The cause of inlet reverse flow was studied in axial flow turbo machinery. A helical inducer, in which neither stall nor significant radial flow was observed, was selected as the experimental model. The flow between the blades was measured by a laser-Doppler velocimeter and investigated using the end-wall boundary-layer theory. Results showed that the inlet reverse flow occurs adjacent to the pressure surface between the blades in the vicinity of the casing wall. Inlet reverse flow, caused by a momentum defect in the axial direction in the boundary layer on the casing wall and a significant pressure gradient in the axial direction adjacent to the inlet blade pressure surface at partial flow rate, was found even though there was no significant radial flow or stall. In this paper, radial flow is defined as flow that causes a nonnegligible increase in the pressure on the casing wall.

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