Numerical Investigation of Negative Flow Characteristics in a Centrifugal Compressor With Vaned Diffuser and Internal Volute

Abstract

Negative flow from the outlet through the volute, diffuser, and impeller to the inlet of the centrifugal compressor can occur continuously as a result of system accidents. A physical comprehension of negative flow dynamics is crucial in evaluating the compressor characteristics under abnormal working conditions and is also important in exploring the compressor aerodynamics over the entire flow range. However, limited research on the negative flow dynamics in centrifugal compressors, particularly with the consideration of vaned diffusers and volutes, can be found. This study aims to determine the compressor characteristics, including the negative flow rates of a centrifugal compressor, and to clarify the negative flow mechanism under the interaction of the volute, diffuser, and impeller. The last stage of a four-stage centrifugal compressor, including an internal volute, a vaned diffuser, and a closed impeller, was simulated under both positive and negative flow conditions using a computational fluid dynamics (CFD) model. The results show that the pressure ratio–negative flow characteristic is almost matched with a parabolic curve. At negative flow rates, the backflow generated on the hub and shroud sides in the impeller expands upstream and causes flow separation in the diffuser. The negative flow enters the impeller at a large incidence angle and results in jet-wall impingement on the pressure surface, flow spillage over the trailing edge, and flow separation near the suction surface. The impeller partially acts as a turbine impeller and performs negative work on the fluid. This work is of scientific significance to enrich the compressor aerodynamics in accident scenarios and of engineering value to improve the advanced design of compressor protection systems.