ИСТИНА

Solid fuel scramjets are considered to be promising propulsion systems for unmanned aerial vehicles. Stability of combustion in solid fuel scramjets is contingent upon proper geometry of fuel grain and availability of a flame holder. Flame stabilization is achieved in the recirculation zone developing due to sudden expansion of incoming air near the inlet. Previous experimental and theoretical works suggest that there exist an optimum range of length-to-diameter ratios for the flame holder. However, due to fuel burnout the channel and flame holder geometry change in the process of scramjet operation, the regression rate being variable both in time and length. The purpose of this work is to develop a computational model of a solid fuel scramjet taking into account the geometry variation, as well as to obtain typical flow patterns at the different stages of combustion. We consider polymethylmethacrylate (PMMA) as a fuel and describe its decomposition under the action of a heat flux from a gaseous combustion products. To describe chemical reactions in the gas phase we consider five mixture components, namely C5H8O2, O2, CO2, H2O, N2. The numerical model is based on a system of fully compressible RANS equations with k-ε turbulence model. Turbulent combustion is described by the Eddy Dissipation Concept (EDC) model. Equations are solved on a Cartesian mesh, with the complex geometry of fuel channel described by the moving embedded boundary based on the level-set approach. The embedded boundary is propagating in the normal direction at the velocity determined by the local regression rate proportional to heat flux incident onto the surface from the combustion zone. Simulations are carried out for a range of inlet parameters and initial fuel channel and flame holder geometries, corresponding to supersonic flow conditions. Variation of fuel channel shape caused by fuel burnout is demonstrated, and its effect on combustion stability in the gas phase is discussed. Parametrical simulations show that the loss of stability occurs in two stages. At the first stage, the fuel extinguishes in the grain channel, while combustion continues in the flame holder. Extinguishing of fuel in the channel can be accompanied by oscillations in pressure and heat release rate. The complete cessation in solid fuel scramjet operation occurs after the fuel is extinguished in the flame holder. A simple analytical method for analyzing the combustion stability in a grain channel is suggested. This research was supported by Russian Foundation for Basic Research, Grant RFFI 16-29-01084 ofi_m