Study On Growth Characteristics And Mixing Enhancement Mechanisms Of Supersonic Mixing Layers

The mixing and combustion process between the high-speed airstream and fuel gas in the combustor has a large effect on the thrust performance of combined cycle engine.However,due to the extreme short residence time,it is difficult to mix the air and fuel completely within the combustor of reasonable length.Meanwhile,the mixing is less efficient in supersonic mixing layer since the compressibility suppresses the growth rate of mixing layer.Thus,a rapid mixing between the high-speed airstream and fuel gas is required.Herein,the flow field of supersonic mixing layer is obtained using the hybrid Reynolds-Averaged Navier-Stokes/Large Eddy Simulation(RANS/LES)approach and the experimental techniques of Nano-tracer Planar Laser Scattering/Particle Image Velocimetry(NPLS/PIV).Through these numerical results and experimental data,the growth and mixing characteristics of mixing layer are analyzed.Afterwards,the influence of upstream forcing,shock wave,streamwise vortices and pressure gradient on the development of supersonic mixing layer is studied.Finally,assessment of the efficacy of these mixing enhancement techniques is discussed.Firstly,the entrainment of large-scale coherent vortices,small-scale turbulence transport and the final molecular mixing are analyzed.Based on the three-step mixing stage,the global growth rate and mixing rate of supersonic mixing layer are proposed.The full evolution process of large-scale vortices in spatially developing supersonic mixing layer is presented clearly.The initial development of mixing layer is dominated by the two-dimensional coherent structure.During the transition process,the hairpin vortices become the predominant large-scale structures.Due to the upper and lower stirring of hairpin vortices,the thickness of mixing layer increases gradually.Further downstream,the hairpin vortices break up into small slender vortices.Meanwhile,because of the formation and evolution of large-scale vortices,the scalar interface is stretched and distorted,which contributes much to scalar mixing.In addition,the statistical scalar preserves a very good self-similarity for the full-developed mixing-layer flow.Secondly,the influence of upstream forcing on the development of supersonic mixing layer is investigated by LES method.Special attention is paid to the effect of forcing frequency and amplitude on the properties of large-scale structures.The forcing frequency yields a predictable and repeatable response,which can organize the eddy rollup and interactions.The size of saturate eddies is determined by the convective velocity and forcing frequency,and is independent on the forcing amplitude.The high forcing frequency and amplitude can increase the initial growth rate of mixing layer.However,it inhibits the growth of other harmonics,which is detrimental to further development of mixing layer.For the mixing enhancement of supersonic mixing layer,the double-frequency forcing with high amplitude is an effective way to perturb the mixing layer.Besides,the thickness of trailing edge has a huge effect on the initial development of mixing layer.the size of eddies in mixing layer is dominated by the shedding frequency when the thickness lies in the proper range.Meanwhile,the sawtooth configuration at the trailing edge can enhance mixing.According to the mechanism of an oblique shock interacting with supersonic mixing layer,a simplified model to solve the post-shock flow parameters of mixing layer is established.This model can be used to estimate the growth rate of mixing layer impinged by shock wave.Afterwards,a shock-induced vorticity variation model is built to assess the maximum vorticity amplification of mixing layer.In the more general case of mixing layer,the post-shock growth rate and vorticity will be amplified.However,the vorticity is likely to be attenuated for the special case of mixing layer.This is because that the velocity-attenuation degree between the two streams is different for mixing layer.Furthermore,the interaction of an oblique shock and mixing layer is studied by numerical simulation and experimental measurement.Results show that the oblique shock has a slight effect on the evolution of large-scale vortices,but affects the small-scale transport in the mixing layer.The turbulent intensity and Reynolds stress will be amplified by the shock wave.The entrainment and mixing characteristics of large-scale streamwise vortices in supersonic mixing layer are studied experimentally.There are two mechanisms to enhance mixing by the streamwise vortices: one is that large amounts of surrounding fluid around the two streams are entrained into mixing zone due to the formation and evolution of streamwise vortices;another is that the large-scale streamwise vortex breaks up into the small-scale structures through its interaction with the spanwise vortices.The former can provide an opportunity to enhance the scalar mixing,and the latter substantially increases the interfacial area of scalar and ultimately contributes to the molecular mixing.In addition,the emergence of the secondary K-H vortices and T-shaped structures in the full-breakup stage of mixing layer suggests that the two structures may be the usual topological structures in small-scale transport.In addition,the mixing mechanisms of a rectangular lobed mixer are analyzed as well.The convoluted shear layer forms downstream of the trailing edge of lobed mixer,and it can increase the scalar interfacial area.Further downstream,the shear layer is stretched and rotated by the streamwise vortices.This process increases the interfacial area,and promotes the scalar mixing.It is suggested that the rectangular lobed mixer can enhance mixing in supersonic mixing-layer flow effectively.Finally,the effect of streamwise pressure gradient on the growth characteristic of mixing layer is studied preliminarily.The confined wall suppresses the growth of harmonic waves in mixing layer and results in the delay of eddy rollup,which causes the thickness of mixing layer is smaller than that of free mixing layer.But the growth rate of mixing layer is not affected by the confined wall.The pressure gradient also puts off the mixing layer to rollup thereby decreases the thickness of mixing layer.However,the pressure gradient affects the development of mixing layer constantly,and it can increase the growth rate gradually.