Incomplete mixing and off-design effects on shock-induced combustion ramjet performance

A shock-induced combustion ramjet ("shcramjet") model is described to inves tigate the effects of incomplete fuel/air mixing and off-design flight cond itions on its performance characteristics. A fully implicit, fully coupled, Newton-iteration, lower-upper symmetric Gauss-Seidel scheme is employed to solve the Euler equations at steady state. This scheme is coupled with a n onequilibrium chemistry model consisting of 33 reactions and 13 species. Ax isymmetric and planar shcramjet flowfields with variable equivalence ratio profiles representing extreme deviations from homogeneous fuel/air mixing a re numerically solved for a range of Eight Mach numbers at a constant dynam ic pressure of 1400 psf. Results show that incomplete fuel/air mixing gives rise to a combination of detonative combustion and simple shock-induced co mbustion. Comparison of overall performance characteristics to schcramjets with homogeneous, stoichiometric fuel/air mixtures demonstrates the degree of performance degradation. The propulsive characteristics of mixed-compres sion ramjets are calculated in off-design operating regimes corresponding t o inlet Mach numbers above and below design Mach numbers of 12, 16, and 20; for external-compression ramjets, the propulsive characteristics are calcu lated for inlet Mach numbers below design Mach numbers of 12, 16, and 20. I t is found that the propulsive properties of the engines deteriorate when t hey are operated at off-design conditions. For mired-compression ramjets op erating at lower-than-design Mach numbers, the degradation in thrust produc tion is due primarily to reduced heat release in the engine nozzle. At high er-than-design Mach numbers, thrust production is reduced only slightly due to a modified nozzle geometry, required to ensure convergence of the numer ical method. Generation of thrust for external-compression ramjets deterior ates at lower-than-design Mach numbers due to a high-pressure zone crested in the combustor by the impingement of the detonation wave on the engine su rface upstream of the design point. Mixed-compression ramjets are found to provide superior performance to external-compression ramjets at off-design operation. External-compression ramjets are found to be more sensitive to o ff-design operation than mixed-compression ramjets. It is concluded that th e engine geometry must be varied as Eight conditions change if degradation in engine performance at off-design conditions is to be avoided.