Blade force model for calculating the axial noise of fans with unevenly spaced blades

In recent years, there has been an increasing demand for quieter cooling fans. It is well-known that the magnitude of the tonal noise that radiates from an axial low-speed fan can be reduced by unevenly spacing the fan blades, and as this practice does not compromise the air flow rate of the fan, it is now widely used in the fans in automobile engines. However, conventional schemes for unevenly spacing the fan blades, such as modulated blade spacing, do not sufficiently reduce the noise in fans. Thus, the focus of this study is to identify quieter blade spacing arrangements by accurately predicting the magnitude of the tonal noise that radiates from a fan when its blades are spaced in a particular manner. As the forces exerted on fan blades play a large role in generating the tonal noises within axial low-speed fans, a computational fluid dynamics-based investigation was conducted to obtain the transient characteristics of the blade forces for fans with unevenly spaced blades. In addition, transient Reynolds-averaged-Navie r-Stokes analyses were conducted for axial low-speed fans with evenly and unevenly spaced blades in order to better elucidate the details of the blade forces. In the analysis, a blade segmentation strategy was employed wherein the blades were treated as sets of point forces, and the various forces exerted on each blade were identified and modelled via a linear approximation based on the blade spacing. The model was then used to predict the sound harmonics that propagate into the far field based on the Lowson model. Finally, a tonal noise reduction model was introduced to evaluate the tonal noise reduction potential of fans with arbitrary and uneven blade spacings. Experiments were conducted to validate the prediction accuracy of the developed model. (C) 2018 Elsevier Ltd. All rights reserved.