The Aerodynamic Interaction of Stator Shroud Leakage and Mainstream Flows in Compressors

Abstract

The aerodynamics of stator shroud leakage and mainstream flow interactions have been investigated using a linear cascade in which leakage mass flow and tangential velocity could be varied independently. Experimental measurements confirm the detrimental effect of increasing leakage mass flow rate, but also show clearly that increasing cavity tangential velocity improves the performance of the blade row. Shroud leakage is also found to modify significantly the spanwise distribution of blockage, flow turning and loss at exit of the stator passage. The mechanisms behind these large-scale effects are identified and a description of the interaction process based on momentum thickness arguments is proposed. Experimental and numerical results show that the amount of leakage is a linear function of the difference between cavity pressure and average pressure on the hub upstream of the stator. This result is shown to be an axisymmetric result, and thus largely independent of the pitchwise variation in blade loading. Departures from linearity at low leakage fractions are associated to the blade loading inducing pitchwise non-uniformities in the structure of the flow at the interface between the shroud cavity and the mainstream. These non-uniformities involve vortical structures, which participate in the exchange of fluid and momentum between the cavity and the mainstream, even at zero net leakage. Experiments also show that these non-uniformities in flow structure contribute to the generation of loss.