Large Eddy Simulation Of Atomization Mixing Enhancement In Scramjet

In order to improve the mixing and atomization performance of the liquid fuel in the scramjet,this paper have studied aerodynamic ramp,coaxial gas flow and rectangular jets.Numerical simulations have been carried out to study the mechanisms how these methods enhance the mixing performance.The Mach number and Weber number are set to 2.85 and 4983,respectively.A coupled Level Set/VOF(CLSVOF)interface tracking method is used in the two-phase simulations,which takes the supersonic flow as a compressible fluid and the liquid phase as an incompressible fluid.The atomization performance under different spray structures is analyzed quantitatively and qualitatively from the spatial distribution of transverse liquid jet and flow field.The conclusions are as follows:The aerodynamic ramp in the supersonic incoming flow has obvious lifting effects on the penetration depth of the transverse liquid jet.And the penetration depth of the transverse liquid jet can be increased by increasing the pressure drop of gas injection or decreasing the gas-liquid spacing.Aerodynamic ramp can significantly change the surface wave structures of the liquid column,resulting in the change of the column’s primary breakup morphology.Aerodynamic ramp can also increase the spray angle and the breakup length,making the jet breakup range wider and more uniform.Besides,aerodynamic ramp has introduced turbulent vortexes,which increases the complexity of the flow field and promotes the atomization of downstream droplets.The injection structure with smaller gas-liquid spacing and larger injection pressure drop has the best effect on improving the jet atomization performance.For the coaxial gas flow,two injection schemes have been set,namely peripheral coaxial gas flow and central coaxial gas flow.And three different injection velocities are designed for the peripheral coaxial gas flow.it is found that the addition of coaxial gas flow jet into supersonic flow can significantly improve the penetration height of the transverse liquid jet,and the performance of peripheral coaxial gas flow injection structure is better.By comparing the coaxial gas conditions with three different injection velocities,it is found that the higher the injection velocity is,the smaller the acceleration of liquid column is,leading to a longer breakup time and a larger penetration depth.Coaxial gas flow injection structure can also significantly improve the spray angle,and the central coaxial gas flow injection structure has better effect.Besides,the liquid column breakup position has been changed,resulting in larger breakup aera and better atomization performance.By comparing the primary breakup morphology,it is found that the central coaxial gas flow has the best mixing and atomization performance.Simulations of five rectangular jets with different aspect ratios were carried out by LES and the simulation results were compared with that of circular jet above.The results show that when the aspect ratio is greater than 1,there will be a more intense breakup and more dispersed droplet distribution from liquid column breakup and surface breakup.By comparing the penetration depth,it is found that the greater the aspect ratio is,the less the penetration depth is.The wavelength and amplitude of surface waves also increase with the increase of the aspect ratio,resulting in nearer breakup position,more intense breakup,bigger spray angle and less breakup length.The rectangular jet with greater aspect ratio will also make the flow field structure more complex,and the separation area in front of the liquid column will become larger,accompanied with bigger horseshoe vortex in the front of the liquid column and the vortex structure in the downstream breakup zone,as well as bigger wake vortex area and inversion vortex.It is concluded that the rectangular jets with aspect ratio greater than 1 have the best mixing and atomization performance.