Beneficial Effects of Guide Fins on Corner Separation

Abstract

Corner separation is a major phenomenon limiting the operability of aeronautical compressors. Passive control devices such as guide fins are envisioned to reduce its deleterious effects. In this paper, guide fins are optimized to reduce the total pressure losses downstream of a subsonic compressor cascade (M = 0.11, Re = 3.2 105, modern stator blades) at design and near-stall incidences. Guide fins shapes are defined using 15 parameters. An optimization strategy relying on the iterative refinement of surrogate model with Reynolds-Averaged Navier-Stokes (RANS) computations is carried out. A database of 1033 guide fins is obtained, a large number of which strongly reduces the near-stall losses. Three guide fins are thoroughly investigated and referred to as Short Fence (short and straight guide fin), Long Fence (long and straight guide fin) and 3D (long guide fin with a strong pyramidal aspect). Their effect on the downstream measurement plane is validated experimentally, as well as their integrated performances. Large reductions of total pressure losses at near-stall incidence are obtained (−2.4 pt to −2.9 pt, or a relative reduction of −50% to −60%) without degrading neither the nominal losses nor the stator deflection. These guide fins are used to highlight three beneficial mechanisms that alter the corner separation development within the blade passage a) Tip Vortex (energizes the passage flow) b) Fence (induces 2 passage vortices) c) Guide (induces a local favourable pressure gradient). The role of these mechanisms in the generation of total pressure losses are analyzed in depth with streamlines visualisations and with a topological analysis. Kriging surrogate models reveal that three parameters drive the guide fins performances : the axial position, height, and thickness value at hub. The performances of the investigated guide fins are robust to significant variations of these parameters, as well as small variation of the inlet boundary layer thickness.