Flow in multistage transonic compressors

Transonic compressors are regularly used as part of the compression system in industrial gas turbines because their high operating speeds make them capable of producing high stage pressure ratios and they have a high efficiency. The flow inside a these compressors is inherently unsteady due to rotor-stator interactions and the flow in transonic compressors is also subject to shock waves leading to further unsteady interactions such as the interaction between the inlet guide vanes and the rotor leading edge shock waves. Despite the presence of unsteady flow, CFD simulations regularly assume the flow to be steady relative to each blade row in order to reduce the cost and time to perform simulations, however, this introduces more assumptions into the simulations compared to unsteady CFD simulations. The first aim of this thesis is to investigate the causes of the differences in predicted efficiency observed between steady and unsteady simulations in order to show the operating conditions at which the steady flow assumptions break down. It is shown that the efficiency in the rotor blade row is over predicted in the steady simulations due to a weaker shock wave. The efficiency in the IGV is also over predicted while the efficiency in the stator blade row is under predicted. The unsteady simulations are also used to show that the stage efficiency characteristic is not affected by the unsteady effects from the downstream blade rows, when the stage is embedded in a multistage machine. In the next part of the thesis the effect of the IGV-Rotor axial gap on the compressor efficiency, and the dynamic blade loading, is investigated in multistage unsteady simulations. This shows that a reduced IGV-Rotor axial spacing gives an improvement in the efficiency but leads to an increase in the dynamic loading which may prove detrimental to the blade life. It is also show that altering the IGV-Rotor spacing has an effect on the loadings of the blade rows in the downstream stages. The final part of the thesis investigates vane rescheduling in a multistage transonic compressor using steady CFD simulations. Vane rescheduling is important in multistage compressors in order to prevent the compressor stalling and to maximise the efficiency when operating at off-design speeds. Vane schedules are optimised for a range of design and off-design speeds and the effect of rescheduling on the radial flow distribution is analysed. It is shown that the flow can be affected far downstream of the rescheduled blade row and that this can affect the location of the separation in downstream blade rows.