Mechanism of circumferential static pressure oscillation in a centrifugal compressor with volute

Under the action of an asymmetric volute structure, a nonuniform flow field is formed in the circumferential direction of the centrifugal compressor. During the throttling process of the compressor at different rotational speeds, the static pressure presents a double-peak distribution of two high static pressure strips, one of which is induced by the volute tongue. However, the formation mechanism of the other high static pressure strip remains unclear. In this regard, computations of the steady and unsteady flows in a centrifugal compressor with and without a volute are performed. The purpose of removing the volute is to simplify the boundary conditions at the diffuser exit, eliminate the circumferential pressure gradient distribution in the volute, and retain the circumferential local high static pressure region induced by the VT; thereafter, the circumferential static pressure distributions in the diffuser and impeller are observed. The results indicate that after eliminating the pressure gradient at the diffuser exit along the rotation direction, only local high static pressure boundary conditions can result in the formation of two high static pressure strips in the diffuser and impeller. The local high static pressure at the exit redistributes the mass flow rate at the impeller outlet, forming two regions with high airflow velocity in the diffuser; this leads to the appearance of two high static pressure strips in the circumferential direction. With the increase in the pressure amplitude of the high static pressure at the diffuser exit, the oscillation amplitude of the circumferential pressure is intensified, and the pressure peaks of the two high static pressure strips increase. However, the circumferential positions of the two static pressure peaks practically remain constant. At large mass flow rates, the pressure reduction along the circumferential direction at the diffuser exit preclude the formation of two circumferential high static pressure strips in the diffuser and impeller.