<p>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. Among them three grooves are discussed in this paper, namely a semi-circular ACG, as well as U and S shaped grooves. Descriptions of the effect of these grooves on the performance and mean flow structure around the rotor tip have been presented in previous papers (GT2017-65099, GT2018-77071, GT2020-14696), including features of the turbulence for the semi-circular ACG (GT2020-15229). Here, we focus on the impact of these grooves on the turbulence in the passage. The turbulence statistics, including the turbulent kinetic energy (TKE), and all the Reynolds stress components are calculated by ensemble averaging at least 1000 instantaneous realizations at selected rotor orientations. The results enable us to examine the evolution of the turbulence as the tip flow interacts with the grooves, e.g. when the downstream end of the groove (their inlet) faces the pressure or suction sides of the blade and when the groove is located between blades. The measurements are performed at flow rates corresponding to pre-stall conditions (referred to as low flowrate) and best efficiency point (BEP) of the untreated end wall. Interactions of the tip flow with the ACGs modifies the magnitude and spatial distribution of the highly anisotropic and inhomogeneous components of the TKE. Owing to the TLV entrainment, at low flowrate the ACGs actually reduce the turbulence in the passage compared to the smooth endwall. The inhomogeneity and anisotropy vary with the groove geometry, and the tip flow-groove interaction 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. However, 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&nbsp;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.</p>

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

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2021/volume/84898

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