is that the boundary layer that does not change proportionally with the combustor scale has different effects on the combustion in the three modes. In the Scram and Ram modes, the combustion scale has little impact on the combustion heat release and only affects the length of the pre-combustion shock train in Ram mode. However, in the Dual mode, slight differences in the relative thickness of the boundary layer are amplified. The increase in the combustion scale will lead to a significant decrease in the combustion heat release. In all combustion modes, the relatively thick boundary layer of the small-scale combustor makes it easier for the flame to propagate upstream, leading to stronger flame oscillations.