| Rocket Based Combined Cycle(RBCC)propulsion system is one of the best candidates for application in single-stage-to-orbit(SSTO)or two-stage-to-orbit(TSTO)launch vehicle.The addition of the rocket jet makes it propel the flight vehicle to takeoff at Mach 0,which means it can operate in the whole flight envelope.The dual-mode,namely both subsonic and supersonic modes,is the primary phase in for the real hypersonic flight process.The ability to achieve a smooth mode transition is related to the success or failure of the engine operation.However,the research of the opearing characteristics of mode transition for RBCC engine is rarely found in open data.Therefore,to enrich the methods of the mode transition and provide the basis for the advantage on the wide envelope in RBCC engine,some exploration should be carried out.The RBCC flow-path which is installed one central strut with large occupied volume ratio(the occupied volume ratio,which means the ratio of central strut cross-section area to flow-path cross-section area is 30%),is employed as the research object in this thesis.Through the methods of experimental measurement and numerical simulation,the characteristics of cold flow field,the model of RBCC pre-combustion shock train,the interaction between isolator and combustion chamber,and the characteristics of different combustion modes were studied.The main work and conclusions in this thesis are as follows:(1)The isolator cold-flow experimental system was established.Based on the showdow experiments and numerical simulation,the study of the regular of the pressure rise and the shock train structure was carried out.At the ground atmospheric pressure,expansion wave is formed at the base of the strut and the subsequent shock appears as the inflow impacts the shear layer.As the back pressure increases,the range of the expansion wave would be decreased and the shear layers turned from converging to the axis to be parallel to the axis.Thus,the leading edge of the impacted shock wave moves upstream.When the starting shock moves into the strut section,many bifurcate shock waves are found in the mixing section.The addition of the primary jets with low mass flow rates,just altered the structure of the recirculation region at the strut base,which leads the shear layer squeezing the inflow.The results for different Mach numbers and back pressures showed that the whole shock train structure in the isolator was composed by both the cylinder shock waves and jet-constrained flow shock waves.(2)Based on the RBCC isolator shock train structure and the ground direct-connect combustion experimental data under different inflow Mach numbers,the classic Waltrup empirical formula was modified by adding the parameters like the equivalence ratio and the strut occupied volume ratio r.Thus a new empirical formula which is appropriate for predicting the RBCC isolator performance was established.By inserting this new correlation to the 1-D model of the flow-path performance,the isolator-combustor performance under Ma=3~6 conditions and the full flow-path(inlet-isolator-combustor)performance under Ma=6~7 conditions were calculated and compared with the 3-D numerical results.It showed that this new correlation could effectively estimate the length of the shock train and the isolator performance at combustion conditions.(3)By injecting liquid kersosene at relatively high equivalence ratio,the ground direct-connect experiments at Ma=5.5 which is characterized for the subsonic/supersonic mode transition was carried out.Based on the value of the combustor inlet Mach number,4 typical combustion modes are obtained: weak combustion mode,rocket-scramjet mode,rocket-subsonic mode and subsonic mode.At the weak combustion mode,only faint pyrolysis reaction was found at the upstream of the rocket jet.A large amount of black smoke was formed in the combustor and the shock train structure was similar to that of the ground atmospheric pressure.The addition of the cavity could enlarge the reactive zone in the wide direction.The heat release would then lead the inflow in both sides thermal choking,which forms the rocket-subsonic combustion mode.When the distance between the fuel strut and the cavity is appropriate,a stable flame source is formed.In this condition,a stable combustion could be maintained when the rocket is turned off.Thus,this combustion mode is called subsonic mode.(4)The influence of the fuel injector position,the position of cavities and the addition of rocket jets on RBCC mode transition were studied.When the rocket jets are turned on,the change of the fuel strut position could cause mode transition.Furthermore,as the rocket is turned off,different axis positions where the cavity is installed could lead to the mode transition as well.When the combustor is operated in scramjet mode,which is characterized in the primary heat release zone being located at the downstream in the combustor,the addition of the rocket jets would make the burning zone jump to upstream abruptly.Thus the combustion was transferred to(rocket-)subsonic combustion mode.The discovery of this phenomenon provides a new method and feasible strategy for the mode transition in dual-mode scramjet.(5)Based on the flow-path configuration where the two cavities were installed in parallel,numerical simulation was carried out under different fuel injecting equivalence ratios and different total temperatures of inflow by using DES(Detached eddy simulation)method.The results showed that under the effects of rocket jets,the mode transition of this flow-path occurs at the range of Eq=0.18~0.28.The analysis of the thrust performance at different equivalence ratios shows that the phenomenon of the thrust jumping was not found during mode transition in RBCC engine.Therefore,the addition of rocket jet can provide an effective solution for realizing the mode transition smoothly and stably.The two-phase flame surface is composed by two parts.One is originated from the fuel strut base and extended to its shear layer.The other one is formed from the shear layer in the central strut base.At rocket-scramjet mode,the reaction zone is concentrated in the core flow from the back wall of central strut to the leading edge of fuel strut,forming the two-stage main reaction heat release zone.At this time,the airflow at two sides is still supersonic and the flame stability manner is in jet-wake stability manner which is leaded by the rocket jet.In the rocket-subsonic mode,however,the high-temperature core reaction zone guided by the rocket jets is separated from the reaction zone which was originated from the fuel strut and the shock train in RBCC isolator is made up by the oblique shock train in the upstream strut section and the quasi-normal shock train at the downstream.The shock train appears in the isolator to reduce the inflow velocity to subsonic.In this combustion mode,the gaseous fuel which is generated by the cracking process of the liquid kerosene would react and produce the third heat release zone under the flame surface originated from the shear layer in the fuel strut.Therefore,the thermal throat would be created in the whole wide direction and the combustion mode is transferred to rocket-subsonic mode.The flame stability manner is the coupled jet wake-cavity shear layer flame stability method in this combustion mode. |
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