The test data of a three-stage L.P. compressor with a solid casing and a treated casing under both uniform and circumferentially distorted inlet conditions are presented in this paper. The effects of two different casings on the air flow, efficiency, constant speed line and surge margin of the L.P. compressor are analyzed. Four different configurations of the treated casing were designed and tested. It was found that the surge margin was improved by means of casing treatment. The best one of all configurations is that with large stagnation cell with blades, which brought about increase by 4 percent of the surge margin of the compressor at design speed. The attenuation of inlet flow distortion is also presented with two types of casings. The treated casing has a greater capability to attenuate the inlet distortion than the solid casing.

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