Impact of unsteady flow processes on the performance of a high speed axial flow compressor

Abstract

This thesis examines the unsteady interactions between blade rows in a high Mach number, highly-loaded compressor stage. Two straight vane/rotor configurations with different axial spacing between vane and rotor are considered. The numerical simulations of the two configurations are used to determine the effect of axial blade row spacing on the level of entropy generation and the flow mechanisms that affect stage performance. The rotor shock waves that impinge on the upstream blade row result in shed vortices that convect downstream through the rotor. At the reduced axial spacing, vortices with larger circulation and entropy are formed. Local entropy generation is assessed using a new numerical technique that allows adequate evaluation of spatial derivatives in high gradient regions, such as shock waves. It is found that the main difference in entropy generation between the two configurations studied is associated with the shed vortices. Entropy production and rotor work input depend on the vortex trajectory within the rotor, which in turn depends on the ratio of time scales: the time for vortex convection between blade rows, and the rotor period (i.e. the time for the rotor to move one rotor pitch), for a fixed geometry and inlet Mach number.