Enhancing compressor stall and surge margin is very important for the development of turbo compressors. Casing treatment is an effective method to expand the stall margin and thus the stable operation range but it generally comes with a drop in efficiency. A new type of casing treatment with self-adaptivity for an unshrouded centrifugal impeller is presented in this paper. The idea of the self-adaptive casing treatment is to automatically control the bleeding flow rate through a set of round holes on the stationary shroud casing. When stall is approached, the casing treatment system will redirect the low-momentum fluid to improve the flow in the impeller and to extend its stable range. However, the system will not bleed or bleed a little flow when the operating point is far from the stall point. This decreases the bleeding-recirculation loss and increases the efficiency compared to bleed-slot casing treatment used before. The experimental results show that the centrifugal compressor with the new casing treatment can extend stable range by about 20% while the efficiency over the whole operating range increases by 0.2 to 1.5%. The theoretical results using CFD analyses agree with the experimental ones in the tendency for both pressure ratio and efficiency. Moreover some possible reasons for further enhancing the stable range are suggested in the paper. Based on our experiences, there should be existed a proper combination of impeller rotating speed, position and numbers of bleed port and other parameters that will maximize both surge margin range and efficiency.

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