On the Mechanism of Unsteady Tip Flow Occurrence in a Transonic Fan

The tip clearance flow, closely related to stable margin and efficiency, influences the performance of fan/compressor in aeroengines. The unsteady tip flow in a high-speed transonic fan rotor is studied by numerical method. This phenomenon is considered to be related to the flow instability. This paper studied the occurrence and development of unsteady tip flow. The unsteady flow structure and DMD mode of pressure fluctuation are also demonstrated. The Richardson number in a turbomachinery environment, Riturbo number, is novelly proposed to study the probability of Kelvin-Helmholtz(K-H) instability inducing unsteady tip flow. It is found that after the fluctuation occurs, as the flow rate decreases, the Riturbo number shows a downward trend of a power function. When the stability provided by the centrifugal force gradually decreases, the tangential inertia force plays a major role, and the internal instability of the shear layer is prone to occur under the large disturbance caused by the vortex wave interference.