Experimental Investigation of Cavity Flame Characteristics for Four-Orifice Fuel Injection at Variable Axial Locations in a Ma 1.6 Supersonic Combustor

Cavity flame holders are core stabilization components for Ma 1.6 low-supersonic scramjet combustors, where the axial location of upstream fuel injection significantly affects fuel-air mixing, flame holding, and combustion performance. Two four-orifice injection schemes (Far-Upstream Injection, FUI: 30 mm upstream of cavity leading edge (CLE); Near-Upstream Injection, NUI: 10 mm upstream of CLE) were experimentally studied under Ma 1.6 inflow (T0=660 K, ϕ=0.2) via synchronized high-speed schlieren and CH* chemiluminescence diagnostics. Results showed that FUI produced greater cold-flow jet penetration, but generated stronger shock structures and flow instabilities. Under combustion, the penetration gap between schemes narrowed substantially due to heat-release-induced thermal expansion, with NUI benefiting more. NUI achieved superior flame stabilization, uniform full-cavity heat release, and suppressed combustion instability through broadband flow fluctuations, whereas FUI exhibited a high-energy discrete dominant frequency and flame oscillation.