Assessment of the Loss Map of a Centrifugal Compressor's External Volute

Abstract

A volute's loss coefficient is highly sensitive to Mach number, circumferential velocity, and flowrate at volute inlet. In case of a backswept impeller, these parameters are coupled to each other. Therefore, in order to investigate the effects of flowrate and flow angle separately, one would have to vary the diffuser width together with the flowrate, keeping the flow angle constant. This corresponds to coupling the volute with aerodynamically similar impellers, designed for higher and lower flowrates. Since this is elaborate, there is no adequate study available in open literature, assessing a volute's global loss map. In this work, a new numerical approach for the prediction of a volute's representative loss map is presented: The volute is calculated by means of steady computational fluid dynamics (CFD) as a standalone component. The inlet boundary conditions are carefully selected by means of one-dimensional calculations (1D) and applied together with different diffuser widths. This allows for the separate investigation of the impacts of flow angle, flowrate, and Mach number. Validation against full-stage CFD confirms the applicability of the standalone model. The results exhibit that minimum losses do not necessarily occur at the theoretical matching point but when the volute is either smaller or bigger, depending on the inlet flow angle. Investigations of the loss mechanisms at different operating conditions provide useful guidelines for volute design. Finally, the validity of these study's findings for volutes with different geometrical features is examined by comparison with experimental data as well as with full-stage CFD.