A volute’s loss coefficient is highly sensitive to its inflow conditions: The Mach number is likely to have an impact, but, according to 1D-theory, the governing parameters are the flow angle and the flow rate at volute inlet. However, in case of a given impeller with constant geometry, operated at constant circumferential speed, these parameters are coupled to each other: According to continuity, an increased flowrate leads to a steeper absolute flow angle due to an increased radial velocity (and, in case of a backswept impeller, also due to a decrease of circumferential velocity). In addition, the absolute Mach number is altered. Since the radial velocity’s kinetic energy dissipates within the volute, its losses increase rapidly if the impeller is operated at higher than design flowrates. This leads to the common misconception that a too small volute generates high losses. However, in order to investigate the effects of flow angle and flow rate separately, one would have to vary the diffuser width together with the flowrate, keeping the radial velocity constant: The volute should be coupled with aerodynamically similar impellers, designed for higher and lower flowrates (i.e. flowcut impellers). In order to keep also the Mach number at volute inlet constant, one would have to couple it with a plurality of different impellers and diffuser widths. Since this is elaborate and costly, a study with a number of impeller-volute combinations, adequate for assessing the volute’s global loss map, is not available in open literature. In this work, the authors present a new numerical approach for the prediction of a volute’s representative loss map: CFD-calculations of the volute as a standalone component are performed. The inlet boundary conditions are carefully selected by means of 1D and applied together with different diffuser widths. This allows for separate investigation of the impacts of flow angle, flow rate and Mach number. Validation against full stage CFD-calculations confirms the applicability of the standalone model. The results exhibit that minimum losses do not necessarily occur at the theoretical matching point but either when the volute is too small or too big, depending on the inlet flow angle. The correspondent loss mechanisms, relevant at different operating conditions, are also identified. Generally, losses show to be more sensitive to proper matching in case of increasing inlet Mach numbers. The validity of these study’s findings for other volute shapes and sizes is discussed. Finally, design guidelines regarding proper volute sizing are presented.


Assessment of the Loss Map of a Centrifugal Compressor\u2019s External Volute

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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