In order to explore the variation of stall mode with changing speed and the mechanism transition of casing treatment under different stall modes

numerical simulation of a centrifugal compressor with solid wall casing and casing treatment under different speeds was performed. The results showed that the stall mode transited from diffuser stall at 100% design speed to inducer stall with the speed decrease. Inducer stall also experienced from end wall stall and blade leading edge stall at 80% design speed to wide-range full stall at 50% design speed. The effect of self-recirculating casing treatment on stall margin and efficiency of centrifugal compressor were closely related to the stall mode

its stability enhancement effect was only valid for the blade leading edge stall at 80% design speed. At about 80% design speed

casing treatment tended to bring about bigger flow rate operating point for the main flow efficiency

thus reducing the efficiency. When the speed was low enough to 50% design speed

with a wide recirculation region in main flow passage

the secondary reverse flow in casing treatment could reduce the load of the impeller to passage recirculation region

so as to improve the efficiency.