Tip leakage flow and aeroacoustics analysis of a low-speed axial fan

Axial fans are widely used in the aerospace field, and new regulations and environmental concerns are prompting manufacturers to design efficient low-noise axial fans. The aerodynamic performance and acoustic emissions of axial fans are substantially affected by the unavoidable tip clearance. Herein, a systematic analysis is performed on axial fans with different tip-clearance sizes to gain a clear understanding of the characteristics of tip leakage flow and investigate the generation mechanism of aeroacoustics. The reliability and accuracy of the numerical predictions are successfully validated through a comparison with experimental data. The unsteady pressure information on the blade surface is examined to clarify the main noise sources. The results show that the enhanced intensity of the blade tip vortex and thereby the enhanced interaction with the blade surface are the main drivers for the extra broadband noise when the tip clearance is increased. For the low-speed axial fans, the low-frequency broadband noise below 600 Hz is mainly related to the blade tip vortex and the interaction of the boundary layer instabilities with the leading edge and trailing edge, whereas the broadband noise above 1200 Hz is mainly due to the turbulent boundary layer fluctuations.