Investigation Of Scramjet Mode In RBCC For A Concept Vehicle With Ascent Trajectory

The Rocket-based combined cycle(RBCC)engines take advantages of the synergistic interactions between the traditional liquid rocket and the air breathing engine.The system has better overall performance in wide flight airspace and full speed bound,which has prominent potential of greatly improving the efficiency of aerospace propulsion system and low cost,and therefore it likely to develop into future reusable spacecraft and propulsion system of hypersonic weapon.The steady and efficient operation of RBCC engine at scramjet mode is a crucial guarantee to achieve climbing trajectory mission.How to achieve efficient combustion organization in scramjet mode is the key technology to be solved urgently.Wide operating envelope of RBCC makes its own work process complicated.In addition to characteristics of strut rocket,changes in fuel status need to be considered.The applicability of the existing scramjet engine combustion organization technology in RBCC also needs to be investigated.In summary,these are more difficult to study on RBCC scramjet mode.In this dissertation,scramjet mode of central-strut RBCC engine for climbing trajectory was studied.RBCC engine configuration for the wide range of Mach number was designed.Numerical simulation method is established.The applicability and accuracy of the numerical simulation method are verified by a variety of examples.Three-dimensional numerical simulation and ground direct-connect test were used in study on pre-injection fuel program.A pre-injection fuel program suitable for strut-rocket was proposed.Influencing factors and law of multi-staged injection combustion technology were studied.The fuel injection strategy under different Mach number is obtained.A better performance was achieved in RBCC full flow channel at the range works in Ma4 to Ma8.The main work and conclusions are as follows:(1)Inner channel of shrinkage scaled engine was designed for RBCC which is used as first-stage propulsion system and horizontal zero speed takeoff two-level orbit aircraft mission.The design point of inlet is Ma6.Three waves mixed pressure structure is used.Capture area is 0.0394m~2.A rectangular cross-section flow channel was used in engine.inlet,strut-rocket,isolation section,combustion chamber and nozzle are binary structure.Isolation section was designed by shock wave length formula and experience.Combustion chamber configuration was designed by classic gas-dynamics relations and experience.The total length of combustion chamber is 1200mm.the expansion ratio is 2.26.(2)To improve the performance of the RBCC engine in scramjet mode,pre-injection fuel program was studied.Mechanism of ignition and stability about pre-injection fuel were obtained.A pre-injection fuel program suitable for strut-rocket was proposed and was verified using ground direct-connect test.The research results show that stable combustion can be achieved by combine upstream fuel pre-injection at strut with downstream fuel injection at pylon for kerosene at ambient temperature.Stable combustion can be achieved by direct fuel pre-injection and no need to inject fuel in downstream of combustion chamber for heating kerosene.(3)For typical flow conditions,law of staging combustion and injection strategy were studied in scramjet mode of RBCC.The study on the influence of the pylon spacing shows that the fuel pylon has a greater impact on combustion efficiency.The selection criteria for the space of pylon are obtained by analyze the relationship between the structural characteristics of the combustion flow field and the combustion efficiency.In the direction of the flow channel it is necessary to place the fuel strut outside the fuel-rich area,this area is formed by pre-injection fuel at bottom of the rocket strut.Fuel injected outside of pylon just contact with the wall flow should be ensured simultaneously.(4)The research of spatial distribution of pre-injection fuel shows that a method of dispersing the pre-injection fuel to different walls could utilize the oxygen in flow adequately,consequently to improve the engine combustion efficiency.While,at the same time,a strong block phenomenon happened,which would cause the inlet unstart for lower Mach cases.Therefore,the distribution strategy of pre-injection fuel should be determined though the incoming flow Mach and the inlet capacity to resist back pressure.For the case of lower Mach or weak capacity to resist back pressure,the program of single wall injection only should be used,adversely,the multi-wall injection program should be chosen.(5)The research of staging combustion injection strategy found that the thrust augmentation brought from the pre-injected fuel from the first stage was caused by the block effect of combustion heat release in isolator section,so the strength of block must be controlled moderately,for the purpose of avoiding inlet unstart or large flow loss while improving the combustion efficiency.When using staged fuel injection strategy,the fuel injected from the second stage come with the strut base of the rocket together make a low-speed region where heat was released,the slow down effect by the fuel from the second stage will weaken the block effect from the pre-injected fuel.(6)Considering the need of the vehicle concept,three-dimesional numerical simulation was employed to investigated the full flowpath.The combustion organization strategy was established based on the law of combustion,and the optimum equivalence ratio was chosen by numerical method.The results shows that at Ma6,the thrust of the full flowpath is 1790N,the specific impulse is 9970N·s/kg;at Ma7,the thrust of the full flowpath is 1771N,the specific impulse is 5616N·s/kg;at Ma8,the thrust of the full flowpath is 1714N,the specific impulse is 3527N·s/kg;the specific impulse or the thrust meets the need of the target of the concept.The full flowpath also has a relatively good performance at Ma4 and Ma5.The numerical results confirm that the designed configuration and the fuel injection strategy can close the climbing trajectory.