Numerical Analysis of an Alternate Stall in a Radial Vaned Diffuser

Abstract

The flow structure in the radial diffuser of a centrifugal compressor is analyzed from steady and unsteady Reynolds-Averaged Navier Stokes (RANS) simulations performed at one rotation speed for which two stable operating ranges separated by an unstable zone have been experimentally experienced. Below a given mass flow rate, close to the peak efficiency point, phase-lagged single-passage simulations do not converge properly anymore. A low frequency appears in the CFD, which cannot be associated with any physical phenomenon. The computational domain is then extended, so that several passages of both the impeller and the diffuser are taken into account. At intermediate mass flow rates, an unstable operating range exists and simulations cannot converge properly either. Nevertheless, if the compressor is further throttled, another stable operating range is obtained at low mass flow rates. The flow structure in that stable operating range is rather unusual: an internal periodicity emerges inside the radial diffuser, involving a two-channel flow pattern. This two-channel flow pattern is found to be stable and fixed in time. Moreover, the phase shift between two adjacent channel pairs happens to be constant. This indicates that a new space – time periodicity is established at low mass flow rates, which involves groups of two passages in the radial diffuser. It is confirmed thanks to a new phase-lagged simulation including one impeller passage and two diffuser passages which shows a good convergence and which gives the same results, both in terms of performance and flow physics.