Numerical Study on Duct Acoustic Modal and Source Flow Structure of Fan Tones With Leaned and Swept Stator

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 of the high-bypass turbofan engine, and the classification, quantification, and ultimate reduction of fan noise components is of major interest. The fan noise is mainly scattered by the rotor-stator interaction, which the fan tone is generated by the non-uniform mean circumferential flow field of the rotor blade wake interact with the downstream blade, and fan broadband noise is generated by the turbulence in the rotor blade wake interact with the downstream blade.  Generally, the fan tone noise is the major sound sources of fan, and to reduce rotor-stator interact tone noise is of great significance for reducing engine noise.

It is well known that the lean and sweep stator blades can significantly reduce the fan rotor-stator interaction tone noise, and many researchers have studied the effects of stator blade lean and sweep on fan tone. However, due to the most researches were based on analytical model or lift surface method, there still is a lack of understanding on the source flow field structure and the acoustic modes distribution of fan tone with lean and sweep stator blade. The present study is focused on the fan tone noise reduction with lean and sweep stator blade. In this paper, a hybrid URANS/Goldstein method is used to calculate the unsteady flow and tone noise of axial-flow fan. A high loaded single stage axial flow fan, namely Hi-Fan2 which consists of 24 rotor blades and 36 stator blades and was working at 7000 rpm with the pressure ratio of 1.24, was used to investigate the flow and acoustic mechanisms of the fan tone noise reduction. Both the aerodynamic and acoustic performance is assessed for the Hi-Fan2 with different lean and sweep stator.

The results show that the influence of blade lean and sweep on fan mass flow is less than 1% within the scope of this study. The negative lean angle or sweep angle of stator blade can improve the aerodynamic performance of the fan to some extent, but the positive lean angle of the stator blade will reduce the total pressure ratio and isentropic efficiency of the fan. It is found that the influence of fan stator blade lean on fan tone is complex. The negative angle lean of stator blade will increase fan tone noise, while the noise reduction using stator blade with positive lean angle is related to the harmonic number and propagation direction. The higher the harmonic number, the better the noise reduction effect with positive lean angle blade. The noise reduction is also different in different propagation directions. The stator sweep can indirectly increase the lean angle of the stator wake at the leading edge of the stator blade, and reduce the wake strength at the leading edge of the stator blade, so noise reduction effect with sweep blade is better than that using lean blade.

The numerical simulation results show that when the stator blade sweeps back 30 degrees, the fan tone noise at the fan inlet can be reduced by 5.5 dB, while the fan tone noise at the fan outlet can be reduced by 9.8dB. It is also found that the integrated design of the fan stator with blade lean and sweep has the best noise reduction effect. When the fan stator blade lean angle and sweep angle are both 30 degrees, the fan inlet tone noise can be reduced by 8.5 dB, while the fan outlet tone noise can be reduced by 17dB.