Investigation Of Ramjet Mode In RBCC For Wide Adaptability Based On Thermal Adjustment

Rocket based combined cycle(RBCC),as one of the most important direction for future space propulsion,has many technical characteristics such as wide operating range,high specific impulse,large thrust and reusability,which is suitable for space transportation,missile weapon,hypersonic flight vehicle and other application fields.With the characteristics of operating in wide envelope and trajectory,RBCC engine needs to realize ejector mode,ramjet mode and scramjet mode operating in the same flowpath.Different from the traditional ramjet engine,the geometry throat is replaced by the thermal throat to realize efficient work in the expansion flowpath,but the performance with thermal throat is lower than geometry throat.The thermal throat is influenced by many factors with complex regularity,thus achieving high-performance with thermal throat is difficult.Moreover,with the wide operating range of ramjet mode itself,realizing the efficient work in the fixed flowpath is extremely hard,taking into account the performance of ejector and scramjet mode at the same time.Therefore,in order to improve the whole performance of RBCC engine for wide flight Mach number,the ramjet mode needs to be investigated based on the flowpath design and adjustment of thermal throat.In this dissertation,taking into account a central-strut RBCC engine which adapt to the wide operation range as the research object,investigation on the ramjet mode of RBCC based on thermal adjustment has been carried out,using research methods contains the thermodynamic and one dimensional model of ramjet mode,three dimensional numerical simulation and direct experiments.The method of flowpath design and thermal adjustment in a wide operating range of RBCC engine were established,and the influencing factors and regulation law of thermal throat,the multistage fuel injection strategy based on the thermal adjustment were obtained.The performance of full flowpath in Ma2-7 range were achieved.The main research contents and conclusions were as follows:(1)Considering the thermal throat formation in the expanding combustor,the thermodynamic cycle model of RBCC in ramjet mode was establish and the configuration parameter design of Ma3-6 full flowpath was conducted.The configuration parameteters of the key cross-section was determined under the different condition of flowpath,and the range of the configuration parameters was cleared.The inlet with variable contraction ratio and the nozzle with expansion ratio of 1.8 and the fixed geometry combustor with expansion ratio of 1.5~2.7 which need optimization design were chosen for RBCC engine operating in Ma3-6.(2)The one-dimensional analysis model of RBCC combustor in ramjet mode was established.The coordinate of exothermic law between the gasous rocket jet and the liquid kerosene was well solved through putting forward a double exothermic model.A method of the equivalent flowpath area correction was established to overcome the traditional one-dimensional model in describing the rocket jet with the difficulties on the three-dimensional flow field.The onedimensional model can be used for the calculation of thermal throat and the parameters of RBCC combustor under the interaction of area variation,exothermic distribution and rocket jet.The model was validation through three-dimensional numerical simulation and direct-connect testing results.The one-dimensional model was adopted to design operation condition in Ma3-6 range.The two-stage combustor with different expansion ratio was put forward to satisfy the operation in Ma3-6 range.The expansion ratio of the first stage was between 1.8~2.2,and the expansion ratio of the second stage was 2.7.(3)The influencing factors of thermal throat was investigated through one-dimensional analysis and three-dimensional numerical simulation.The results of one-dimensional analysis showed that the smaller of expansion ratio,the more of exothermic quantity,the slower of heat release,the more back of injection location or the smaller of rocket exothermic quantity,the latter thermal throat forms in the flowpath.The more after of thermal throat position was,the bigger of integral thrust could be obtained when the expansion ratio of combustor and total exothermic quantity were the same.Therefore,in the process of practical operation,the performance of engine could be improved through adjusting the thermal throat to generate at the end of the combustor with the optimization of fuel injection position which match with the equivalence ratio.The influencing law of exothermic quantity,injection position and rocket flow rate on the thermal throat was validated by the results of three-dimensional numerical simulation.The performance was improved by 11.8%~23.3% through thermal adjustment.For the condition of Ma3 and Ma4 in which the total pressure of the incoming flow was relatively low,adopting the two-stage injection strategy could effectively balance the operation characteristics of ramjet mode and thermal throat adjustment.The performance of Ma3 and Ma4 were improved by 12.3% and 14.6% respectively.The influencing factors and adjustment method of thermal throat were obtained through the research.(4)The direct-connect experiments of thermal throat were performed.The experimental results of Ma5.5 showed that the engine performance was improved by 3.0%~8.3% through optimizing the fuel injection position.The experimental results of Ma4.0 showed that the thermal throat developed from nothing and generated gradually backward with the increase of equivalence ratio.The comprehensive performance of thrust and specific impulse achieved the optimal value when the thermal throat located at the end of the combustor.The efficiency of improving the engine performance through thermal adjustment was verified by experiment research.The suitable position of cavity was determined according to the the experimental results of Ma5.5 and Ma3.0 with the purpose to realize the flame stability and better combustion performance.The radiation spectrum method was used for temperature measurement in the flow field of thermal throat.Through the temperature and pressure measurement along the flowpath,the Mach number calculated in experiment had the same variation trend with the numerical simulation results in the flow field of thermal throat,which characterized the generation process of thermal throat where the flow accelerating from subsonic to supersonic.(5)The flowpath design method was established based on the thermodynamic and one dimensional analysis.The design of RBCC full flowpath operating in Ma3-6 was carried out,which adopting a 2-D inlet with the variable throat of two contraction ratio 3.8 and 7.6,two stage combustor with different expansion ratio of 2.1 and 2.7 and the nozzle with expansion ratio of 1.8.The fuel injection strategy was determined based on the thermal adjustment and the numerical calculation of full flowpath was performed.The results showed that the matching work and 869s~1115s performance of full flowpath in Ma3-6 were achieved through the combustion organization in the region of different expansion ratio(Ma4-6 in the first combustor,Ma3 in the second combustor),with multi-stage fuel injection and composite flame stability,which verified the efficiency of flowpath design method and thermal adjustment.Combination the calculation results of Ma2 and Ma7 conditions,showed that the full flowpath of RBCC engine designed in this dissertation could obtained better performance in the range of Ma2-7.The full flowpath and fuel injection strategy with wide adaptability were obtained through the research.(6)The different combustion organization of full flowpath was investigated through threedimensional numerical simulation.The results showed that,for the operating condition of Ma5 and Ma6 which the total pressure of the incoming flow was relatively high,the better performance of large equivalence ratio can be achieved through adopting the more front of injection location with the recirculation region of rear-faced step at the entrance of combustor.For the operating condition of Ma3 which the total temperature of the incoming flow was relatively low,the rocket jet of high temperature shorten the interval of mixing and combustion and realized the stable and efficient combustion of fuel in the expanding flowpath with thermal throat.The performance of RBCC operation mode was better than pure ramjet.