Effects of Inlet Disturbances on Fan Stability

This paper investigates the effects of inlet disturbance caused by crosswind on a fan blade operation and addresses the possible aerodynamic instabilities, which can arise for such a fan-intake system. The work is carried out by using a three-dimensional unsteady computational fluid dynamics (CFD) model (AU3D), and for a modern low-speed fan rig for which extensive measured data is available. The computational domain includes the fan with outlet guide vanes for the bypass flow, engine-section stators for the core, and a symmetric intake upstream of the fan (a whole low-pressure domain). The unsteady full annulus simulations under crosswind are performed to analyze the effects of inlet disturbances on the operation of this blade. It was observed that, for sufficiently high amplitudes of crosswind, the intake lip separates, and results in a significant loss of stall margin. Moreover, even in the absence of lip separation, the blade can still stall prematurely due to nonhomogeneous flow caused by the two contra-rotating trailing vortices. In the second phase of this study, the effects of fan loading on the suppression of flow separation in the intake, and the consequent stall margin of the fan blade, were explored. The results indicated that, as the fan speed increases, it becomes more capable of reducing the inlet distortion levels, and consequently, the loss in stall margin decreases.