Tandem configuration is an effective methodology to reduce flow separation on compressor blade suction surface and to improve blade loading. However, in modern highly loaded cases, corner separation remains as its single blade counterpart. In this study, non-axisymmetric endwall profiling (NAEP) was utilized in a highly loaded tandem cascade (diffusion factor D = 0.69), aiming at reducing its severe corner separation and revealing the unique flow mechanism while NAEP is utilized in tandem cascade. NAEP was designed in both forward (F) blade and rare (R) blade separately, and was investigated numerically in tandem environment. Results show that, NAEP in F blade passage can effectively eliminate the corner separation and reduce loss generation, whereas NAEP in R blade passage has no positive effect on corner separation and even promotes loss production. The optimal NAEP approximately removes the corner separation completely, with loss coefficient reducing by as much as 37.8%. The optimal NAEP for the tandem cascade features optimal axial location at the origin of corner separation. There is an optimal NAEP height (0.02 of blade height), under which NAEP can achieve pretty good control effect while the peak of NAEP varies in a large axial location range. In the tandem configuration, it is found that NAEP transfers blade loading from R blade to F blade; the static pressure increases significantly for the entire cascade, but the static pressure distribution of F blade does not exhibit as the design intent of NAEP. In addition, it is interesting to find that the flow turning near endwall reduces after endwall profiling, which is unique in tandem cascade and is contrast to the view on conventional configuration. On the contrary, NAEP in R blade has no influence on the corner separation of the tandem cascade; due to the decrement of cross-passage pressure gradient for R blade, the flow overturning near endwall reduces.
Air injection is an effectively methodology to suppress flow separation and to improve blade loading of airfoils and compressors. In order to remove corner separations in a cascade, investigation of endwall slotted injection was carried out numerically in this study. Based on endwall slot schemes of other flow control methods, six different endwall slots were designed, aiming at revealing the axial location effect and pitchwise location effect. For each endwall slot, numerical simulations were performed with six different injection directions to uncover the injection direction effect. Results showed that endwall slotted injection can effectively remove the corner separation. The overall loss coefficient and endwall loss coefficient of the cascade were reduced by 10.3% and 36.8% at most, respectively. Injection from leading edge and mid-chord can reduce endwall loss; however, the optimal axial location of endwall slot is near the trailing edge, where the corner separation is located. Different with other flow control methods, in general, the optimal pitchwise location of endwall slot is not close to suction surface but 0.16 pitch away from it. Injection near the suction surface is more sensitive to injection direction compared with injection at 0.16 pitch away from suction surface. Injection with velocity components both downstream and toward suction surface promises optimal control effect on corner separation. However, at mid-span, trailing edge separation is deteriorated and the flow turning angle is reduced, the flow mechanism being that the low-momentum fluid migrates along spanwise.
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