Influence of Tip-Clearance, Aspect Ratio, Blade Loading, and Inlet Boundary Layer on Secondary Losses in Compressor Cascades

Abstract

Within the scope of a compressor research program on endwall flows a highly loaded linear compressor cascade of NACA 65 profiles was investigated. Detailed study of the three-dimensional flow field was carried out for three different cases of tip-clearance including zero clearance with a variation of blade loading, blade height, and inlet boundary layer thickness. Using a small five hole probe measurements have been performed inside and downstream of the blade passage. Additional information about the formation and development of the passage and tip-clearance vortices is obtained from static pressure tappings at midspan and on the endwall, and a surface flow visualization technique. The cascade performance is presented in terms of turning angle and loss coefficient. It is found that in addition to the frequently investigated effects of tip-clearance and blade loading, the displacement thickness of the inlet boundary layer has a significant influence on the radial distribution of the losses and the outlet angle. However, the overall loss behavior remains almost unaffected by the inlet boundary layer. Only for low values of the aspect ratio around one, zero tip-clearance, and high values of incidence the influence of the aspect ratio becomes important. In this case the endwall flow regions from both blade ends are linked. The experimental results provide an extended basis for the improvement of the known correlations on aerodynamic losses and flow angle deviation as well as for the validation of 3D-Navier-Stokes calculations. An improved approach for loss correlations is presented in this paper.