Complicated flows featuring with highly three-dimensionality and intense rotation/curvature in turbomachines have made the steady Reynolds-averaged Navier-Stokes (RANS) simulation very difficult. In this paper, based on the widely used xbased shear stress transport (SST) model, two modifications (one is denoted as shear stress transport model with curvature correction (SST-CC), the other is denoted as shear stress transport model with reattachment modification (SST-RM)) are assessed in predicting the tip leakage flow in the rotor passage and the three-dimensional separating flow in the stator passage of a low-speed axial compressor. Based on the comprehensive experimental data, including the compressor characteristics lines, inter-blade-row velocity profiles, and also detailed three-dimensional flows inside both rotor and stator passages, the abilities for capturing both compressor performance characteristics and specific flow details for simulations with different turbulence models were analyzed. The results indicate that the SST-CC model could simulate an approximate operating line with the one simulated by the SST model, which is very close to the experimental results, while the SST-RM model over-predicted the compressor pressure rise characteristics at all mass-flow-rate conditions. For the tip leakage flow inside the rotor passage and the three-dimensional separating flow inside the stator passage, the SST-RM model predicts a more accurate result, while for the outlet flow profile of the rotor and the stator passage the SST-CC model could simulate a more anastomotic result. In conclusion, the SST-RM model has advantage in predicting the flow fields within the blade rows, while the SST-CC model is adept in predicting the inter-row flow profiles. However, neither of them could simulate great comprehensive results in the investigated axial compressor.

Three-dimensional corner separation is common in axial compressors, which can lead to large flow loss and blockage especially when it evolves into the corner stall (open separation). In this paper, the evolution of the three-dimensional flow structures inside a cantilevered stator of a 1.5 stage low-speed highly loaded axial compressor as the stator hub clearance varies, and its effect on the whole compressor performance are investigated experimentally. Firstly, when the stator hub is sealed, the hub corner stall will occur at small mass flow rate conditions. Then, when a very small stator hub clearance is introduced, the leakage flow tends to strengthen the hub corner separation at large mass flow rate conditions and prompts the occurrence of hub corner stall as the mass flow rate decreases. This is mainly caused by the fact that the leakage flow has relatively low energy due to the viscosity effect in the clearance and large flow loss generation as the clearance flow comes across and mixes with the transverse secondary flow. Finally, when the stator hub clearance increases, the effect of the flow viscosity becomes very weak and could be ignored, so the enhanced leakage flow can suppress the transverse migration of the low energy flow near the hub, and the hub corner separation at large mass flow rate conditions could be weakened and the hub corner stall at small mass flow rate conditions could be removed or delayed. As the stator hub clearance varies, the flow structures inside the stator passage could be summarized into five typical flow structures, and this is closely associated with the performance of the compressor.

Flow separations in compressor blade passages are common and can cause significant flow blockage and loss production. This paper investigates experimentally the three-dimensional (3D) flow separations in a highly loaded low-speed large-scale compressor facility. Oil flow visualizations, stereoscopic particle image velocimetry (SPIV), and five-hole probe measurements are conducted at certain conditions from the near-chock to near-stall condition along the compressor operating line. The 3D separation and vortex flow structures in the stator at different operating conditions are analyzed. By changing the size of the rotor tip gap, six groups of oil-flow pictures are obtained to study the effect of the rotor gaps on the 3D separating flows in the downstream stator. The variation of the corner separation scale is almost linear with the rotor tip gap size. Along the compressor operating line, four typical 3D flow structures are found inside the stator passage. Between the second typical 3D flow structure and the third typical 3D flow structure, an unstable stage exists on the compressor operating line; during this stage, the hub corner separation becomes an open separation from the closed type. A smaller rotor tip gap corresponds to an earlier unstable stage. Finally, a critical rotor tip blockage state usually existed for the transform of corner separation types at a certain rotor tip gap configuration. This discovery is valuable for the study of multistage compressor matching problems at off-design conditions.

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