Numerical investigation of aeroacoustic noise reduction of large diameter axial flow cooling fans through tip clearance reduction

Past studies have hypothesized that the noise emissions caused by large diameter axial flow fans are mostly attributed to turbulence ingestion noise, with trailing edge noise being less pronounced. This study investigates the effect of reducing the blade tip clearance on the noise emission profile of large diameter axial flow fans. A numerical study is conducted on an eight bladed 1.542 m diameter, axial flow cooling fan, with constant solidity and hub-to-tip ratio. The numerical study comprises of computational fluid dynamics and MATLAB code to predict the noise profile of the fan. Three tip clearances were tested: 0.26%, 0.13%, and 0% of the fan diameter or 2.29%, 1.14% and 0% of the blade chord. Results show a consistent noise reduction with decreasing tip clearance, while improving the total-to-static performance of the fan. Conclusively, reducing the tip clearance at constant solidity and hub-to-tip ratio lowers noise emissions for large axial flow cooling fans. The numerical prediction tool utilised also gives good overall agreement in capturing the effect of tip clearance reduction.