Leakage and secondary flows i compressor cascades

Abstract

This paper describes the effects of leakage and secondary flows on lift, outlet angles, induced drag and loss coefficients in a rectilinear cascade of compressor blades in which spanwise gaps simulated the clearance spaces of an axial compressor. The resultant leakage flow was studied under three different conditions, (i) with uniform inlet flow (ii) with non-uniform inlet flow near the gaps (iii) with severely non-uniform inlet flow and an end wall within the gaps. In the first experiment only leakage flows were present. Analytical predictions of blade pressure distributions and induced drag agree fairly well with the experimental values. A semi-theoretical formula for induced drag coefficients is suggested for use by designers. For the range of gap/chord ratios used in practice, leakage produces a slight increase in the average lift. In the second experiment an interaction between leakage and secondary flows was found to be generally favourable. The leakage and secondary flows are opposite in direction and this results in the angle distribution becoming more uniform and less variation in spanwise pressure distribution. Analytical predictions of outlet angle agree qualitatively with experimental values. The leakage flow was found to have very favourable effects in the third experiment. The severe separation in the corner formed by the wall and blade suction surface that is observed when there is no clearance is reduced by the leakage flow. The outlet angle distribution becomes more uniform and the loss coefficients are reduced. An optimum gap/chord ratio, at which separation on the blade and wall surfaces is a minimum, is also observed.