Investigation Of Characteristics Of Combustion Organizations And Mixing In An Axisymmetric Rocket-based Combined-cycle Engine Operating In Ejector Mode

Ejector mode is most important to a rocket-based combined-cycle(RBCC)engine.The engine consumes a lot of fuel in this mode,and its performance directly affects the overall performance of the engine.In this paper,two different combustion modes of ejector mode,namely,SMC mode and DAB mode are studied.Theoretical analysis,numerical simulation,engine thrust test are used to make a systematic study.Finally,the mixing characteristics of the RBCC engine and the main performance characteristics and influencing factors of the two kinds of combustion organization are obtained.Through modeling and analysis,it is found that there is a big design contradiction in the intake and exhaust system of the wide-Mach region engine,and it is easy to produce the problem of insufficient thrust under the condition of low mach number.It is found that the duct area is the main factor affecting the wide Mach range of the engine.It can effectively improve the thrust and overall average specific impulse performance at low Mach number,and make the engine meet the performance index in a wide speed range,by appropriately reducing the Mach number of the engine design.The design point of RBCC engine needs to be optimized and designed according to the flight trajectory and mission requirements.The SMC mode is better for RBCC engine with supersonic cruise.It is not highly dependent on the duct area of the engine,and can make the engine obtain certain thrust gain,furthermore the structure is simpler.For RBCC engine with wide-speed maneuvering target,DAB mode is better.It has higher specific impulse gain and higher combustion efficiency in the ejector mode,and has obvious advantages over the rocket engine in the low Mach number flight.The entrainment ratio is the key parameter to determine the combustion modes.The RBCC ejector mode was measured in simulated high altitude environment,and the ultimate thrust performance of RBCC was obtained.The performance of DAB mode is closely related to the entrainment ratio of the engine.With the increase of the secondary flow air mass flow rate,the specific impulse gain of the engine will increase significantly.Because of the interaction between rocket gas plume and air stream in the duct,there will be aerodynamic choking in the engine.When the aerodynamic choking is in the third type of aerodynamic choking,the thrust performance of the engine is better,greater thrust gain.In the calculation and experiment,when the length of the straight mixing section reaches 5-6 times the diameter,the mixed air flow can burn stably in the secondary combustor,and the pressure in the combustor does not affect the air flow rate,when the simulated flight condition is Mach 1.2,the specific impulse gain of the engine is 16.5%.If the inlet area of the engine decreases or the rocket flow rate increases,the ejection coefficient of the RBCC engine decreases.The aerodynamic choking state in the mixed channel gradually changes into the second kind of aerodynamic choking,and the specific impulse gain performance of the engine decreases.The performance of RBCC engine with SMC mode is related to the combustion in the mixing layer.The air mass flow rate is no longer the main factor.The SMC mode is mainly related to the mixing effect of rocket gas plume and air stream.Enhancing mixing is the key to improve the performance of the engine in SMC mode.The results show that 8% specific impulse gain can be obtained at low Mach number,and the specific impulse gain is related to the equivalent ratio of rocket engine.In order to enhance the understanding of the flow mechanism in RBCC engine,the visualization experiment of axisymmetric compressible mixing layer was carried out,and the turbulent flow field structure in axisymmetric mixing layer was obtained for the first time using NPLS method.When the main flow is over-expanded,the mixing layer thickens and the mixing speed becomes faster,and when the main flow is under-expanded,the Mixing Layer thins and the mixing speed decreases.The higher the Mach number is,the faster the vortex structure moves and the slower the mixing speed is.The bigger the nozzle exit area is,the bigger the primary and secondary flow contact area is,the faster the mixing speed is.In order to enhance the mixing,the main flow nozzle should adopt the special-shaped nozzle,increase the contact area of the main flow and the secondary flow,and improve the mixing efficiency.The results of numerical simulation show that the mixing efficiency can be improved by decreasing the Mach number of primary and secondary flow and increasing the total temperature difference.For thermal protection,increasing the thickness of the rocket wall has little effect on the mixing rate.The main flow should be in the state of complete expansion or under-expansion to avoid the loss of total pressure.