Low pressure ratio transonic fan stall with radial distortion

Numerical methods that can predict stall behaviour with non-uniform inlet conditions allow assessment of the stable operating range across flight conditions during the design of fan stages for civil aircraft. To extend the application of methods validated with clean inflow, the effect of a tip low radial distortion on the stall behaviour of a low pressure ratio transonic fan has been investigated using both high speed experiments and 3D URANS computations. The distortion is generated in the experiment using a perforated plate and this is fully represented within the computational mesh. This enables computations to reproduce the full range of flow conditions accurately without adjusting the inlet boundary condition. Both the calculations and measurements show that the presence of the distortion decreases the stall cell rotational speed and increases the cell circumferential extent. In the calculations, the cell speed reduced from 87% to 67% of shaft speed, compared to a change of 82% to 58% in the experiment. With and without distortion, the computations show how stall inception stems from blockage formed by flow separation from the tip-section suction surface, behind the shock. In the distorted case, the more forward shock position produces the blockage further upstream, causing a greater reduction of flow to adjacent passages. This leads to a stall cell in the distorted case that is around 80% larger.