EFFECTS OF TIP GAP SIZES AND OPERATING CONDITIONS ON THE FLOW STRUCTURES AND THE COMPLEX TURBULENCE IN THE TIP REGION OF AN AXIAL COMPRESSOR ROTOR
Johns Hopkins University
The flow and turbulence in the tip region of an axial compressor rotor are studied experimentally in an optical refractive index-matched facility. Flow visualization and stereo-PIV measurements are performed with two different tip gap sizes at two flow rates. Increasing the tip clearance causes a significant reduction in overall performance, hence altering the blade load distributions and characteristic flow structures, which consist of multiple instantaneous vortices. The tip leakage vortex (TLV) is a 3D structure after ensemble-averaging with elevated vorticity in all directions. Decreasing tip clearance or flow rate increases the blade loading and leakage velocity near the leading edge, causing earlier TLV rollup and migration away from the blade suction side. This migration is governed by both the tip leakage jet and the flow induced by the ‘image vortex’ across the endwall, the latter dominating deeper in the passage. TLV breakdown occurs in the aft part of the passage with a rapid expansion of the vortex core and a decrease in peak vorticity. Increasing the tip clearance delays the breakdown at high flow rates, but causes earlier breakdown at pre-stall conditions due to the propagation of more prevalent secondary structures across the gap. The endwall boundary layer with opposite-sign vorticity separates at the point where the backward leakage flow meets the passage flow. This layer is entrained radially inward by the TLV for the narrow gap but remains above the TLV for the wide gap. Elevated turbulence with high spatial anisotropy is observed in the vicinity of these characteristic vortical structures. Some trends in the distributions of Reynolds stress can be related to their local production rates, but others are not, requiring analysis of other processes including transport by mean-flow and turbulence as well as dissipation. The measured stress and strain rate tensors show a poor correlation due to the prevalent non-equilibrium condition in the tip flows, making proper modeling particularly challenging. Results from different axial turbomachines and conditions reveal similar features in the vortical structures and turbulence as well as the non-equilibrium condition. Surprisingly, similar spatial distributions are observed for individual stress and strain rate components and, accordingly, the eddy viscosity.
tip leakage flow, turbulence, axial compressor