Effects of distributed injection on inlet unstart and combustion performance of a model scramjet under thermal nonequilibrium conditions

Distributed injection in scramjet engines has been shown to improve combustion efficiency, flame stability, and operational range by dispersing fuel through multiple locations rather than a single point. In this study, to relieve the fuel jet induced blockage responsible for inlet unstart, an equal diameter, pressure reduced distributed injection strategy is proposed. By lowering the local jet momentum flux, this strategy further weakens the bow shock and associated wall pressure rise. Three key parameters of the distributed injection are systematically evaluated at flight M a = 10 , namely fuel flow allocation, streamwise injector spacing, and spanwise injector offset. The thermal nonequilibrium effects on the reacting flow field under the high Mach conditions are assessed with unsteady Reynolds-averaged Navier–Stokes simulations coupled with Park’s two-temperature model and a vibration-chemistry coupling model. Among the fuel allocation modes examined at M a = 10 , the balanced fuel allocation mode provides the most favorable observed compromise between combustion performance and center plane pressure relief by regulating the heat release distribution and injection induced blockage. Among the streamwise arrangements examined, the bow shock influenced configuration achieves the highest combustion efficiency without an additional center plane pressure penalty. Within the spanwise offset configurations examined, the moderate offset configuration improves three dimensional mixing and combustion efficiency while avoiding the severe mass flux weighted pressure rise observed for the excessive offset configuration. With the combined settings identified from the parametric studies, the moderate offset distributed configuration increases the nozzle-exit combustion efficiency from 48.5% for the single orifice baseline to 53.0% under the adopted numerical framework. It also provides the least negative uninstalled thrust among the tested cases, changing from −3.61 N for the baseline to −2.28 N, although positive uninstalled propulsion is not achieved at the selected nozzle-exit plane. The results therefore identify comparative trade-offs among combustion development, pressure response, total pressure recovery, and uninstalled performance for the present configuration and selected operating conditions.