Research On Supersonic Turbulent Flow And Mixing Mechanism Of Gaseous Fuel Injection In Circular Cross-section Flow Channels

Scramjets with circular cross section were widely used in hypersonic aircraft.There are many curved structures in the flow channel of scramjets,which have significant influence on the fuel mixing.The curruret study focued on the mechanism of the fuel mixting,scale effects,interaction of jets with several orifices in the circular scramjets.Several methods were conducted to reveal the development mechanism of the secondary flow and its effects on fuel mixing charateristics,including numerical simulation,data fitting,NPLS,PIV and oil-flow experiments.Analysis and discussion about the interaction between secondary flow and transverse jets and the interaction were given,and the mechanism of fuel mixing in the circular scramjet was obtained.In this paper,the mixing characteristics of jet from plat are studied by numerical method and theoretical derivation.The results show that the turbulent boundary layer has great influence on the jet penetration and mixing efficiencey.Thicker turbulent boundary layer can provide larger expansion space for the jet gas,and then promote jet penetration and mixing.The dimensionless penetration height and mixing efficiency remain unchanged when the boundary layer and the orifice change in equal proportion simultaneously.According to theoretical derivation,the boundary layer thickness does not change linearly with the scale of the aircraft,but has a 4/5 power relationship with the change of the aircraft size.For some specific flow field structures,such as bow shock wave and recircualtion zone,the change of orifice scale or injection pressure can only influence the size of them,not the bow shock angle and separation angle.The flight speed would change the flow field structure sharply.For the parallel multi-injection,the interactions of jet plume and shock wave are quite different as the jet distance changes.This paper will reveal the development mechanism of jet plume.The curved pipe structures are very common in the front section of the inner passage of the aircraft.In this paper,NPLS experiments and large eddy simulation(LES)were combined to reveal the secondary flow characteristics and the turbulent boundary layer development mechanism of supersonic flow in the curved pipe.The secondary flow in the central slice of the curved pipe is mainly affected by centrifugal force and pressure gradient.Under the action of centrifugal force,the secondary flow in the central slice flows from inner part to outer part.On the contrary,in the near-wall region,the fluid moves from the outer part to the inner part under the action of spanwise pressure gradient and viscosity.Affected by the wall curvature in the curved part,the development of boundary layer is asymmetry significantly,with the outer boundary layer gradually thinning and the inner boundary layer gradually thickening.This varation is accelerated by the secondary flow,which draws the circumferential low-energy fluid to the inner side of the curved part,resulting in a low-energy zone in near wall regiom of the inner part.By analyzing density gradient and baroclinic production distribution,this paper reveals the development mechanism of the supersonic turbulent boundary layer in curved circular pipes and its interaction characteristics with oblique shock waves.The flow field structure and mixing mechanism of gaseous fuel injection in a limited a circular flow interval channel were studied by means of LES,and the NPLS experiments were supplement.The bow shock generated would reflect repeatedly in the pipeand interact with the jet shear layer to promote mixing.For the single jet in the circular channel,the bow shock interacts with turbulent boundary layer on the opposite wall to generate a separation zone,and a separation shock formed upstream the separation zone.The separation shock and the bow shock interact with each other in the downstream region,leading to total pressure loss and promoting jet mixing finitely.For the double jets,the bow shocks intersect with each other and reflect on the shear layer of the jet plume,forming an oblique shock wave train that propagates downstream.The interaction between the bow shock wave and the jet shear layer results in a bulge recirculation zone in the jet leeward,which significantly promotes gas mixing.Combined with the study on the flow field structure of secondary flow in curved pipe and jet flow in straight pipe,the effects of S-bend with different deflection angles on jet mixing is compared.The complex secondary flow structures and separation zone characteristics downstream the S-bends are revealed.The direction of the Dean Vortices generated in the upstream part of S bend is opposite to that of the dean vortex generated in the downstream part,and the Dean vortex in the upstream part is surrounded by the Dean vortex in the downstream part.The double jets appear obvious asymmetry due to the secondary flow,which becomes more and more obvious with the increase of deflection angle.By comparing the interaction and mixing characteristics between jet and secondary flow at different positions,the optimal jets location for S post-jet injection scheme was proposed.