Reduction of Secondary Flow Losses in Transonic Nozzle Guide Vane Through Axisymmetric Endwall Profile Optimization

Abstract

Improvement in performance and size of gas turbine engine pave the way to the design of high blade loading, low aspect ratio and the small axial gap in the initial stages of turbine. This gives rise to the substantial amount of losses especially secondary flow losses and at the same time, endwall contouring showed its effective part in reduction of secondary flow losses. The intended purpose of the present numerical investigation is to compute the secondary flow mechanisms occurring inside the nozzle guide vane of high pressure turbine stage, a typical representative of modern aero engine design and to optimize its endwall to reduce these losses. Axisymmetric variation in endwall profiles are achieved by functional approximation and genetic algorithm based numerical optimization method. Initially, nozzle guide vane endwalls are parameterized with control points of Bezier curve. A subset of control points are considered as the design variables having a constraint to move in radial coordinate. Statistical tool latin hypercube sampling is adopted to explore the design space. Based on the values of objective functions obtained from the numerical CFD calculations, functional approximation model is construed using the artificial neural network. Finally, a genetic algorithm is used to obtain the optimum solution and analyze the effect of the axisymmetric endwalls on secondary flow losses. Design of endwall profile is studied in three different approaches by confining the axisymmetric variation near hub, shroud and both endwalls. Based on the obtained optimized profiles, secondary flow mechanisms occurring inside the NGV passage are investigated. Reduction in total pressure loss coefficient and improvement in isentropic total-total efficiency are observed for the optimized profiles.