The effect of applying a vaned recessed casing treatment to a single-stage axial flow fan has been investigated. The influences of the axial position of the recess relative to the rotor leading edge and other geometric modifications of the vane passage have been examined. Significant improvements in stall margin were observed without (in some builds) loss in peak efficiency. Slow and fast frequency response yawmeter probes have been used in the study to examine both the steady flow conditions and the unsteady flow caused by rotating stall.

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