Investigation of Rotating Stall in Vaneless Diffuser of Single-Stage Centrifugal Blower Using Wall-Resolved Large Eddy Simulation

Abstract Centrifugal turbomachinery, including centrifugal blowers, the focus of this study, is utilized across a wide range of fields and applications, from industrial plants to home appliances. The market demands higher efficiency and a wider operating range. A sudden increase in loss generation impacts the slope of the performance curve and can cause intense fluctuations in pressure within a centrifugal blower, which typically defines the lower limit of the operating range for turbomachinery. To gain a comprehensive understanding and extend the blower's operating range, we applied wall-resolved Large Eddy Simulation (WR-LES) with 14.6 billion grids to a small-scale model of a single-stage centrifugal blower. This model blower was equipped with a vaneless diffuser and a scroll casing. Experimental measurements were also conducted on the model blower to validate the accuracy of the WR-LES. A particular emphasis was placed on clarifying the relationship among the onset of rotating stall, intense fluctuations in static pressure, loss generation, and separations of the meridional flow in the vaneless diffuser. The onset of rotating stall was accurately captured by the WR-LES, whereas under-resolved Large Eddy Simulation predicted it at a lower mass-flow ratio than the experiment or could not predict it at all. At reduced mass-flow ratios, repeated separations of the meridional flow from the hub and shroud sidewalls occur in the diffuser at a mass-flow ratio of 0.61, preceding the onset of rotating stall at 0.41. These separations are induced by a strong adverse pressure gradient in the radial direction rather than by rotating stall. Furthermore, these separations in the meridional flow are accompanied by the occurrence of intense static-pressure fluctuation as well as an increase in loss generation in the diffuser. As rotating stall develops in the vaneless diffuser at a further-reduced flowrate, loss generation intensifies at the mid-span plane, particularly near the diffuser inlet. This is driven by localized acceleration of the tangential velocity, leading to stall-cell formation. Rotating stall also localizes and amplifies the aforementioned separations of the meridional flow in the diffuser.