Investigation On Mixing Enhancement Mechanism In A Rocket-Based Combined Cycle Engine Under Ejector Mode Operation

In this thesis,the operating process of rocket-based combined cycle(RBCC)engine in ejector mode was taken as the research background.By means of numerical simulation and wind tunnel test,the flow and mixing characteristics of mixing layer with complex background waves formed by the rocket jet and the ejected air in the confined mixing duct were studied,and the mixing enhancement mechanism of different mixer configurations was revealed.On this basis,the effects of backpressure on the mixing behavior of primary and secondary flows were investigated,and the design guidelines of mixing and combustion section of RBCC engine under ejector mode operation were proposed.The matching relationship between parameters of rocket jet and entrained air has important influence on the flow structures and mixing characteristics in a constant-area mixing duct.As the total temperature of rocket jet increases,there emerges obvious thermal choking phenomenon in the mixer,and thus the mixing efficiency is significantly enhanced.When the rocket ejector operates in the over-expanded condition,the separation shocks created in the upstream mixer interact strongly with the mixing layer,which accelerates the mixing rate of two flows.However,the severe over-expanded rocket jet will lead to boundary layer separation in the downstream region,and the total pressure of the mixed flow is sharply decreased.When the rocket jet is in under-expanded state,the mixing efficiency is remarkably increased.It is advised that the primary and secondary flow pressure ratio should be controlled within2.5 in order to ensure enough air mass flow rate and avoid spillage phenomenon in the inlet.When the secondary air flow enters the mixer at transonic speed,it can be mixed more sufficiently with primary rocket jet.Besides,the LES calculation results show that due to the heat release process of high-temperature rocket jet,the transition in the mixing layer with large temperature difference is delayed,but after the occurrence of flow instability,the typical K-H vortex structures rapidly roll up,and the entrainment of large-scale coherent structures in the downstream flowfield promotes the mixing.It is also found that the development of RBCC subsonic-supersonic mixing layer is more restricted by the wall surfaces,and the large-scale vortex structures break into small-scale vortex structures after impingement,which is conducive to improving the mixing uniformity of primary and secondary flows.The converging-diverging mixing duct can effectively promote the rapid and fully mixing of primary and secondary flows in the RBCC ejector mode.Increasing the contraction ratio can significantly strengthen the interactions between background shock waves and mixing layer in the mixer,and thus the mixing efficiency is greatly enhanced.Shortening the length from the mixer inlet to the throat position can accelerate the mixing rate of two flows in the upstream region to some extent.However,in the downstream far-field region,the variations of throat position have little effects on the scalar mixing process in mixing layer.Besides,increasing the converging angle has no obvious influence on intensifying the mixing in the mixer.On the contrary,it may lead to a sharp decrease in total pressure recovery.Compared with the constant-area mixer,the converging-diverging mixing duct achieves better mixing performance.The oblique shocks formed at the compression corner and the separation shocks produced in the divergent section interact with the mixing layer,which induces mixing enhancement of primary and secondary flows.The vorticity of flowfield is significantly increased near the shock wave/mixing layer interaction point.Based on the analysis of vorticity transport equation,it can be found that the enhanced baroclinic effect caused by the interactions of shock waves with mixing layer is the key mechanism of rapid vorticity generation in the converging-diverging mixing duct.The mixing performance of mixer in the RBCC ejector mode can be considerably improved by using the wall-mounted cavity.When the cavity position is close to the mixer inlet,the large-scale separation bubbles formed by boundary layer separation intensify the scalar mixing process between primary and secondary flows downstream of cavity.As the cavity length-to-depth ratio increases,the acoustic disturbances emitted from the cavity rear wall obviously strengthen the interactions of cavity shear layer with mixing layer,and thus the mixing efficiency is significantly enhanced.However,when the length-to-depth ratio goes up to 11,the total pressure losses in the mixer are increased,and meanwhile there will be a slight decrease in mixing efficiency.Increasing the cavity aft ramp angle can trigger the instability of cavity shear layer,which is beneficial to promoting rapid and sufficient mixing.The wall-mounted cavity arranged in the upstream mixing duct is considered as an actuator without external energy addition into the flowfield.The acoustic waves induced by cavity self-sustained oscillations propagate downstream along the flow passage,resulting in strong interactions between cavity shear layer and mixing layer.The separation bubbles emerging near the wall surfaces keep squeezing the mixing layer,which considerably promotes the mass,momentum and energy exchange processes between upper and lower flows,and thus contributes to mixing enhancement in the cavity-based mixing duct.Compared with the non-backpressure condition,when the backpressure at the combustor exit rises,the interactions of shock train with mixing layer significantly promote the scalar mixing of primary and secondary flows in the constant-area mixer.By contrast,the downstream backpressure has less effects on the scalar mixing uniformity of two flows in the cavity-based mixing duct.Under the given inflow conditions,the resistance to backpressure of the constant-area mixer is the best,the converging-diverging mixing duct is the worst,and the cavity-based mixing duct is medium.By comparison,it is also found that the total pressure recovery performance of the constant-area mixer is most affected by the backpressure.When designing the mixing and combustion section of RBCC engine in practice,it is recommended to arrange the wall-mounted cavities in parallel in the upstream straight portion of mixing duct,which not only effectively achieves mixing enhancement of primary and secondary flows in the RBCC ejector mode,but also appropriately reduces the impacts of backpressure on the total pressure recovery performance of mixer.