Characteristics Of A Variable Geometry Combustor In RBCC

A rocket-based combined-cycle(RBCC)propulsion system is a combined propulsion system that integrates a rocket engine and an air-breathing engine into one flow path.RBCC engines represent one of the major propulsion solutions for space transportation and hypersonic flight vehicles in near space.Previous schemes have generally employ a fixed geometry combustor,and the continuous and stable operation of the combustor requires the expense of performance in the ejector and ramjet modes for the coordination of the multiple modes and to widen the flight envelope of the RBCC engine.In order to make the wide range RBCC engine have superior performance throughout the operating envelope,it is necessary to adopt feasible technical approaches to make the engine as optimal as possible in different flight modes.In view of the beneficial effects of variable geometry in optimizing and improving the performance of the RBCC whole trajectory and solving the technical problems of RBCC development,this dissertation thoroughly studies the feasibility of variable geometry combustor to improve engine performance,the influencing factors and variation law of variable geometry combustor flow combustion process and engine performance through the combination of theoretical analysis,three-dimensional CFD numerical simulation and ground direct-connect test.Through the structural realization of the variable geometry combustor and the hot test to verify the engine performance,the gas and combustion dynamics of the variable geometry combustor are revealed,and the variable geometry control law is obtained.It provides theoretical basis and technical means to solve the problems of intake and exhaust and combustion stability under multi-factor coupling in the model transition process.The main research contents and conclusions of the dissertation are as follows:(1)Investigation of RBCC engine performance and combustor profile adaptation based on variable geometry combustor.The effects of flight Mach numbers,dynamic pressure,equivalence ratio,and drag in inner flow path on the geometry of the combustor were studied.The results showed that the expansion ratio of combustor should be adjusted according to the flight Mach number,and the combustor area should be determined according to different combustion organization.The variable geometry combustor enlarged the range of fuel equivalence ratio.The specific thrust and specific impulse performances of the engine could be obtained by matching the fuel equivalence ratio and the combustor profile.It meant that the specific impulse performance of the engine can be improved effectively while adjusting the thrust.The change of flight dynamic pressure affected the heat release of combustor.Different flight Mach numbers had their most suitable flight dynamic pressure boundaries,which can be used to optimize flight trajectory.Any drag in the combustor would result in a decrease in engine performance,and especially an increase in the drag coefficient in the scramjet mode results in a sharp drop in engine performance and an unreasonable demand for the combustor area,and efforts should be made to reduce the drag of the combustor.For the ramjet combustor of variable geometry RBCC engine with a wide operating range,the range of combustor absolute area ratio was determined to be 0.2~0.91,and the mode transition in the combustor can be reliably transited between Ma6~Ma7.(2)Investigation on the role of geometric throat of combustor.The geometric throat of combustor can be utilized up to flight Mach number Ma7 with the limiting temperature of 3000 K.After the flight Mach number Ma>7,the supersonic combustion mode was basically maintained in the combustor,and the combustor needed a full expansion flow path.The numerical simulation of the RBCC variable geometry combustor showed that the throttling of the geometric throat made the combustor in the subsonic state,which increased the residence time of the fuel and was beneficial to the combustion and heat release of the secondary fuel.It can effectively control the combustion heat release region and concentrate between the fuel pylons and the geometric throat.The variable geometry combustor can greatly improve the overall performance of the combustor under the same combustion organization.(3)Investigation on the influencing factors of variable geometry combustor characteristics.The primary rocket played a role in supporting combustion and increasing thrust.With the increase of the flow rate of the rocket,the effect of a rocket on the improvement of combustion efficiency was weakened,mainly due to the increase of thrust.The change in the geometric throat area of the combustor affected the strength of the pre-combustion shock train in the isolate section,and a certain amount of heat release in the combustion chamber needed to have a matching geometric throat area.The total temperature of the incoming flow affected the length requirement of the combustor.The combustion heat release length of the fuel was not too long at a higher total temperature,which was beneficial to shortening the length of the combustor.The use of concentrated injection fuel for concentrated combustion and heat release had higher combustor performance.When the flight dynamic pressure was low,the primary rocket had obvious combustion-supporting and thrust-increasing effect.At the same time,a proper amount of secondary fuel injection can ensure that the specific impulse of the engine remained unchanged and further increase the thrust of the engine.In addition,the combustor pressure was controlled by adjusting the geometric throat area and the combustor expansion ratio.On the one hand,it improved the start-up performance of the inlet when the secondary fuel in the lower Mach number combustor was burned with high equivalence ratio and high efficiency.On the other hand,it can increase the ability to resist the external inflow and the interference of secondary fuel combustion in the combustor,and improve the stability of the inlet start-up state.(4)Investigation on variable geometry combustor via ground direct-connect tests.The ability of the variable geometry combustor to operate in the wide range of Ma2~6 was verified.By replacing the rocket jet with a small amount of fuel in the isolator to stabilize the flame,the combustor realized the transition of the rocket-ramjet combustion mode with a high thrust to the ramjet combustion mode with a high specific impulse,which can be used as an effective technical way to improve the engine specific impulse.The validity of the design of the hydraulic drive mechanisms and the dynamic seals under high temperature condition was verified by the dynamic adjustment test of the combustor profile change,as well as the reliability of the hydraulic drive system in the hot tests.The system had been subjected to more than ten thermal shock tests.The 0.5 s geometry gradual adjustment control law can smoothly realize the change of engine operating condition,and the combustor pressure responded quickly without overshoot.(5)Investigation on full flow path integrated matching of a variable geometry RBCC engine.By coupling the combustion organization of variable geometry combustor with intake and exhaust,the matching between the combustor and the intake and exhaust in the wide range of Ma2~7 was realized,and the superior engine performance was obtained.Compared with the engine with fixed combustor,it was found that the specific impulse of the engine increases by 55% under the inflow conditions of Ma 2,the specific impulse performance of the rocket ramjet mode and the pure ramjet mode increased by 13.5% and 30.2% respectively under the inflow conditions of Ma 3,the specific impulse of the engine increases by nearly 10% in the range of Ma4~Ma6,and the specific impulse of Ma7 increases by 2.6%,which fully demonstrated the feasibility of improving the engine performance by changing the combustor profile according to the operating mode.