Numerical Simulation of the Effect of the Tip Clearance Flow on Rotor-Stator Interaction Tone Noise in Axial-Flow Fan

Aero-engines of today’s transport aircraft are the dominant noise sources for most flight conditions, and one of the most commonly known noise source of aero-engine is the fan noise, and the classification, quantification, and ultimate reduction of fan noise is of major interest. It is well known that a smaller rotor tip clearance(TC) is beneficial for the aerodynamic performance of axial-flow turbomachines. The pressure rise increases and the onset of rotating stall moves to lower flow rates as the TC is reduced. In addition, the TC flow is recognized as having an influence on fan noise as a result of blade-tip-vortex interactions and tip-vortex-stator interaction. The nature of a tip leakage vortex observed in a rotor tip region has been studied by many researchers because of its important role on the aerodynamic performance, while other researchers focused on the random broadband noise generated by the TC flow interfering with the rotor blade itself.

The present study is focused on the sound generation due to the rotor tip clearance flow interaction with stator in an axial flow fan. In this paper, a hybrid URANS/Goldstein method is applied to calculate the unsteady flow and tone noise in axial-flow fan with different rotor TC. A high loaded single stage axial flow fan, namely Hi-Fan2 which consists of 24 rotor blades and 36 stator vanes and was working at 7000 rpm with the pressure ratio of 1.24, was used to investigate the flow and acoustic mechanisms of the TC flow noise in this study. Both the aerodynamic and acoustic performance is assessed for the Hi-Fan2 with different rotor TC.

The numerical simulation results show that the main sound sources of fan tone noise are concentrated in the leading edge of downstream stator blades. It is found that when the fan operates at a specific speed and pressure ratio, the mass flow rate and isentropic efficiency of the fan decrease with the increase of rotor tip clearance of the fan. The results indicate that when the rotor tip clearance increases from zero to 2.5 mm (0.94% relative blade height), the mass flow of the fan decreases by about 2% and the efficiency of the fan decreases by about 1 percentage point. Further analysis of the flow field structure indicates that the wake width and wake strength at the rotor exit increase with the increase of tip clearance, but the wake phase distribution hardly changes. When the wake of the rotor blade interferes with the downstream stator blade, the change of tip clearance mainly affects the unsteady load intensity on the surface of the stator blade, but has little effect on its phase distribution.

The results show that the noise level at 1BPF increases with the increase of fan rotor tip clearance, whether it propagates forward or backward in fan flow duct. The numerical results show that when the tip clearance of the rotor increases from zero to 2.5mm, the sound power level of 1BPF tone noise propagating forward of the fan increases by 1.47dB, and that of 1BPF tone noise propagating backward increases by 0.65dB. However, the influence of tip clearance variation on the tone noise intensity at 2BPF and 3BPF is more complex, and there is no specific rule, and the variation range is less than 1dB. Further analysis of the calculated results shows that the rotor/stator interaction tone is mainly caused by the unsteady pressure on the downstream stator blade surface, and the unsteady pulse force is mainly affected by the outlet conditions of the upstream rotor. The change of rotor tip clearance significantly changes the inflow condition of stator inlet. The tip secondary flow caused by rotor clearance seriously affects the circumferential inhomogeneity of stator leading edge inflow conditions. The amplitude and range of velocity loss increase with the increase of rotor tip clearance. These two factors lead to the increase of fan tone noise level.