Axial-flow compressor stall and stability

The research described in this dissertation is a computational study aimed at investigating blade row stability and stall inception in a high-speed compressor. Three-dimensional, steady and unsteady Navier-Stokes flow solvers were used to identify and understand the differences in the off-design low mass flow performance and stalling inception mechanism of a high-speed rotor blade row when it operates in isolation and as part of a single-stage (rotor-stator) build. The main conclusion from steady flow analysis is that when the rotor operates in the presence of a downstream stator, it is able to maintain stable axisymmetric performance down to a lower mass flow compared to when it operates in isolation. The rotor’s stability is extended when operating in the stage environment primarily because the stator acts to redistribution the radial pressure variation imposed at the rotor exit, thereby unloading the sensitive tip region. Unsteady flow analysis of the isolated rotor reveals that stall inception is associated with reversed flow at the tip region of a single rotor blade spilling forward of the leading edge. This results in the development of a spike-type disturbance that grows rapidly into a single finite stall cell. The reversed flow at the rotor tip and the resulting increase in tip-clearance related blockage that are both associated with the initial development of the spike disturbance are attributable to modal perturbations promoting localised flow separation at the rotor tip and to spanwise migration of fluid within the suction surface boundary layer. When the rotor operates as part of a stage, a qualitatively different type of stall inception mechanism is observed. Unsteady flow analysis reveals a one-dimensional breakdown of the rotor tip-clearance flow, with the flow observed to ‘surge’ upstream in an axisymmetric stalling pattern. Unlike the development of traditional rotating stall, no evidence of modal or spike-type disturbances are detected prior to the onset of stall and no stall cell structure is formed.EThOS - Electronic Theses Online ServiceGBUnited Kingdo