Incomplete Mixing and Off-Design Effects on Shock-Induced Combustion Ramjet Performance

Abstract

A shock-induced combustion ramjet (“ shcramjet”) model is described to investigate the effects of incomplete fuel/air mixing and off-design e ight conditions on its performance characteristics. A fully implicit, fully coupled, Newton-iteration, lower ‐uppersymmetric Gauss ‐Seidel scheme is employed to solvethe Euler equationsat steady state. This scheme is coupled with a nonequilibrium chemistry model consisting of 33 reactions and 13 species. Axisymmetric and planar shcramjet e owe elds with variable equivalence ratio proe les representing extreme deviations from homogeneous fuel/air mixing are numerically solved for a range of e ight Mach numbers at a constant dynamic pressure of 1400 psf. Results show that incomplete fuel/air mixing gives rise to a combination of detonative combustion and simple shock-induced combustion. Comparison of overall performance characteristics to shcramjets with homogeneous, stoichiometric fuel/air mixtures demonstrates the degree of performance degradation. The propulsive characteristics of mixed-compression ramjets are calculated in off-design operating regimes corresponding to inlet Mach numbers above and below design Mach numbers of 12, 16, and 20; for externalcompression ramjets, the propulsive characteristics are calculated for inlet Mach numbers below design Mach numbers of 12, 16, and 20. It is found that the propulsive properties of the engines deteriorate when they are operated at off-design conditions. For mixed-compression ramjets operating at lower-than-design Mach numbers, the degradation in thrust production is due primarily to reduced heat release in the engine nozzle. At higher-thandesign Mach numbers, thrust production is reduced only slightly due to a modie ed nozzle geometry, required to ensure convergence of the numerical method. Generation of thrust for external-compression ramjets deteriorates at lower-than-design Mach numbers due to a high-pressure zone created in the combustor by the impingement of the detonation wave on the engine surface upstream of the design point. Mixed-compression ramjets are found to provide superior performance to external-compression ramjets at off-design operation. External-compression ramjetsarefound to bemoresensitiveto off-designoperation than mixed-compression ramjets.Itisconcludedthat the engine geometry must be varied as e ight conditions change if degradation in engine performance at off-design conditions is to be avoided.