Research On Combustion Mode Transition And Its Control Method For Scramjet Engines

Having a high performance in a wide flight Mach number range is the inevitable requirement of the development of scramjet engine. In a low Mach number range, ram mode operation has a higher specific impulse. When the flight Mach number is high enough, the total pressure loss result ing from the compression of the air from supersonic speed to subsonic speed becomes excessive and the dissociation of the air occurs due to the high static temperature caused by the strong compression. At this time, the performance of the ram mode operation degrades seriously and the scram mode operation should be employed. Hence, to meet the conflicting requirement of high performance and wide operating range of flight Mach number, combustion mode transition must be embraced for the scramjet engine operation. Aiming at the characterist ic of the combustion mode transition and its control problem, theoretical and experimental studies are carried out as follows:Analyses of the supersonic combustion process are carried out and it is found that scramjet has a strong distr ibuted parameter characteristic and is multimodes operation. This means that the existed analyses method based on the zerodimensional model is no longer applicable and thereby the strong demand of the one-dimensional model is elicited. A cycle performance analyses method of scramjet engine considering one-dimensional model is established. The basic governing equat ions of the one-dimensional transonic reacting flows are given, which consider lots of physical effects that affect the engine performance. The governing equations of the one-dimensional transonic flows are rewritten by using a variable transformation method, and in this way the singularity of the governing equations is eliminated. Subsequently, a forward iterative shooting method is introduced to deal with the numerical problem appearing in the flow that includes finite-rate chemistry and high-temperature gas properties. The comparative analyses show that the proposed method has a wider applicability, better stability and higher solution precision than previous methods.Due to the restriction of the thermodynamic cycle, the scramjet engine wit h single-staged fuel injection is not able to exert its potential of thrust production. The necessit y and importance of the multi-staged fuel injection for the scramjet is elaborated. Based on the analysis method of reheat cycle, a parametric model is proposed to analyze the performance of the scramjet engine with mult i-staged fuel injection and its combustion mode is discussed. The effects of combustor area expansion ratio, reheat times and distribution proportion of area expansion on the engine performance are analyzed. The results show that increasing the combustor area expansion ratio is the most effective way to improve the thrust production capacit y of scramjet and the number of fuel injection should not be too much.To maximize the performance within the entire flight trajectory, combustion mode transition should be implemented at an applicable Mach number for the scramjet engine. Therefore, a strategy for combustion mode optimum selection is presented in this paper, in which the cycle performance and flight missio n of scramjet engine should be considered together. The variation rule of engine performance with flight Mach number is analyzed for a scramjet with constant area combustor. The results show that for a scramjet with the acceleration flight mission, combustion mode transit ion should be designed to take place within the range of flight Mach number 6-7 for maximizing acceleration performance. For a scramjet with the cruise flight mission, it should be operated as much at the ram mode cycle as possible.The combustion mode transit ion boundary is described and the effects of geometric configuration, heat release distribution, wall temperature, wall friction and freestream component on the combustion mode transition boundary is studied. These influence laws can provide a theoretical foundation for the engine design and the design of combustion mode transition control scheme. Catastrophe and hysteresis phenomenon which occurs in the mode transition is discussed and based on the catastrophe theory, two kinds of forma tion mechanism of the catastrophe and hysteresis characteristic are proposed: the unstability of the combustion and the unstability of the flow separation. These mechanisms are validated by the one-dimensional model. The influence of fuel flow rate discontinuity caused by injector switch on the performance is also discussed.Detailed analyses of the combustion mode transit ion process reveal that the nature of the combustion mode transit ion control is the protection control. The basic task of combustion mode transit ion control is to protect the scramjet from the instability due to the catastrophe and hysteresis characteristic caused by mode transition and to make the mode transition process smooth. According to the strength of thrust catastrophe in the mode transit ion, corresponding control scheme of combustion mode transit ion is presented and validated by simulation. Thrust regulation and inlet unstart protection control for the scramjet are analyzed and switching control method based on strategy of integral initial values resetting is proposed. The proposed control method is validated by ground experiment on the direct-connecting combustor. The results show that the proposed method can achieve the coordination control among several control objectives and ensure the efficient and safe work of scramjet engine.