A unique vortex nozzle facility has been conceived and developed to simulate the exit flow from a high pressure ratio centrifugal compressor impeller. Visual studies and performance measurements have been made for three vane sets representing common designs for vaned radial diffusers. Motion pictures show the progression from choke through operating to surge conditions as the back pressure on the diffuser is increased. The films, together with total and static pressure measurements, indicate that surge is an instability triggered by flow separation in the vaneless or quasivaneless space ahead of the diffuser throat. A geometrical criterion for the onset of surge is identified. The surge-to-choke operating range of the three diffusers appears to be a function of the number of diffuser vanes only.

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