Effect of the Axial Casing Groove Geometry on the Production and Distribution of Reynolds Stresses in the Tip Region of an Axial Compressor Rotor

Abstract

Stereo PIV measurements performed in a refractive index matched facility examine the effect of axial casing grooves (ACGs) geometry on the turbulence in the tip region of an axial compressor rotor. The ACGs delay the onset of stall by entraining the Tip Leakage Vortex (TLV), and by causing periodic changes to incidence angle as their outflow impinges on the rotor blade. To decouple these effects, measurements have been performed using a series of grooves having similar inlets, but different outflow directions. The performance and flow structure associated with three grooves, namely a semi-circular ACG, as well as U and S shaped grooves have been presented in several recent papers. This paper focuses on the impact of passage flow-groove interactions on the distribution, evolution, and production rates of turbulent kinetic energy (TKE) and all the Reynolds stress components. The analysis is performed at flow rates corresponding to pre-stall conditions and best efficiency point (BEP) of the untreated end wall, and for different blade orientations relative to the groove. Interactions of the tip flow with the ACGs modifies the magnitude and spatial distribution of the highly anisotropic and inhomogeneous turbulence in the passage. Owing to TLV entrainment into the grooves, at low flowrate, the ACGs actually reduce the turbulence in the passage compared to that in the smooth endwall. However, the geometry -dependent tip flow-groove interactions introduce new elevated turbulence centers. In all cases, the TKE is high in the: (i) TLV center, (ii) corner vortex generated as the backward tip leakage flow separates at the downstream end of the groove, and (iii) shear layer connecting the TLV to the rotor blade suction side tip. The location of peaks and the dominant components vary among grooves. For example, the axial component is dominant for the semicircular ACG, and its peak is located in the shear layer. The radial component is the dominant contributor for the U and S grooves, and it peaks inside the grooves at different locations. The circumferential component peaks in the TLV for the U and semicircular ACG, but inside the S groove. The shear layers generated as the flows jet out from the upstream ends of the grooves also bring varying elevated turbulence. At BEP, interactions of the TLV with secondary flows generated by the U and semi-circular grooves, for which the outflow is oriented in the negative circumferential direction, generate high turbulence levels, which extend deep into the passage. In contrast, the interactions associated with the S grooves are limited, resulting in a substantially lower turbulence level. Many of the various trends can be readily explained by examining the corresponding spatial distributions of the turbulence production rates. Such understanding elucidates the different mechanisms involved and provides a unique database for modelling turbulence in the passage.