Recovery Process from Rotating Stall in a Fan

An implicit, time-accurate, three-dimensional, compressible Reynolds-averaged Navier-Stokes solver, based on a cell-vertex finite volume methodology and mixed-element unstructured grids, was applied to the study of the recovery process from rotating stall on a transonic fan. It is well known that a larger throttle opening is required to move an operating point from rotating stall to an unstalled condition than for the throttle closing at stall inception. This phenomenon, referred to as hysteresis, has a significant influence on the compressor operation, since the fan may not recover from stall in some cases, in spite of a large opening of the throttle or nozzle. The objectives of this paper are to demonstrate the feasibility of using computational fluid dynamics for capturing the hysteresis loop of a fan and to explore the flow features during this process. A wide-chord fan blade, typical of modern civil designs, was used as the benchmark geometry for this study. Rotating stall at 70% of the design speed was simulated using a whole-annulus model by decreasing the nozzle area. As the mass flow rate was decreased beyond the near-stall condition, a part-span stall was seen to develop near the casing. The nozzle was then opened gradually and the internal flow recovered from rotating stall to an unstalled operating condition. As expected, the numerically predicted final operating condition had higher mass flow rate and lower total pressure ratio than those at near-stall condition. The numerical results during recovery were compared against the measured data, which were obtained on a test rig with an analogous geometry and flow conditions. The comparison showed qualitatively good agreement.