End-wall flows and blading design for axial flow compressors
The flow in multistage axial flow compressors is particularly complex in nature because of the proximity of moving bladerows, the growth of end-wall boundary layers and the presence of tip and seal leakages and secondary flow. The problems associated with these phenomena are at their most acute in the latter, subsonic stages of the core compressor, where Reynolds numbers are modest and the blading has low aspect ratio. Indeed, much of the inefficiency of axial stages is believed to be associated with the interaction between blading and end-wall flows. The fact that the end-wall flow phenomena result in conditions local to the blade which are quite different from those over the major part of the annulus was appreciated by many of the earliest workers in the axial turbomachinery field. However, experiments on blading designs aimed specifically at attacking the end-loss have been sparse. This thesis includes results from tests of conventional and end-bent blading in a four-stage, low-speed, axial compressor, built specifically for the task, at a scale where high spatial measurement resolution could be readily achieved within the flowpath. Two basic design styles are considered: a zero a0 stage with DCA aerofoils and a low-reaction controlled-diffusion design with cantilevered stators. The data gives insight into the flow phenomena present in 'buried' stages and has resulted in a much clearer understanding of the behaviour of end-bent blading. A 3D Navier-Stokes solver was calibrated on the two low-reaction stators and was found to give good agreement with most aspects of the experimental results. An improved design procedure is suggested based on the incorporation of end-bends into the throughflow and iterative use of the 3D Navier-Stokes solver.