Using Isolated Diffuser Simulations

In compression systems the range of stable operating is limited by rotating stall and/or surge. Two distinct types of stall precursors can be observed prior to these phenomena: the development of long-wavelength modal waves or a short-wavelength, three-dimensional flow breakdown which is typically known as a "spike". The cause of the latter is not well under-stood; in axial machines it has been suggested that over-tip spillage flow has a significant role, but spikes can also occur in shrouded vaned diffusers of centrifugal compressors, where these flows are not present, suggesting an alternative mechanism may be at play. Unsteady Reynold's Averaged Navier-Stokes simulations are performed for an isolated vaned radial diffuser from a highly loaded centrifugal compressor. Key to their success is the definition of pitchwise "mixed out " averaged inlet conditions derived from the impeller exit flow field from separate single passage stage calculations. This guarantees the relevan