Numerical Investigation On Mechanism Of Jet Mixing In Lobed Mixer With Swirl Of Radially Different Distribution

Since1960s, lobed mixer have been successfully applied in worldwide turbofan engine with medium or low bypass ratio, and exhibited its various advantages as increasing thrust, decreasing specific fuel consumption, suppressing jet noise and infrared radiation etc. During recent fifty years, lots of research have been done on lobed mixer, mainly focusing on physical mechanisms of jet mixing and the variation law of overall mixing performance by geometrical and aerodynamic parameters, while little attention had been paied to the variation of mixing performance of lobed mixer with swirl. In particular, almost no results have been reported on the study of jet mixing in lobed mixer with swirl of radially different distribution. From this perspective, this paper chose the1:0.7scaling model of lobe-mixing-exhaust-system employed in certain aero-engine as research object, incorporated vanes into the upstream of lobed mixer to produce the swirl of radially different distribution, and utilized fully3-D numerical simulation to study the variations of jet mixing performance under this situation after a thorough understanding of basic mixing mechanism in lobed mixer. The research focuses were placed on vortex dynamic process, jet mixing process and the variation of aerodynamic performance. Furthermore, this paper considered including blade into lobed mixer to deduce the swirl from upstream flow and analyzed the effect of blade. In addition, further discussion was invlolved on the influence of blade on vortex dynamic process, jet mixing process and aerodynamic performance. The main conclusions are drawn as follows:1) The study on physical mechanism of jet mixing in lobe mixer showed that the specific geometry of lobe induces large-scale streamwise vortex, whose development and functions with normal vortex control the jet mixing process in lobed mixer. After its initial development, streamwise vortices twist and stretch normal vortex to strengthen convections between bypass flows and core flows, and then indirectly enhancing jet mixing. With the development of jet flows, normal vortex and streamwise vortex begin to break down and release high turbulent kinetic energy to fasten jet mixing.2) Radially different distribution of swirl angle enlarges the difference of intensity between two streamwise vortices with adverse whirling directions, strengthens the entrainment between these two streamwise vortices, and then fastens their dissipation. Correspondingly, difference begins to rise on locations, intensities and numbers of functions between normal vortices and streamwise vortices, and the dissipation of normal vortex is also accelerated. Overall, the mixing performance has been largely refined. Meanwhile, the remained swirl intensifies the effect of shear around centerbody and improves the jet mixing in this area.3) Swirl angle at different radial gradients have different influence on jet mixing in lobed mixer. Larger radial gradient expands the difference of intensities between two streamwise vortices with adverse whirling directions, partially intensifies the effect of shear around centerbody and improves the jet mixing. However, larger swirl angle on the bottom of vane deduces this radial gradient, weakenes this difference, decreses the improvement of jet mixing, and to large extent, it strengthens the effect of shear around centerbody and enhances the jet mixing at this part.4) The incorporation of blade in lobed mixer could weaken the swirl of upstream flows, reduce the difference of intensity between two streamwise vortices with adverse whirling directions, resulting in the reduction of overall jet mixing performance to cetain kind of extent. The fluid around centerbody showed the similar characteristics as the overall performance.5) Reasonably controlling the swirl angle of flows around centerbody could effectively suppress backflow, which may occur around centerbody. Then the pressure loss and thrust loss could be largely lowered. The lobed mixer itself has the ability to resist upstream swirl. Tiny increase of swirl angle (no larger than30°) will not cause large loss of thrust and pressure, but may possibly improve jet mixing. The incorporation of vane unavoidably causes friction loss in lobed mixer, and increases pressure loss, but it also effectively weakens the azimuthal movement of core flow and brings obvious improvement of thrust.