UNDERSTANDING THE FLOW FIELD IN A HIGHLY LOADED TANDEM COMPRESSOR CASCADE

Large flow turning in compressor cascades with single airfoils requires an effective control of the boundary layer growth under the diffusing flow. An alternative approach consists of distributing the loading over two subsequent airfoils, using a so-called tandem blade, to, in some measure, restart the boundary layer before flow separation occurs. It is, however, not always clear whether the benefits of the two-blade setup justify the additional manufacturing complexity. The present work explores the tandem blade concept using a gradient-based optimization method to produce an efficient, highly loaded compressor cascade blade directly. A comparison between two-dimensional single and tandem configurations is first presented to clarify the benefits of one over the other. The geometry is optimized for each concept using a gradient-based optimization technique to improve the pressure loss coefficient at multiple operating points for a given flow-turning constraint. While the optimized single and tandem blade designs have similar performance, the lower solidity of the latter provides a lighter compressor stage for the considered operating range. A three-dimensional tandem compressor cascade based on the two-dimensional study is then optimized to account for secondary flows. The aerodynamic performance and the operating range are assessed and compared, along with a study of the physical phenomena surrounding the tandem configuration. The resulting geometry presents similar non-conventional features observed during the two-dimensional study that exploits the flow mechanism of two-airfoil configurations.