Flow Structure of Short-Length-Scale Disturbance in an Axial-Flow Compressor

Abstract

Short-length-scale disturbances, also called spikes, are often responsible in triggering rotating stall in axial-flow compressors. One hypothesis suggests that spikes can be the consequence of dynamic interaction among forward-spilled tip-leakage flow, the main throughflow, and the reversed flow. However, the transit process of such a dynamic interaction in the vicinity of the rotor tip clearance and, thus, the physical images of the flow structure of a spike are still unknown. In this paper, we present a numerical study with a novel scheme for a low-speed axial-How compressor by incorporating rotating inlet distortion. Because the inlet distortion will overload a portion of the blades while keeping the rest working normally, the short-length-scale disturbances can be observed without advocating the computational difficulty of simulating a fully stalled compressor. Two unsteady simulations using a commercial, three-dimensional, time-accurate, Reynolds-averaged Navier-Stokes solver are performed: one for a one-fourth rotor annulus with finer grids and the other for the entire rotor annulus with coarser grids. After the code is validated by comparing experimental results with the simulation for the entire rotor annulus, the one-fourth-annulus simulation is used to unveil the flow physics. As elucidated from the computational results, the complete birth-to-decay process of the short-length-scale disturbances is captured for the first time. The corresponding 3-D flow structure is also revealed. It is shown that the effects of dynamic How interaction at the tip extend beyond the tip region and deeply into the blade span. A horn-shaped vortex with one end at about 30% of the blade span and the other end at the casing is formed, which generates a low-pressure dip in the casing-pressure measurement. The spike, as identified from casing-pressure measurement, corresponds to a flow image in which the vortex rotates around the annulus.