Analysis of Cavity Leakage Effects on Coupled Non-Axisymmetric Endwall-Airfoil Optimization in a Low-Speed Compressor Tandem Stator

Abstract

This paper aims at describing the effects of the hub leakage flow on non-axisymmetric endwall contouring and airfoil optimization in a highly-loaded tandem vane, in order to give a better understanding of the potential of the approach, independently of the seal leakage. Since its first applications on highly-loaded compressor stages, it has been highlighted how the non-axisymmetric contouring can be effective in manipulating the secondary flow behavior close to the endwalls. Many cases have proven its positive influence on the flow field behavior for single and tandem vanes, drawing general guidelines for the preliminary design phase, thanks to the usage of optimization tools. An interesting question still to be addressed in a general sense is at which specific conditions designers should rely on endwall contouring and when they should not, considering the obvious additional manufacturing expenses associated. First studies on this subject, assumed the interchangeability of non-axisymmetric endwall contouring with radial stacking laws. Nowadays, fully 3D optimized airfoil geometries being state of the art, it appears evident that new general guidelines are needed. In this context, the influence of the hub leakage flow on the definition and effectiveness of the endwall contouring is investigated, isolating its beneficial influences, which will be described independently of the topology of the low momentum fluid present close to the endwalls. Therefore, the reference geometry is varied in mass leakage flow by varying the geometry of the cavity of the shrouded tandem stator arrangement. Then, the new configuration is optimized via non-axisymmetric endwall contouring and hub airfoil profile optimization, carried out by means of a meta-heuristic approach. The investigation based on total pressure losses and static polytropic efficiency will show the break-even point of the beneficial influence of the non-axisymmetric contour with respect to the reducing leakage mass flow. Moreover, the change of design strategies adopted by the optimizer among the different flow field configurations shall serve as a reference guide for every future design choice incorporating non-axisymmetric endwall contouring.