Tonal Noise Prediction of Unevenly Spaced Blades Axial Fans Based on Blade Force Model

2.3 Design for Low-noise Fans

Tonal noise is the main noise source of the automotive engine cooling fans. Many methods are adopted to reduce the tonal noise level, such as uneven-spaced fan, swept and leaned fan. The uneven-spaced fan technique is widely used in the automotive engine cooling fans. As normal, the uneven-spaced fan is designed applying phase modulation principle according to adjusting the blade spacing angles, that is the Modulated Blade Spacing method. For that method, the blade forces were not taken into account, and the blade forces have much effect to the noise prediction. In this paper, a detailed investigation was conducted on the transient characteristics of blade forces within uneven-spaced fans.
To obtain the blade force, CFD modeling of two axial low-speed fans, one unevenly-spaced and another evenly-spaced were carried out. Discrepancies and characteristics in blade-forces' within uneven fans were identified and thus modeled. The blade segmentation strategy was used to abstract the blade force from unsteady Reynolds-averaged Naviers Stocks simulation, propagated into the far field using point-force model derived from Lowson model. Then the noise modulation relation was set up based on the blade spacing angle and blade force. Verifications are done with comparing results with experimental data, Duncun model, as well as the prediction done by applying the Lowson model directly to the calculated forced items get from CFD. The comparison results show that the modified noise modulation prediction model agreed well with the experiment.