Optimization Of Fuel Jet Flow Field And Investigation On Its Mixing Enhancement Mechanism In Supersonic Flows

The scramjet(supersonic combustion ramjet)engine as the next-generation engine airbreathing is one of the most promising propulsion systems,and it produces power for hypersonic vehicles.It is the most effective engine cycle when vehicles fly in or beyond the supersonic speed.In the scramjet combustor,the mixing process is the initial phase for all the physical ones,and it is the primary factor to restrict the combustion process.Hence,a sufficient mixing is the key to realize the airbreathing hypersonic propulsion system.It is a challenge for a sufficient mixing between the supersonic incoming air and the fuel due to the short residence time of airflow within the scramjet combustor being the order of milliseconds for typical flight conditions.It attaches profound importance to conduct a survey on the mixing augmentation mechanism in the transverse gaseous injection flow field and design a good injection strategy with large penetration depth,high mixing efficiency and low stagnation pressure loss within the scramjet combustor.Firstly,transverse jet in supersonic crossflows is introduced.Then,the research progress of mixing techniques for transverse injection flow fields in supersonic crossflow are summarized from several aspects,namely the single injection flow field,the multi-port injection flow field,the air jet flow field,the pulsed jet flow field and the transverse injection flow field with a vortex generator.Secondly,the numerical approaches utilized in this thesis are introduced.The results predicted by the three-dimensional Reynolds-average Navier–Stokes equations coupled with the two equation k-? shear stress transport(SST)turbulence model are compared with the available experimental data in the open literature.It is concluded that the mathematical and computational models can be used with confidence to investigate the transverse gaseous injection flow fields.Thirdly,the mixing enhancement induced by a micro-ramp is evaluated numerically.The influences of the micro-ramp height and the jet-to-crossflow pressure ratio on mixing efficiency,penetration depth,and stagnation pressure loss are investigated.Then,the combination of a micro-ramp with pulsed gaseous jets is proposed.The obtained results show that the pulsed jet flow field with the periodic variation of the jet-to-crossflow pressure ratio of the fuel injection owns better comprehensive performance,and the penetration depth and mixing efficiency are improved simultaneously.Subsequently,the dual transverse injection system with a front fuel porthole and a rear air porthole arranged in tandem is proposed.This configuration is investigated in terms of variation in the aspect ratio of the air porthole in a parametric study.The influence of the aspect ratio of the air porthole on the supersonic mixing process between the fuel and air is evaluated.The air jet not only enhances the mixing efficiency,but also improves the fuel penetration depth when the aspect ratio is large enough.However,the dual injection system has more stagnation pressure losses than the single injection system.Then,the extreme difference analysis approach is utilized to obtain the deep information in the supersonic jet-to-crossflow flow field with a micro-ramp vortex generator,and three optimization micro-ramp configurations corresponding to three different objectives,i.e.the minimization of the mixing length,the maximization of the penetration depth and the minimization of the stagnation pressure loss,are obtained and verified.Finally,a new injection strategy made up of a micro-ramp and an air porthole is proposed and integrates the advantages of the micro-ramp and the air jet.The transverse injection flow field with lower stagnation pressure loss and shorter mixing length is realized.