Suppression of Combustor-Inlet Interaction in a Scramjet Engine under M4 Flight Conditions

A sidewall-compression-type scramjet engine was tested under M4 flight conditions. Tested engine had an inlet, a constant cross-sectional area isolator, a constant cross-sectional area combustor (designated as combustor-1), a diverging combustor (designated as combustor-2), and an internal nozzle. In the previous study under M4 flight conditions, the maximum thrust increment by the fuel injection within the combustor-1 was 1,380N at an equivalence ratio of 0.31, and further fuel injection resulted in the combustor-inlet interaction (designated as CII). To suppress the CII, we attempted (1) a two-stage fuel injection within the combustor-1 and the combustor-2 and (2) a boundary layer bleed on the top wall, in the present study. The former was to suppress heat release around the first-stage fuel injectors in the combustor-1 at a given total fuel mass flow rate, and the latter was to decrease interaction length by decreasing a boundary layer thickness on the top wall. With the two-stage fuel injection, the maximum thrust increment was 2,230N at an equivalence ratio of 0.63. Next, the boundary layer bleed was carried out, and the maximum thrust increment was 2,300N at an equivalence ratio of 0.66. Thus, both two-stage fuel injection and boundary layer bleed led to 60% higher maximum thrust increment than that obtained in the previous study. Finally, both two-stage fuel injection and boundary layer bleed were applied simultaneously to obtain the largest thrust performance, and the maximum thrust increment was 2,560N at an equivalence ratio of 0.95. As a result, we obtained 80% higher maximum thrust increment than that in the previous study by these methods. The thrust achievement factor, which was defined as the ratio of the maximum thrusts obtained from experiment and theoretical prediction, under this condition was estimated as 70%.