This paper concerns the role of air injection method in stabilization and stall control in centrifugal compressors. The main aim is to find the best arrangement of air injection parameters such as injection angle and injection mass flow rate in order to optimize the injection performance for stabilizing the compressor and increasing the surge margin. Numerical model was built to simulate high speed transonic centrifugal compressor working at an operating point close to surge. Air was injected at 12 locations at the vaneless region between the impeller and the diffuser at shroud surface with 5 different injection angles and 3 different injection mass flow rates. Results showed that the best injection method is when using an injection angle of 30° with injection mass flow rate of 1.5% of the design mass flow rate and the worst injection method is the injection at angle of 180° (reverse tangent injection). Results also indicated that by using air injection, the number of stalled diffuser passages is decreased compared to the case of no injection. The most significant result of this paper is that using an angle of injection around twice the value of the diffuser vane angle gives the best results and makes the ideal correction of the fluid kinetic energy and fluid angle at the diffuser inlet. It was found that injecting air at an angle less than the diffuser vane angle weakens the effect of injection and doesn’t increase kinetic energy of the fluid at diffuser inlet. It was also found that injecting air at an angle larger than the diffuser vane angle corrects the fluid direction but, at the same time, decreases the fluid kinetic energy at diffuser inlet.

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