In axial compressor or fan one of the major sources of losses is uncontrolled development of casing boundary layer and its interaction with the rotating blades. These losses are aggravated in an axial compressor or fan with low aspect ratio blades due to strong secondary flow development.
The recess vaned Casing treatment has been used in a low speed contra-rotating axial flow fan unit with low aspect ratio blades. The casing treatment aims at removing casing boundary layer from near the second fan tip and inject the flow back near the first fan tip. The treatment works on a principle of automatic suction and injection. The boundary layer control mechanism is expected to be automatically adjusted by axial static pressure gradient across the vanes.
The recess vaned casing treatment has yielded significant improvement in contra-rotating fan unit performance, and has increased the surge margin, essentially through fluid flow control near the casing.