Effects of Housing Geometry on the Performance and Noise of a Two-Outlet Centrifugal Fan - A Numerical Study

E3 - Flow Characteristic of Centrifugal Fans

Noise is a common nuisance of domestic products that contain fans. Centrifugal fans are commonly used in these applications, especially in household appliances and lifestyle products. Centrifugal blowers are able to obtain high pressure, large flow rate, relatively low noise and small dimensions compared to the other types of air-moving devices. However, depending on the different functions imposed by the customer needs, the fan designs of these products is very diverse. Usually the flow within a centrifugal fan is highly three dimensional and unsteady. Therefore, analysis using computational fluid dynamics (CFD) software is frequently adopted to uncover the key noise-generating flow dynamics within a centrifugal fan. In the present study, a centrifugal fan design with two flow outlets is investigated. This two-outlet centrifugal fan design is aimed at providing high mass flow rate but very low noise level, which is suitable for air-purifier applications in quiet premises such as bedroom. Two dimensional unsteady flow simulations with CFD code, Fluent 6.3, are carried out to analyze the fan flow dynamics and assess its subsequent noise radiation. The calculations were done by solving the unsteady Reynolds averaged Navier Stokes (URANS) equations in which effects of turbulence were accounted for using k-ε model. Although a two dimensional simulations are adopted in the study, all relevant key flow dynamics and key fan performance parameters calculated are consistent with the three dimensional situations. Therefore, the results of the calculations suffices to provide an insight in the dominant noise source mechanisms of the two-outlet centrifugal fan and an indication of the variations of noise level with key fan geometrical parameters, such as the ratio between cutoff distance and the radius of curvature of the fan housing. Four fan design variations are simulated and compared with existing design. The results of simulations show that the unsteady flow-induced forces on the fan blades are the main noise sources. The blade force coefficients are then used to approximate the dipole source terms in Ffowcs Williams and Hawkings (FW–H) equation for assessing the noise effects. It is found that one design is able to deliver a mass flow 34% more, yet with sound pressure level (SPL) 10 dB lower, than the existing design. This founding is verified by testing at the manufacturer laboratory.