The technique of smoke-flow visualization has been used to show clearly the action of the flow field upstream of a single rotor during rotating stall. The flow processes on the blades and in the blade passages of a stationary compressor cascade also have been observed using the same technique. An audio method of detecting rotating-stall patterns has been developed and has indicated that some compressor operating conditions at which no periodic flow disturbance was previously thought to occur actually contained rotating-stall patterns whose number of stalled regions changed very quickly from one value to another. The absolute speed of rotation of a stall zone was reduced to zero and its over-all shape clearly outlined by means of smoke visualization. The qualitative results of all these investigations have given a good physical picture of the rotating-stall phenomenon. Severe reverse flows were found to exist during rotating stall and these caused the formation of reversed flow regions extending upstream of a blade row. Furthermore, the distortions of the flow field in the vicinity of a blade row were found to be of such large magnitude that it does not seem likely that they can be described adequately by any linearized theory. Quantitative investigations were made into the effects of guide-vane turning and axial position on the rotating-stall characteristics of an axial-flow-compressor rotor. The absolute direction of the fluid entering such a configuration was found to affect nearly all facets of rotating-stall behavior.

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