The vaneless diffuser downstream of a centrifugal impeller is probably the simplest geometrical turbomachinery flow passage that can be described by three parameters. However, the flow through it is as complex as in any complex geometrical passage. A series of experiments were carried out at three different speed lines on a centrifugal compressor with two vaneless diffusers with parallel walls and different width. The performances in terms of pressure ratio, efficiency and flow range are compared and the unsteady phenomenon encountered at low mass flows studied. These tests are part of an extended program of testing of different diffusers type at MSU.
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