Research On Design And Optimization Of Scramjet Engine Flowpaths

The scramjet and components were designed and optimized by thermodynamic analysis,the quasi-one-dimensional estimated analysis and numerical simulations.The effect of design variables on system performance and components'behaviors was investigated.A design method including scramjet internal flow conceptual design,component design,performance evaluation and optimization was developed.The stream thrust method has been adopted to calculate each subsystem's in-/out-area and physical parameters.A mixed external and internal compression system,constant-area isolator,constant-area plus expansion combustor and a minimum length nozzle were adopted to design scramjet engines.A quasi-one-dimensional engineering estimation program and an exergy analysis programe were developed to evaluate scramjet engines.Optimizations with objectives of specific impulse and exergy efficiency separately and then together were conducted.The exergy loss distribution of scramjet engines and the effect of design variables on optimized engines were examined.Using this method,three-dimensional scramjet engines with four different static temperature ratios,pressure ratios and offset dimensions were designed,which contain Busemann inlets,constant-area isolators,combustors with a cavity and asymmetric truncated nozzle.Three-dimensional supersonic asymmetric truncated nozzles were designed and optimized.With the specific pressure ratio of nozzle's exhaust to ambient,reserved initial expansion ratio and average compression ratio,an axisymmetric minimum length nozzle was firstly designed using the method of characteristics.Then,streamlines trace technique with an offset circular entrance was adopted to extract the three-dimensional asymmetric nozzle.Nonlinear compression method was applied to compress the nozzle to a suitable length.Afterward,a surrogate based optimization of three design variables was performed with the objectives of thrust and lift force,and a Pareto optimal front was therefore obtained.Numerical simulations were also made at six selected Pareto front cases to offer an insight into the flow fields.An infection point was observed in the Pareto front due to the maximum constrained length.The pressure ratio was found to be the most influential parameter,and the middle parts of Pareto front were revealed better uniformity of exit flow.Transverse fuel injection upstream of a cavity in supersonic combustors were analyzed and optimized.The effect of four fuel injection position and different cavity dimensions on the combustor performance were investigated.Combustor with distance of 35 mm between fuel injection and cavity has better combustion efficiency and total pressure recovery coefficient.The Pareto front obtained by optimization of the cavity dimension can be divided into rapid change section,gentle change section and extraordinary change section,the corresponding shape of cavities are narrow and deep cavities,shallow and long cavities and medium deep and medium long cavities.The combustors in the gentle change sections has more uniform pressure and higher total pressure recovery coefficient.The length,depth and aft ramp angle of cavities in this section can be varied to a certain degree.Then combustors in gentle change section are preferred when there is no need of high combustion efficiency and total pressure recovery coefficient.