Research on the operating boundary of the dual mode scramjet with a constant area combustor through thermodynamic cycle analysis

The maximum pressure ratio of the isolator is important for studying the operating boundary of the dual mode scramjet. In this paper, the pressure rise mechanism of the dual mode scramjet engine and the characteristic of the isolator maximum pressure rise are analyzed. The operating range and performance of scramjet under the pressure rise limit are investigated using the constant cross-sectional area combustor. The maximum pressure ratio of the isolator confines the back pressure from the combustor so that the scramjet engine has a minimum freestream Mach number of each equivalence ratio. When the freestream Mach number is smaller than the minimum freestream Mach number, an expansion section may be arranged between the combustor and the isolator to balance the pressure of the combustor and that of the isolator. The expression of the minimum freestream Mach number is derived and it is found that the minimum freestream Mach number of the dual mode scramjet in both modes increases with the increase of equivalence ratio and compression ratio of the inlet. A variable divergent section is suggested to balance the pressure under different freestream Mach numbers and equivalence ratios.