The paper describes experimental investigations of an alternative shrouded stator concept in a 2.5 stage low speed compressor. The idea of this new concept is to raise the stator hub line by a small amount, thus decelerating the flow upstream of the shroud cavity due to the into wind step and raising the static pressure. Downstream of the cavity the out of wind step changes the streamline curvature thus lowering the static pressure locally. As a result, the static pressure difference across the shroud is lower and the shroud flow is reduced. Tests were done at three seal gap heights under stator 1, both with a “neutral” (in–line) hub and a six percent “bump shroud”, i.e. the hub is raised by six percent annulus height.

Performance measurements show the impact of the “bump shroud” geometry on the overall behavior of the compressor, i.e. efficiency and pressure ratio and the variation of these quantities with varying seal gap height. While the efficiency and pressure ratio of the compressor inevitably reduce with increasing seal gap height, the sensitivity of both is reduced by using “bump shrouds”. At small seal gap heights the “bump shroud” design behaves similarly to the neutral one, while at the design seal gap height it is superior. Thus, both the efficiency and the pressure ratio are less sensitive against seal gap height variations if the compressor is equipped with a raised hub line — leading to a more robust product. A similar behavior is seen at near stall conditions.

The analysis of five hole probe measurements reveals the reason for the improved efficiency. The stator 1 losses were significantly reduced by the introduction of the “bump shroud”. This is mostly due to the reduced amount of shroud flow and the subsequent reduction of hub cross–flow in the stator. A comparison of losses with and without the raised hub line show not only a reduction of the losses near the hub, but also adjacent to the suction side of the stator due to reduced migration of hub boundary layer fluid onto the vane.

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