Aerodynamic Performance of Low Aspect Ratio Turbine Blade in the Presence of Purge Flow

Abstract

In aero engines, purge flow is directly fed from the compressor which bypasses the combustion chamber and introduced into the disk space between blade rows to prevent the hot ingress. Higher quantity of purge gas fed through the disk space can provide additional thermal protection to passage endwall and blade surfaces. Moreover interaction of the purge air with the mainstream flow can alter the flow characteristics of turbine blade passage. The objective of the present investigation is to understand the secondary vortices and its aerodynamic behavior within a low aspect ratio turbine blade passage in the presence of purge flow. An attempt is made to understand the influence of velocity ratios and purge ejection angles on these secondary vortices. The objective is broadened by investigating the unsteadiness generated by upstream wakes over the secondary vortex formations in th presence of purge flow. Further the thesis aims to judge the feasibility of implementing endwall contouring to curb the additional losses generated by the purge flow. To accomplish these objectives, a combination of experimental measurements and computational simulations are executed on a common blade geometry. The most reliable commercial software ANSYS CFX which solves three dimensional Reynolds Averaged Navier Stokes Equations together with Shear Stress Transport (SST) turbulence model has been used to carry out computational simulations. Along with steady state analysis, in order to reveal the time dependent nature of the flow variables, transient analysis has been conducted for certain selected computational domains. The numerical results are validated with experimental measurements obtained at the blade exit region using five hole probe and Scanivalve. The experimental analysis is conducted for the base case without purge (BC) and base case with purge (BCp) configurations having flat endwalls. vi In the present analysis, it is observed that with an increase in the velocity ratio, the mass averaged total pressure losses also increases. In an effort to reduce the losses, purge ejection angle is reduced to 350 from 900 with a step size of 150. Significant loss reduction and improved endwall protection are observed at lower ejection angles. Numerical investigation of upstream disturbances/wakes explore the interaction effects of two additional vortices, viz. the cylinder vortex (Vc) and the purge vortex (Vp). Steady state analysis shows an increase in the underturning at blade exit due to the squeezing of the pressure side leg of horseshoe vortex (PSL) towards the pressure surface by the cylinder vortices (Vp). The unsteady analysis reveals the formation of filament shaped wake structures which breaks into smaller vortical structures at the blade leading edge for stagnation wake configuration (STW). On the contrary, in midpassage wake configuration (MW), the obstruction created by the purge flow causes the upper portion of cylinder vortices bend forward, creating a shearing action along the spanwise direction. Investigation of contoured endwall geometries shows that, endwall curvature either accelerate or decelerate the flow thereby a control over the endwall static pressure can be obtained. Out of three contoured endwalls investigated, the stagnation zones generated at the contour valleys has resulted in the additional loss generation for the first two profiles. Reduced valley depth and optimum hump height of the third configuration has effectively redistributed the endwall static pressure. Moreover an increase in the static pressure distribution at the endwall near to pressure surface has eliminated the pressure side bubble formation. Computational results of URANS (Unsteady Reynolds Averaged Navier Stokes) simulations are obtained for analyzing transient behaviour of pressure side bubble, with more emphasis on its migration on pressure surface and across the blade passage.