Flow Performance Analysis Of Turbine Stage Cascades

With the development of modern highly loaded turbine, secondary flow losses have taken a larger proportion in turbine overall losses. The research of secondary flow in cascade has always been an important subject in the field of turbomachinery. Optimizing 3D flow patterns in turbine cascades is significant to control and reduce the secondary flow losses.Firstly, numerical simulations of the first stage of a power turbine are performed with a three dimensional viscous code. The flow details in this turbine stage are obtained in the present dissertation. The computational results display the flow characteristics of endwalls and blade surfaces of these cascades and the structures of the three dimensional flow fields in these cascades. The evolution of the secondary flow vortex structures in cascade passage, i.e., horseshoe vortex, passage vortex, corner vortex, trailing vortex, tip leakage vortex etc., are revealed by analyzing secondary flow phenomenon and loss mechanism, and the effects of vortex structures on losses distribution are also studied, the distribution of primary loss in this turbine cascade has been also confirmed.Secondly, the effects of different meridional endwall profiles on flow performance of the turbine cascade are studied through numerical simulations. The shroud which used concave-convex profile gains the lowest energy loss and avoids effectively the formation of diffusion flow and separation with the great diffused angle casing, and finally improves the flow condition near shroud region. After using concave endwall contouring for hub profile, both the transverse and radical pressure distributions are inflected, the transverse pressure gradient of the majority of middle region is reduced, and the root load also moves downwards which slowed down the boundary layer growth in the hub nearby, and also reduced crosswise secondary flow loss in a certain degree. In the radial direction, the pressure of root region is enhanced, and the low pressure area aparts from hub, at the same time, low energy fluid of hub nearby move towards mispan due to radial negative pressure gradient, which also improved the flow of hub region.In this paper, the influence on the flow performance of different stacking lines, such as positive lean, positive curved, negative curved, S-type blades, is investigated through numerical simulations. Positive lean, positive bow, S-type blades couldn't decrease the overall loss effectively for this stage cascade. Comparatively, the J type curved blade is more suitable to this type cascade, which is effective to improve both the blade-root and blade-tip flows and has the lowest flow loss.Finally, the effects of the combination design of endwall contouring and blade stacking line on flow performance are also studied in this paper. At the base on the optimized endwall contouring, it is difficult to further improve the flow near shroud region by changing the blade stacking type. Thus, the combination of concave at hub and positive curved blade could further decrease the energy loss of root region; especially the intension of low passage vortex descends obviously. In this case, the combination design of optimized endwall contouring and J type blade has the lowest energy loss.