Research On Gaseous Fuel Mixing Mechanism Of Transverse Jet In Scramjet Engine

Due to the high speed of the supersonic flow in the scramjet combustor,the residence time of fuel remains only a few milliseconds.Thus it has been one of the key challege to achieve rapid mixing and combustion of fuel and air in the engine design.Wall jet is one of the mainly used technology for fuel injection in scramjet combustor.Via experimental observation and numerical simulation methods,this study focuses on the mixing mechanism,penetration and lateral diffusion characteristics of wall jet in supersonic combustor.The jet mixing enhancement by using vortex generator are explored.The flow and mixing characteristics of the transverse jet upstream to the cavity are studied.The main findings and conclusions of this paper can help to improve the understanding of the mixing process in the scramjet combustor,and be useful to the engine design.To investigate the jet in supersonic crossflow(JISC)with different inflow conditions,two supersonic wind tunnel were disigned.A low noise supersonic wind tunnel is designed to provide supersonic flows with laminar boundary layer.Meanwhile,a direct connected wind tunnel specially for transverse jet is also accomplished so as to obtain a fully developed turbulent boundary layer.The Nano-tracer planar laser scattering(NPLS)technology and stereoscopic particle image velocimetry(SPIV)are employed for experimental measurement.Quantitative measurement data is acquired from JISC flow field with different incoming boundary layer conditions,JISC flow field under the control of vortex generator and JISC flow field upstream of cavity.Developing process and inherent mixing mechanisms of flow structure are analyzed based on experimental results.The flow fields of the JISC are further studied numerically by using the hybrid RANS/LES methods,the penetration and total pressure loss characteristics are discussed based on the numerical results.Fractal dimension analysis is used to evaluate breakup process of jet plume based on NPLS grayscale image.Two stages of jet breakup process are discovered.It is found that the jet plume with turbulent boundary layer breaks down more intensely than the one with laminar boundary layer.Empirical formula of jet penetration and total pressure recovery coefficient are fitted according to experimental and numerical results.Penetration and total pressure characteristics are evaluated for inclined injection.It turns out that there is an optimal value for injection angle between 45 degree and 90 degree to enable highest jet penetration as well as lowest total pressure loss.Velocity as well as vorticity field are obtained by SPIV technology.Evolutionary of counter-rotating vortex pair(CVP)and intensity changes of streamwise vortices are revealed based on the SPIV measurement.Each vortex of CVP is observated to become a surface vortex structure with elliptical velocity circulation distribution in the far field mixing region.The gap between outer vortex surfaces decreases gradually;the mass and momentum exchange between jet and surrounding fluid is quite few.The intensity of CVP keeps as constant in a long distance of JISC flow field,which results in poor mixing efficient in the far field mixing region.Since the mixing efficient of wall jet is not sufficient,mixing enhancement device(e.g.,vortex generator,VG)is recommended.However,momentum and total pressure loss caused by the parasitic device becomes a current problem.So the flow field with the implementation of VG are studied in this paper.Flow structure,penetration,lateral diffusion,mixing enhancement performance and mechanism of JISC under the control of VG are investigated.It is found that developing process of jet CVP is greatly affected by the VG wake flow.Both the formation and the consumption of CVP are accelerated;the velocity circulation increases at first and decreases immediately without a steady stage.This feature can enhance the mixing enhancement of jet and air significantly.Two different mechanisms are obtained for the interaction between the VG wake and jet plume with different distances between VG and jet orifice.When the VG is close to the jet orifice,the VG affect the jet plume mainly by enhancing both the penetration and the lateral diffusion in the near field mixing region.When the VG is far away from the jet orifice,the lateral diffusion is reduced,while jet penetration is greatly improved.Additionally,jet penetration has an optimal value as optimization design methods are used among VG configuration parameters,VG position and jet/mainstream dynamic pressure ratio.Experimental design and quantitative observation are carried out for flow past cavity with upstream transverse jet.Three modes of jet and cavity interaction are found via experimental observation,defined as strong interaction,moderate interaction and weak interaction.In the strong interaction mode,the CVP entrains lots of cavity fluid into jet plume in strong mode of jet and cavity interaction;in the moderate interaction mode,the mainstream fluid with high velocity enters into cavity so as to enhance mass and momentum exchange between mainstream and cavity;in the weak interaction mode,the mass and momentum exchange is not improved.Nonlinear programming analysis is carried out in order to determine the boundary lines of three interaction modes based on the experimental results.It is notable that the jet/cavity interaction modes are corresponding to the flame stabilization modes in the cavity-stablized combustor.The corresponding relationship is revelatory to ignition mechanisms and flame stabilization mechanisms.It is also valuable for fuel supply methods and injection schemes inside cavity combustor,which needs further exploration and research.