Session: 35-02/10-06 Joint Session Fan & Turbomachinery noise and operations

Submission Number: 175804

Research on Optimal Shroud Modification and Aerodynamic Noise Reduction of Axial Flow Fan in Air Conditioner Outdoor Unit Based on Acoustic Modal Analysis 

With the increasing societal demands for energy conservation, environmental protection, and sound quality, the noise from household air conditioner outdoor units—particularly the aerodynamic noise generated by the axial flow fans—has become a critical factor constraining product competitiveness. The uneven airflow induced by the L-shaped inlet of outdoor units often leads to pronounced tonal noise. Since outdoor unit axial fans operate at low speeds with fewer blades, their Blade Passing Frequency (BPF) noise typically falls below 50 Hz, which has historically drawn little attention. However, as outdoor units grow more compact, with tighter upper and lower wall dimensions, the inlet flow becomes more irregular, and tip vortex turbulence worsens. This amplifies discrete tones at BPF and its harmonics, raising the overall sound pressure level and degrading noise quality. Understanding the root causes of this aerodynamic noise is now essential.

This study examines the axial fan in air conditioner outdoor units, using aeroacoustic theory and acoustic modal analysis to identify noise sources and explore noise-reducing shroud designs. First, the prototype fan’s far-field noise spectrum and directivity are measured. The CS method with dual-uniform sampling point (DUSP) distribution then maps the circumferential acoustic modes and sound power levels at the fan outlet, with particular emphasis on capturing the energy attenuation patterns of low-order dominant modes. Next, experimental studies on various modifications of the fan shroud are carried out to preliminarily explore their noise reduction capabilities. Combined with acoustic mode propagation theory, the physical mechanisms by which shroud modifications regulate noise are revealed.

The experimental results demonstrate that reasonable modifications to the shroud of the outdoor unit (such as designs with circumferential grooves and axial straight grooves) can significantly suppress the energy radiation of low-order dominant acoustic modes at the 1st to 3rd BPF. The modified shroud optimizes the uniformity of the intake flow, reduces the interference intensity between the rotating blades and inlet distortion, and decreases the amplitude of unsteady aerodynamic forces caused by rotor-stator interaction. Consequently, the energy input to low-order tonal modes is reduced, ultimately achieving the effect of lowering tonal noise.

Presenting Author: Anni Lin Shanghai Jiao Tong University

Presenting Author Biography: Ms. Anni Lin is a Master's Candidate at the School of Mechanical Engineering,Shanghai Jiao Tong University, under the supervision of Professor Hua Ouyang. Her research focuses on flow instabilities and aeroacoustics in rotating machinery, with particular emphasis on noise reduction in axial fans of air-conditioning outdoor units. She employs computational fluid dynamics (CFD) to simulate complex flow fields and investigate noise generation mechanisms.

Authors:

Anni Lin Shanghai Jiao Tong University
Zhaoyin Li Shanghai Jiao Tong University
Yitong Wu Shanghai Jiao Tong University
Jie Tian Shanghai Jiao Tong University
Hua Ouyang Shanghai Jiao Tong University