Numerical Research On The Flow Mechanism In Ultra-highly Loaded Turbine Cascades With And Without Tip Clearance

Adding the flow turning angle is an effective way to raise the blade loading, increase the trust-weight ratio and then enhance the overall engine performance. However, numerous research results indicate that there are serious flow separations and strong vortexes, which would increase the secondary loss and reduce the flow efficiency sharply in highly/ultra-highly loaded turbine cascades with large blade turning angle.Tip clearance is an important factor of turbine efficiency reducing. Experimental and numerical studies show that the tip clearance has great effect on the aerodynamic performance in turbomachinery. Tip clearance flow can not only cause leakage loss, but also interfere with mainstream and form complex vortex structures, which bring new flow losses and cause flow stability problems.In this paper, an ultra-highly loaded turbine blade with160degree turning angle is studied with numerical simulation method. Based on the numerical simulation results, topologically analysis method is used to investigate the flow features in ultra-highly loaded turbine cascades with and without tip clearance, which is in order to give out the mechanism of flow losses, providing reference and consulting for the ultra-highly loaded turbine aerodynamic optimization design.Firstly, numerical simulation has been performed on ultra-highly loaded turbine without tip clearance. The formulation, development and evolution of horseshoe vortex, passage vortex and corner vortex are described in details. In addition, the interaction mechanism of different vortexes is presented. Then impact of the incidence angle on flow field is discussed, the study result shows that the flow efficiency fall sharply with the increase of the incidence angle.Subsequently, numerical simulation has been performed on ultra-highly loaded turbine with different tip clearance heights. The structures of leakage vortex and tip separation vortex are presented in details, the impact of tip clearance height and boundary layer thickness on the flow field are studied. The results show that there are several forms of flow separation/reattachment in tip clearance, and the leakage flow directly affects the formation and development of upper passage vortex, making passage vortexes move downward. With clearance height increasing, the intensity and scale of vortexes are both reinforced and the blade loading especially the tip loading decreases gradually. However the boundary layer thickness has much less influence of the flow field, the scale and intensity of different types of vortexes, and the total pressure loss coefficient increase slightly with the boundary layer thickness increasing.