Challenges facing accurately predicting turbulence in the tip region of turbomachines include anisotropy, inhomogeneity, and non-equilibrium conditions, resulting in poor correlations between the Reynold stresses and the corresponding mean strain rate components. The geometric complexity introduced by casing grooves exacerbates this problem. Taking advantage of the unique refractive index-matched liquid facility at JHU, we have established an extensive database containing performance, visualization of vortical structures by cavitation, and high-resolution velocity distributions obtained using stereo-PIV. Ensemble-averaged data of mean flow and turbulence parameters for varying rotor orientations, covering the entire tip region including the space within the casing grooves, enables us to evaluate the structure of turbulence and its relationship to mean flow parameters. The one and a half stages compressor with acrylic blades has been described in previous publications (e.g. GT2016-57054). For the present analysis, we compare the tip region turbulence without grooves to that developing when semi-circular axial casing grooves, skewed by 45° in the positive circumferential direction are installed. The groove geometry is based on Müller et al. (GT2011-45364), and is described in detail in GT2017-65099. The analysis focuses on differences in distributions of turbulent kinetic energy (TKE), Reynolds stresses and the corresponding anisotropy tensor, and well as in-plane (meridional and radial) contributions to TKE production, and distributions of mean strain rate. The on-going analysis demonstrates that in general (i.e. with or without casing grooves), the TKE peaks in the tip leakage vortex (TLV) center, and in the shear layer connecting it to the blade suction side tip corner. The turbulence is highly anisotropic and inhomogeneous, with the anisotropy tensor shifting from one dimensional (1D) to 2D and to 3D over small distances. Furthermore, the correlation between the strain and Reynolds stress tensors is also poor, especially in areas occupied by the large scale vortical structures. At low flow rates, when a significant fraction of the TLV and backflow vortices are ingested into the grooves, the turbulence in the remaining TLV residues is significantly lower, but the area with elevated TKE expands. Interactions between the tip leakage flow and the groove generate several secondary flows, e.g. a large corner vortex that rotates in the opposite direction to that of the TLV. At low flow rates, these secondary structures remain within the groove, become scrambled, and are reinjected from the upstream end of the groove. The reinjection generates a layer of elevated turbulence that propagates back to the rotor. In contrast, at high flow rates, the turbulence generated in the grooves is entrained into the passage by the TLV, especially when the blade passes the downstream end of the groove, significantly increasing the turbulence level in the tip region. Interestingly, in all cases, the grooves appear to increase the axial velocity fluctuations and reduce the radial component of the TKE. Specific contributors to these trends can be identified by examining corresponding (shear and contraction) terms in the TKE production as well as transport by mean flow and turbulence. These findings introduce new insight that might assist future development of Reynolds stress models in tip flows with casing grooves.


Effects of Axial Casing Grooves on the Structure of Turbulence in the Tip Region of an Axial Turbomachine Rotor

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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