The present work analyzes the unsteady flow structure of a low-speed low-pressure ratio axial fan stage operating at stall conditions. Unsteady simulations were conducted with the elsA software, which solves the Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations on a full-annulus calculation domain for the rotor and the stator. The analyses will focus on a near stall operating point, the stall inception mechanisms, and the final rotating stall pattern. Numerical simulations are confronted with experimental measurements from high-frequency Kulite transducers mounted on the casing near the rotor and stator blades. The results show that boundary layer separation occurs on the stator blade close to the hub region at near stall conditions. The reverse flow interacts with the main flow to form a vortex shedding phenomenon. The frequency is not clocked with the blade passage frequency, requiring unsteady full-annulus methodology even for a stable operating point. It is also shown that the final state into the rotating stall region is composed of a single part-span cell that occupies three-quarters of the circumference and rotates at 60% of the shaft speed. This pattern agrees with the experimental measurements in terms of both the number of cells and the rotational speed. Finally, the paper demonstrates that without inlet distortion, the onset of the rotating stall is wrongly predicted by the simulation. However, by imposing a distortion on an angular sector in the rotating frame, that permits to observe the same inception that is observed experimentally.

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