Enhancement Of Fuel Injection And Mixing By Using Outflow Vortex Characteristics Of A Three-dimensional Inlet

Three-dimensional inward turning inlet is an ideal inlet configuration for hypersonic propulsion systems.However,there are always evident complex vortex structures on its exit plane.Previous investigation on scramjet combustor indicates that a well-organized inflow vortex flow pattern is helpful to enhance fuel injection and mixing efficiency.This proposal therefore suggests an integration design between a three-dimensional inward turning inlet and a scramjet combustor.Numerical investigation on fuel injection and mixing performance of the three-dimensional inlet/scramjet combustor integration schemes and injection schemes with uniform incoming flow has been carried out,which helps to analyze effects of complex vortex pattern of the inlet on the mixing performance of the combustor,and provides a further understanding of the turbulent flow physics in three-dimensional scramjet internal flow path and the dominant factors of mixing process.First of all,flow features and vortex characteristics of three-dimensional inward turning inlet at the design point were analyzed.The chosen inlet can capture almost all the incoming flow.The design of rectangular-to-elliptical section and interaction between three-dimensional circular reflected shock/boundary layer is helpful to generate streamwise vortices.Lateral pressure gradient of the inlet forces these streamwise vortices to flow towards the long axis sides of elliptical duct,then merge.Secondly,a scramjet combustor with uniform incoming flow was employed to study effects of momentum flux ratio,boundary layer thickness,injector diameter,wall curvature and shock reflection on fuel injection and mixing characteristics.It can be found that there is a critical momentum flux ratio for every specific boundary layer status.Only when the momentum flux ratio is less than that value,increasing boundary layer thickness could effectively improve the jet penetration and mixing efficiency.Otherwise,the variation of the boundary layer rarely affects the fuel penetration and mixing performance.Effects of wall curvature on mixing performance are highly related to injector diameter.For the case with large injector diameter,decreasing the wall curvature enhances mixing efficiency of the fuel.However,for the small diameter case,variation of wall curvature rarely affects the mixing performance.At the same momentum flux ratio condition,similarities are observed regarding the flow characteristics,penetration depth and mixing efficiency of different diameter cases.Additionally,interaction between reflected shock and mixing layer in the combustor substantially reduces penetration depth,but enhances mixing efficiency due to the intense shearing motion caused by the interaction.Finally,fuel injection considering outflow vortex characteristics of the inlet was proposed and investigated numerically.Fuel injection in the inlet/combustor case has better mixing efficiency and speed than the schemes with uniform incoming flow,since the outflow vortex structures of the three-dimensional inlet greatly enhance fuel/air mixing performance in the combustor.Excluding the mixing increment caused by the boundary layer,the streamwise vortex structures of inlet contribute approximately 65%enhancement of mixing efficiency.Injection positions for the inlet/combustor schemes are more favourable for fuel mixing by considering the effective averaged vorticity magnitude of the inlet cross-section.Larger effective averaged vorticity leads to higher mixing efficiency.