Design And Performance Study Of A Ma0-7 Over-Under TBCC Exhaust System

TBCC combined propulsion system has become a research hot spot in recent years for its high specific impulse,reusability,and ability to take off and land horizontally.As the main thrust generating component of combined propulsion system,exhaust system plays a very important role in the overall performance of aircraft.In this paper,the design procedure and method of an over-under TBCC combined exhaust system with speed up to Ma=7 will be explored by theoretical analysis,numerical simulation and wind tunnel experiment.Firstly,considering the geometry size limitation of aircraft,a general size constraints of hypersonic vehicle exhaust system are summarized through literature research.The ramp and flap contour is obtained via the maximum thrust nozzle design method and then be truncated according with the specific size limitation.On this basis,the throat adjustment scheme for the ramjet flowpath has been determined to use a cowl plate rotation composition.Then the author uses numerical computation means to compare the performance of two different throat adjustment schemes for the turbo flowpath,i.e.the schemes of splitter plate rotation and rotation around rear shaft.The results show that the splitter plate rotation scheme can obtain better aerodynamic performances in all working condition,especially approach to the modal transition process.After the structure scheme of TBCC exhaust system has been ultimately determined,Ma=7.0 is selected as the finest design point of exhaust system.The thrust performance can get a relatively balance under different flight conditions,and the thrust coefficient reaches above 0.96 at cruise point.Secondly,the three-dimensional(3D)numerical simulation of typical working conditions in flight envelope has been conducted for the designed over-under TBCC exhaust system,and the variation rules of flow field characteristics and aerodynamic performance have been obtained.The results show that the thrust coefficient of TBCC exhaust system remains above 0.9 throughout the flight envelope,the value keeps above 0.96 during ram mode process and can reach 0.961 at the cruise point.Yet,due to the appearance of serious over expansion phenomenon,the thrust performance is relatively poor during transonic flight process,and the lift of the exhaust system varies greatly with different working modes.With the purpose of further understanding the performance variations in the process of mode transition,a quasi-dynamic three-dimensional numerical simulation of the exhaust system has been carried out.The results indicatives that the lift varies greatly before and after mode transition process,and the thrust coefficient can be maintained above 0.9.In order to verify the accuracy of numerical calculation results,a scaling model of over-under TBCC exhaust system has been designed and a series of cold-flow wind tunnel experiments were carried out.The pressure distribution on the walls which provide most thrust for the propulsion system and schlieren images of flow field under different working conditions were obtained.The good agreement of the experiment and numerical calculation results has proved the accuracy and reliability of both results.Finally,for gaining a more comprehensive and profound understanding of the dynamic characteristics of the exhaust system's aerodynamic performance and flow field structure during a specific mode transition process,unsteady numerical simulation of the complete transition process has been carried out by using the dynamic grid technology.The results show that the structure of the exhaust flow field is rather complex and the wave-shock structure produced by the interaction of different flow is very obvious.In the process of mode transition,the thrust coefficient of exhaust system keeps above 0.9,and the low thrust performance of ramjet nozzle at the initial period has little influence on the total thrust.Due to the combined effect of rapidly growing nozzle pressure ratio(NPR)of ramjet nozzle and the rotation of cowl plate,the lift varies greatly during the mode transition process.