Abstract: To investigate the effects of fuel injection on inlet starting characteristics, a numerical study of hydrogen injection into the GK01 two-dimensional hypersonic inlet was conducted at a freestream Mach number of 8. The results show that, as the fuel injection equivalence ratio increases, the separation region at the inlet shoulder gradually extends upstream. When the equivalence ratio reaches a critical value, large-scale flow separation occurs at the shoulder, leading to inlet unstart. When chemical reactions are additionally included in the simulation of the fuel injection flowfield, combustion is found to significantly increase the boundary-layer displacement thickness downstream of the shoulder separation bubble, thereby increasing the characteristic length scale of the shock-wave/boundary-layer interaction at the shoulder. Consequently, the critical equivalence ratio for inlet unstart is lower under reacting conditions. During the restart process, the Damk?hler number in the separation region is three to five times that in the started state at the same equivalence ratio. Chemical reactions in this region are intense, and combustion heat release sustains a strong adverse pressure gradient in the internal contraction section. This makes the unstarted state strongly self-sustaining and substantially increases the difficulty of restarting the inlet. An investigation of the effects of backpressure on inlet starting characteristics shows that near-wall combustion decreases wall shear stress while increasing boundary-layer thickness and shape factor, thereby reducing the inlet’s backpressure tolerance. At an equivalence ratio (ER) of 0.25, near-wall combustion reduces the maximum sustainable backpressure of the inlet by 30.2% compared with the case without fuel injection.

Key words: hypersonic inlet, fuel injection, flow separation, starting characteristics, shock/boundary-layer interaction