| Rocket Based Combined Cycle(RBCC)engine has a broad application prospects and important strategic significance.Flight vehicles can take off at the sea-level state and return to base site with propulsion regulating by RBCC operating in the ejector mode.In general,the RBCC engine configuration mainly considers demands on the combustion organization and propulsion performance in the ramjet and scramjet mode,and thereby the engine performance in ejector mode is mainly controlled by the primary rocket operating parameters and its jet state,confronting that the inlet is unstarted in the ejector mode.Unfortunately,the high propellant consumption in ejector mode currently constraints RBCC applications.Aimed to improve the engine performance in ejector mode,this dissertation studied matching of operation of the primary rocket with the flow and thermodynamics bounded in whole inner passageway and focused on uncovering the influence of primary rocket design parameters,jet state,secondary heat release on bypass ratio and engine performance.In this dissertation,the mechanism of the ejector mode engine was studied by means of thermodynamic cycle analysis,one-dimensional aerodynamic analysis and CFD simulation for the whole flow-path including the fore-airframe and aft-airframe of a flight vehicle.The matching principle of the secondary combustion organization strategy,the primary rocket mixture ratio,throttle ratio and jet state were obtained.The ejector mode specific impulse performance increased by 35.1%,and the conclusions can guide the design of the primary rocket.The main work and conclusions of the dissertation are as follows:(1)Mechanism of improving ejector mode performance.The basic physical process and thermodynamic cycle in the ejector mode under the constraints of flowpath configuration for a multi-mode RBCC were analyzed: the suction process of the primary rocket jet does a negative work.The pressure in the ramjet combustor is lower than that ambient,and a negative work encounters in the thermodynamic cycle of entrained air when there is no secondary combustion in the ramjet combustor,so that specific impulse performance of RBCC in ejector mode is not better than that of a conventional rocket engine.The higher bypass ratio and secondary heat release measures a better specific impulse performance in RBCC than that in a conventional rocket is obtained.(2)Influencing factors on bypass ratio.Increasing the bypass ratio is beneficial to improve the ejector mode engine specific impulse.Secondary heat release distribution,combustion heat distribution between the primary rocket jet and the air flows,the primary rocket throttle ratio and jet state have significant impacts on the bypass ratio:(a)The secondary fuel injection position should be moved downstream to weaken the inlet overflow and increase the bypass ratio,the bypass ratio could increase by 23.6%;(b)With the increase of the primary rocket mixture ratio,the combustion heat release intensity between the primary fuel-rich rocket jet and air flow,and the primary fuellean rocket chamber pressure are gradually reduced,the bypass ratio could increase by 99.5%;(c)Raising the primary rocket throttle ratio could increase the under-expansion degree of the primary rocket jet and the extrusion degree of the primary rocket jet to entrained air.The bypass ratio could decrease by 56.0%;(d)Increasing the primary rocket chamber pressure and nozzle expansion ratio can increase the bypass ratio.The pumping performance is enhanced and the bypass ratio could increase by 27.7% at the sea-level state.The total pressure of the mixed airflow is increased and the degree of inlet overflow is reduced,and the bypass ratio could increase by 10.0% when the ramjet combustor pressure is higher than that isolator outlet.(3)Matching principle of operation of the primary rocket with the flow and thermodynamics bounded in whole inner passageway.The studies of the matching of operation of the primary rocket with the flow and thermodynamics bounded in whole inner passageway reveals the matching principle between the high primary rocket combustor pressure and the low ramjet flow-path pressure,and that between the low incoming flow total pressure and the high ramjet combustor pressure.Understanding the function and design principles of the primary rocket,and the regulation strategy between the primary rocket throttle ratio and the secondary fuel mass flow rate.(3.1)Matching between the high combustor primary rocket pressure and the low ramjet flow-path pressure.The primary provides the main engine thrust,and the higher momentum jet could obtain the higher bypass ratio.The primary rocket with a higher combustor pressure could improve the total pressure of the mixture flow,increase the anti-pressure ability and then reduce the inlet overflow.The primary rocket jet with the high combustor pressure and limited blockage ratio could produce Fabri choking,therefore the combustor pressure of primary rocket is limited in the RBCC engine.The results of thermodynamics analysis and 3-D CFD simulations show that increasing the primary rocket combustor pressure and expansion ratio at the same time could avoid the Fabri choking,and then match the high primary rocket combustor pressure and the low ramjet flow-path pressure,finally increase the bypass ratio and engine performance.The engine thrust and specific impulse could increase by 35.4% and 35.2%,respectively.(3.2)Matching between the low total pressure of the incoming flow and the high ramjet combustor pressure.In addition to increasing the primary rocket jet momentum to increase the bypass ratio,but also should be stoichiometric and the secondary fuel should be injected in the secondary ramjet combustor to match the inlet overflow and the high ramjet combustor pressure.Secondary fuel injection principle of ejector mode: the secondary fuel should be injected in the downstream of the ramjet combustor in the premise of ensuring high combustion efficiency to reduce the inlet overflow and locate the thermal throat at the secondary ramjet combustor,therefore engine thrust and specific impulse could increase by 25.8% and 26.3%,respectively.In addition,the stoichiometric primary rocket jet with the downstream secondary fuel injection could match the low incoming total pressure and the high pressure of heat release.The fuelrich primary rocket jet could increase the upstream heat release,and then increase the inlet overflow,finally reduce the bypass ratio and engine performance.However the concentrated heat release between the oxygen-rich primary rocket jet and the secondary fuel could increase the ramjet combustor pressure,and then the pressure spread to the upper stream and increase the inlet overflow,therefore reduce the bypass ratio and engine performance.(3.3)The main function of the primary rocket could be summed up as:(a)Providing thrust.It is difficult to organize a strong secondary combustion heat release for the inlet is unstarted in the ejector mode.The primary rocket have to provide main thrust,especially in the ejector-suction stage;(b)Suctioning air.The primary rocket jet should suction the air when the ramjet pressure is lower than that isolator outlet,and then improving the circulating efficiency ramjet combustor by organizing secondary combustion;(c)Resisting the back pressure.The primary rocket jet could resist the ramjet combustor pressure and reduce inlet overflow,and then increase the bypass ratio,finally increase engine performance by increasing secondary combustion heat release.(3.4)Design principle of ejector mode primary rocket and the regulation law under the off-design point.Design principle of ejector mode primary rocket: Regardless of self-starting or boost,the design point should be self-starting or relay Ma,the primary rocket should be stoichiometric,the primary nozzle expansion ratio should be able to expand the jet completely under the limited blockage ratio,meet the engine thrust demand by adjusting the primary rocket throttle ratio and the secondary injection strategy at the off-design point.The general principle is increasing the degree of rocket throttling with the increase in the flying Ma,and then make full use of the entrained air in order to organize the secondary combustion in the downstream,the secondary fuel equivalent ratio should be 0.8 to obtain a higher specific impulse.(4)The high inlet contraction ratio and the large nozzle expansion ratio for the ramjet and scramjet mode are not conducive to the operation of ejector mode.The inlet area has an current-limiting effect on incoming air,excessive inlet contraction ratio reduces the bypass ratio and increases inlet resistance.Due to the low ramjet pressure and the high environmental pressure,the mixture flow would over-expansion for the excessive nozzle expansion ratio.The results of this dissertation show that it is necessary to adopt the variable structure inlet and exhaust system to improve the multimode RBCC engine performance. |
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