Rotating stall in variable geometry compressors

The design and operation of gas-turbine engines is heavily influenced by the off-design stability of the compressor, which limits obtainable performance and may result in aeroelastic vibration issues. Whilst Variable Stator Vanes (VSVs) are widely used to mitigate this problem, research considering the mismatching effect of VSVs away from their optimal settings is limited. In this thesis, a high-speed variable geometry compressor is studied at part-speed conditions and VSV setting adjustments are made to deliberately trigger stable rotating stall and study its behaviour. Examination of unsteady measurements reveals two "families'' of rotating stall, each at different frequencies, where the dominant behaviour depends upon the VSV settings. Stall in the front stage is shown to consist of a spatially non-uniform and time-varying pattern of short lengthscale cells, which couple with rotor vibration and propagate as noise. Second stage stall is a longer lengthscale uniform disturbance consisting of fewer stall cells. The stalling pressure amplitudes are also found to correlate well to blade loading parameters from a one-dimensional meanline model. Steady and unsteady CFD simulations at these stalled conditions confirm that the behaviour is due to regions of stall in the front stage tip region together with the hub of the second stage. These CFD calculations naturally result in the formation of stall cells and give a credible match to the experiment. Inviscid reasoning explains how this flowfield is due to spanwise static pressure gradients arising from part-speed closure of the VSVs. Finally, the non-dimensional cell propagation speed (Vstall/U) for each family of stall is shown to be uniquely determined by the VSV settings. This appears to be linked to the axial flow velocity local to the cell and suggests that cell speed may be restated in a more universal non-dimensional form. Furthermore, simulations show the importance of flowfield coupling mechanisms in determining the number of stall cells, which are also driven largely by the VSV settings.Open Acces