Axial compressor stall.

This dissertation describes the experimental investigation of rotating stall in compressors of high hub-tip ratio. Measurements were obtained in builds of one to four stages, covering a wide range of design flow rates and also a change in design reaction. The purpose of the experimental work has been to gain a better understanding of the details of the flow in the stall cell, so as to make possible the prediction of the in-stall performance of axial flow compressors. A new, on-line, phase-lock sampling technique has been developed, which, for the first time, allows the quasi-continuous recording of instantaneous velocity, pressure and flow direction measurements from within the stall cell. The technique provides results of superior quality to any hitherto available, and has been used to study the details of the flow under various test conditions. In particular, the axial and radial profiles of the stall cells in multi-stage compressors of different design flow rates were considered. The effects of changes in design reaction, blade row spacing and the influence of the number of stages in the compressor were also investigated. Finally, measurements were obtained to try to determine the differences between full-span and part-span stall. From a study of the results obtained, a fundamentally new picture of the flow in the stall cell has been built up, which shows features that are at variance with conventional ideas about stall cell structure. It has been shown that within the compressor itself, the edges of the cell do not follow streamlines in the unstalled flow, as might be expected of a dead wake, but rather assume an attitude implying tangential flow across the cell. The presence of this tangential flow is easily detectable from the measurements obtained within the cell, and is supported by the observation of extremely high whirl velocities just upstream of the rotor blades. Comparatively little flow is shown to pass axially through the stalled region, but, because of the opposing accelerating and decelerating influences of the rotor and stator blades, the axial movement of this small amount of fluid has been shown to lead to the dissipation of large amounts of mechanical energy. This results in the overheating of the compressor during stall. Centrifugal effects in the swirling flow ahead of the rotor blades are shown to contribute to strong radial pressure gradients in the stall cell, and are thought to be responsible for the fact that the pressure rise across a stage in the stalled region occurs ahead of the rotor blades. This observation is included in an overall model of compressor performance which is then used to explain the observed relationships between the various time-averaged compressor characteristics. This performance model is also used to provide some explanation of the observation (first made by McKenzie) that the total-to-static pressure rise across a stalled compressor appears to be independent of the blading used, but increases by a fixed amount for each stage in the compressor. Time-averaged measurements were obtained from all the compressor builds tested, and were used in conjunction with the overall flow model to formulate a new correlation for predicting the performance of a stalled compressor. The correlation makes use of the above idea that the pressure rise during stall is independent of the blading used, for both part-span and full-span stall, and relies on the concept of a critical level of cell blockage to distinguish between operation in either of these two modes. The work reveals the effect of the number of stages and the design flow coefficient on the behaviour of the compressor, and demonstrates the unexpected influence of system parameters (such as throttle line slope) on the stalling performance. The correlation also makes it possible to predict the occurrence of either full-span or part-span stall at stall inception from a knowledge of the unstalled characteristic, and allows estimates to be made of the size of the stall/unstall hysteresis loop. Data obtained from the literature are successfully correlated by this approach, and a new basis is therefore established from which to view the stalling behaviour of all axial flow compressors