Design of a rear-stage subsonic axial compressor with casing treatments

Abstract

The aerodynamic design of a 3.5-stage high-pressure axial compressor, representing a modern rear stage design with low aspect ratio blades and large relative tip gaps is presented. The subsonic rotor and stator airfoils feature a controlled-diffusion (CDA) type of sectional design, obtained from 2D viscous-inviscid simulations using the MISES flow solver. Improvements of the main gas path, as well as the 3D blade design, are based on 3D steady Reynolds-averaged Navier–Stokes simulations (ANSYS CFX). Sweep and dihedral were applied to improve the design and off-design performance of the compressor. The matured set of blading, with smooth casing, still exhibits large rotor tip vortices and double leakage, limiting the compressors operability towards stall point. Consequently, the application and development of axial slot casing treatments (CT) into the smooth casing is performed, aimed at improving local stage and overall stall margin with particular emphasis on avoiding efficiency penalties within stages. In order to investigate and optimise the geometric properties of the CTs with respect to efficiency a DoE (Design of Experiments) is executed. Based on the DoE a CT-geometry is deduced and applied to the critical rotor. Due to the fact, that the stage operability with casing treatment is limited by the downstream stator, a first adaption of the existing blade design is considered. In a first step, the downstream stator’s incidence is adjusted, resulting in a slightly increased stall margin. The second step incorporates the modification of rotor blade count, following the idea of trading rotor operability for an efficiency increase, due to reduced friction. However, no notable efficiency benefit was achieved, as the reduced friction loss is counterbalanced by increased secondary flow loss resulting from higher blade loadings.