A numerical study and experimental investigations have been performed in order to compare different boundary layer suction slot geometries and suction velocity ratios and rate their performance. The numerical study, also performed with secondary flow suction, has been done with a RANS solver using two different setups. In the first setup the suction slot itself is not calculated, but only present as an outflow region with constant outflow velocity - no recirculation can appear. The second setup contains a fine resolved suction slot with representative height and width to clarify the influence of in-slot recirculation. The calculations are used as a basis for the comparison of suction slots used in high speed wind tunnel experiments. All investigations have been performed at a Mach number of Ma_1 = 0.67 and a Reynolds number of Re_c = 560.000. In the simple numerical simulation, the cascade total pressure loss is a function of the suction rate and is not affected by the suction slot size and therefore the suction velocity. In the experiments, a strong influence of the in-slot suction velocity to the efficiency is present. This can also be detected from the calculations of the cascade with attached suction slot. The authors state that for the boundary layer suction to increase the compressor performance effectively, the velocity from passage to slot must be positive throughout the orifice and recirculation from slot to passage must not appear.
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