Scramjet Engine Flowpath That Improves Specific Impulse Using JP-7 Fuel

A flowpath geometry was computed that improves the specific impulse of a dual-mode scramjet engine in a generic X-51 hypersonic vehicle. Six parameters were varied: inlet contraction ratio, the diameters and numbers of fuel injectors, divergence angle of the combustor wall, nozzle flap deflection angle, and flight Mach number. The maximum specific impulse Isp was 2296 s in the ram mode and 832 s in the scram mode. A reduced-order model simulates the finite-rate chemistry of JP-7 fuel and the unstart limits. Results show that both combustion efficiency and Isp drop to unacceptably low levels when the finite-rate chemical reaction rates are weakened by flame strain-out due to the large air velocities, or when the flame becomes longer than the combustor. Small Isp occurs when the following are too small: the inlet contraction ratio, the inlet compression ratio, the number of fuel injectors, and the diameter of fuel injectors. When these parameters are too large, excessive heat release causes unstart. The operating range was identified between these limits. For JP-7 fuel, it was found that the inlet should raise the pressure to above 5 atm. Results are explained by the interactions between reactions, mixing, and flame strain-out.