Numerical Investigation on Tip Clearance Flow in a Transonic Fan Rotor

摘要

Unsteadiness of tip clearance flow appears at operating points near the stability limit of the compressor with a relatively large tip clearance of the rotor blades whether in axial or centrifugal compressors/fans. Unsteady tip clearance flow is not only the source of undesirable noise but also a potential indicator for critical operating conditions with rotating stall or surge. It can also induce blade vibration, which would cause premature blade failure when the vibration is strong enough. The objective of the present study is to give more physical insight into the characteristics of unsteady tip clearance flow fields, its formation mechanism, and the relation between leakage vortex and rotating stall in a transonic axial compressor. NASA Rotor 67 is chosen and investigated in this paper. Unsteady single- and full-passage simulations of NASA Rotor 67 are performed using a commercial, three-dimensional, time-accurate, Reynolds-averaged Navier-Stokes solver. After validating the CFD data, we make a detailed analysis of the flow structure associated with tip clearance flow based on the numerical results. It is found that unsteady tip clearance flow appears when the compressor operates at the small flow rate condition near stall. The evolution of the tip leakage vortex plays a decisive role in the disturbance of the flow filed. The trajectory and position of the leakage vortex directly determine the stable state of the compressor. Further analysis indicates that disturbance is caused by the unsteady periodical oscillation of tip leakage vortex. With the mass flow rate further throttled, the trajectory of tip leakage vortex moves upstream. The results of the unsteady characteristics at the tip region are shown detailed.