Mid-Span Stall Inception in Low-Pressure-Ratio Transonic Fans

Abstract

In this paper, steady and unsteady computational fluid dynamics (CFD) have been used to investigate stall inception for a modern low-pressure-ratio transonic fan. The computational results are validated against measurement data from a high-speed test facility. CFD validation was approached as a blind test case. The results show good agreement between the experiments and computations. Stall is triggered by the growth of a suction surface separation behind the shock around the mid-span of the rotor blade. As the fan is throttled, the separation grows, leading to increased blockage in the blade passages. At the point of instability, the separation grows further, locally increasing incidence and leading to the formation of a stall cell. It is shown that changes to the tip leakage flow leave the stall inception mechanism unaffected. A computational case with a suction surface slip patch between 25% and 75% span shows that the reduction in blockage around the mid-span increases the stall margin by 25%. This demonstrates that for cases with mid-span initiated stalls, it is important to consider the flow away from the tip as well as the flow in the tip region. A redesigned fan is used to illustrate that design changes around the mid-span can be effective in improving flow range. The redesigned fan increases the stall margin by 6.7% while maintaining the design point efficiency within 0.1%.