Combined Power Ejector Ramjet Channel Design And Operating Characteristics Study

The turbine-based combined cycle engine has the advantages of high specific impulse,wide range of flight speed and reusability,which is an ideal propulsion system for full-speed hypersonic flight.The introduction of the ejector ramjet channel structure in the combined power ensures a continuous,stable and sufficient thrust output during the thrust succession between the turbine engine shutdown and the ramjet engine startup,and compensates for the serious thrust deficiency problem during the switching between Ma2～3 turbine and ramjet power in the practical application of the turbinebased combined power.Firstly,through modeling analysis,the effects of different rocket design back pressure and flow ratio parameters selection on the performance of ejector ramjet channel are compared.The performance of the rocket decreases with the increase of back pressure when it is installed in the channel,and the magnitude of thrust gain is related to the flow ratio of the rocket and the incoming flow,which shows a trend of first increasing and then decreasing with the increase of the flow ratio.When the flow ratio is relatively low,the thrust of the channel can be doubled on basis of the ejector ramjet thrust by secondary fuel injection combustion.However,as the flow ratio increases,the oxygen concentration of the mixed gas decreases,the temperature rise decreases,and the relative proportion of the thrust increase caused by the secondary fuel injection combustion gradually decreases.Then,according to the analysis results,the ejector ramjet chamber was designed under the given parameters,and the three-dimensional simulation of the designed combustion chamber was carried out.It was found that the ejector ramjet chamber could achieve uniform mixing at the nozzle inlet when the ejector ramjet was working in the rocket ejector mode,and the rocket back pressure matched well with the static pressure of the mixer,and the introduction of the rocket could effectively improve the total pressure ratio of the chamber.The rocket in the same incoming flow condition can obtain a greater ducted rocket thrust gain for a low working condition,which ensures that a smaller flow ratio incoming flow condition corresponds to a larger ducted rocket thrust gain when the rocket working condition is certain.In the ramjet mode,the rocket back pressure and the ejector ramjet combustion chamber pressure match directly affect the ejector ramjet combustion chamber performance under secondary fuel injection,and the nozzle exit back pressure should be increased simultaneously after the combustion chamber pressure is increased.Rocket is used to assist the ramjet work by rocket ignition stabilization flame at Ma3.0,which can obtain better thrust performance.The full flowpath simulation yields a net thrust gain of nearly double in the ejector ramjet channel compared to the rocket high-altitude environment when the rocket starts to relay turbine’s operation.The net thrust and mass flow ratio impulse of the ejector ramjet channel increase significantly as the rocket working condition increases in the rocket launch mode,and the propellant flow ratio impulse and net thrust gain coefficient decrease in the high working condition compared with the lower working condition of the rocket,so in order to obtain higher channel thrust performance,secondary fuel injection afterburning is needed in the afterburning chamber.Under the premise of matching the rocket back pressure with the combustion chamber pressure,a smaller rocket operating condition with a larger secondary fuel injection volume is preferred to obtain a higher propellant specific impulse performance with a certain amount of fuel consumption.The matching strategy for ejector ramjet is as follows:the rocket starts at a low operating condition in order to relay the turbine’s propulsion,and a higher starting condition may cause the backpressure forward transmission to push out the inlet terminal shockwave,resulting in inlet surge or non-starting.It is advisable to adopt a lower expansion ratio of the nozzle to reduce the effect of transient backpressure precession on the normal operation of the combined inlet during rocket start-up.With the increase of flight altitude and Mach number,the rocket gradually increases the working condition to meet the thrust demand to relay the turbine channel and climbing process,and after the normal operation of the afterburning chamber,the rocket working condition can be appropriately reduced to increase the amount of secondary fuel injection to ensure the effect of combustion stabilization while obtaining higher fuel specific impulse performance.Finally,a ground-based direct-connected test sample was designed and a preliminary test program was developed.