Study On The Mixing Enhancement Mechanisms Of A Rectangular Lobed Mixer For Supersonic Mixing Layer

The mixing process between the high-speed airstream and fuel gas in the combustor has a large effect on the thrust performance of combined cycle engine.Herein,a series of simulations based on Large Eddy Simulation(LES)and experiments using Nano-tracer Planar Laser Scattering(NPLS)are conducted to analyze the mixing mechanisms of plate and lobed mixer at the flow field of supersonic mixing layer.Through these numerical results and experimental data,the mixing enhancement mechanisms ofrectangular lobed mixer are analyzed.Firstly,the growth and mixing of the plate layer are analyzed.The incoming flow in a certain thickness of the clapboard trailing edge will produce different shedding frequency.Shedding frequency dominates the development and evolution of large-scale vortices structure in the mixer layer.The smaller clapboard thickness induces a larger vortices shedding frequency,which causes the vortices to roll up quickly,but the vortices structure size is smaller;The vortices shedding frequency induced by the larger clapboard thickness is smaller,the flow distance required for vortices rolling is longer,and the vortices are aligned with each other in the interaction of high frequency harmonics,and finally the vortices structure scale is larger.In addition,the effects of temperature ratio on the plate mixing layer are reflected as follows.Increasing the temperature ratio can increase the shedding frequency of the plate trailing edge,and the shedding frequency still dominates the evolution of the large-scale vortices structure of the mixed layer.In the case of low temperature ratio,there are main frequency and high frequency in the flow-field and vortices pairing phenomenon appears.With the increase of temperature ratio,the dominant frequency selection amplification is obvious,vortices pairing phenomenon disappears,and the temperature ratio plays a positive role in the influence of the convective mach number on the thickness of the mixed layer.Secondly,flow-field characteristics and the mixing mechanisms of large-scale streamwise structures shed by the trailing edges of lobed mixer are analyzed.There are two mechanisms of mixing enhancement of the streamwise vortices: one is that large amounts of surrounding fluids around the two streams are entrained into the mixing zone due to the formation and evolution of the streamwise vortices;the other is that the large-scale streamwise vortices breaks up into small-scale structures through its interaction with the spanwise vortices.Four large-scale streamwise vortices are generated in the flow.The results indicates that the rectangular lobed mixer can enhance mixing of supersonic mixing-layer effectively.Additionally,the geometric parameters of lobed mixer have a large effect on the flow field of supersonic mixing layer.The distribution of streamwise vortices and the thickness of mixer layer have been influenced by the wavelength.The circulation of streamwise vortices increases with the increase of the lobe penetration angle.The pitot loss decrease with the decrease of the lobe penetration angle.There is a best wavelength and penetration angel to maximize the mixing rate.