Investigation On The Design And Flow Mechanisms Of The De-swirling Lobed Mixer

The aeroengine with lobed mixer exhibit its various advantages on increasing thrust,decreasing specific fuel consumption,suppressing jet noise and infrared radiation etc,lobed mixer is extensive used in medium or low bypass ratio turbofan engine mixing exhaust system accordingly.For years,researchers focus on the accelerating jet mixing mechanisms of lobed at axial inlet case.However,the loaded of low pressure turbine augment with the increase aeroengine thrust weight ratio,while lobed exhaust system inlet flow does exists core inlet swirl for high-performance engine with highly-loaded low pressure turbine,which significant decrease the performance of exhaust system,and then decrease aeroengine performance.Some attention had been paied to the influence of inlet swirl on the mixing mechanisms and performance of lobed,while no results have been reported on the study of altering lobed geometry to adapt the core inlet swirl.This paper works on a scaled lobed exhaust system employed in a certain turbofan engine,the design method of de-swirling lobed which could adapt core inlet swirl is studied by consulting the design theory of turbomachine blades.Based on the co-annular,low speed,wind tunnel experiment rig at Institute of Engineering Thermophysics,Chinese Academy of Sciences,the seven-hole probe and hot-wire are used to measure the downstream flow,and the numerical simulation offer detailed information of jet as an assistant,the detail research on baseline and de-swirling lobed is carried out.In this paper,the mixing mechanisms of baseline lobed are studied,firstly,and then,the effects of de-swirling lobed geometric parameters and swirl angle of inlet flow are analyzed,the mixing mechanisms difference between baseline and de-swirling lobed is compared,Further more,the influence of scalloping for both lobed on mixing mechanisms and performance are researched too.The main conclusions are drawn as follows:Based on the research on the mixing mechanisms of baseline lobed mixer,the lobed special alternate geometry induces the large scale streamwise vortices,normal vortices at downstream of lobed with the same contour as lobed exit is also formed,the interaction between streamwise vortices and normal vortices and their break-down jointly dominate the jet mixing.At core inlet swirl cases,the leakage flow induced by the gap between lobed and centre-body also play a role in jet mixing near the trough,the performance of lobed exhaust system decrease at the same time.meanwhile,a suction surface with large low pressure zone formed at the lee side of lobed inner surface,the core flow near the suction surface deteriorate consequently.Based on the research on the mixing mechanisms of both lobed,at various core inlet swirl cases,the de-swirling lobed rectify the lobed suction surface static pressure distribution and improve flow condition near lobed suction surface,while the strength of leakage flow is almost not affected.The bend angle of de-swirling lobed determine the lobed load and de-swirling ability,the de-swirling lobed geometric outlet angle determine the medium radius swirl angle of lobed outlet flow.The swirl angle of outflow at lobed exit determine the formation and development of downstream streamwise vortices,which means that the bigger swirl angle flow at medium radius restrain the formation of clockwise streamwise vortices near the crest,enhance the formation of counterclockwise streamwise vortices,meanwhile,the de-swirling lobed magnify the radial pressure gradient of lobed suction surface adjacent flow,the strength of clockwise streamwise vortices lobed crest nearby is enhanced consequently.Further more,the de-swirling lobed changes the relative distance between clockwise and counterclockwise streamwise vortices,the stretching of normal vortices is enhanced which accelerate jet mixing.At various core inlet swirl cases,the de-swirling lobed could improve the flow condition of the flow nearby lobed suction surface,and it give a better performance on total pressure losses and thrust than baseline lobed.The core and bypass flow begin to interact at scalloped geometry for both scalloped lobed mixer jet,scalloped lobed induce streamwise vortices in advance,enhance the core and bypass jet mixing.While the baseline lobed flow movement from core to bypass near the downsegment scalloped geometry is inhibited at core inlet swirl cases,the de-swirling lobed could lessen the negative effect.Two additional streamwise vortices are formed at the down notch of scalloped geometry for both scalloped lobed,scalloped lobed weaken the de-swirling ability of both lobed,and the de-swirling lobed weaken more,therefor,the same scalloped geometry brings different flow swirl angle at lobed exit,and the strength of additional streamwise for scalloped de-swirling lobed is larger than scalloped baseline lobed.At aggressive core inlet swirl cases,the thrust decrease due to scalloped for both lobed,but the thrust of scalloped de-swirling lobed is still bigger than scalloped baseline lobed.