| Axial flow fans are widely used in various kinds of automotive cooling,environmental ventilation and household appliances,but their main drawback is the high aerodynamic noise produced by rotating fans.With the demand of comfort working and living environment,it is very necessary to reduce the noise of axial fan.The sound radiation of low speed cooling fan can be divided into two parts: tonal noise and broadband noise.High tonal noise is the main reason for human ears to feel uncomfortable,so how to reduce the tonal noise and explain its noise reduction mechanism is one of the main directions of fan noise research.In this paper,an automotive axial flow cooling fan is taken as the research object,and different control strategies are proposed to suppress the aerodynamic noise of the fan.The flow field and far-field noise of the fan are studied in detail by combining numerical simulation and experimental measurement,and the noise reduction mechanism of the control method is revealed.The research results can provide important theoretical basis and technical support for noise reduction of axial fan.The main contents of this paper are presented as follows:1.The computational and experimental models of an asymmetrical airfoil-strut model and an axial cooling fan with downstream struts are designed and manufactured.The asymmetric airfoil-strut model is a two-dimensional simplified approximation of the axial fan model.For the airfoil-strut model,the flow configuration of the model is changed by adjusting the separation distance(L)between the airfoil and the downstream strut and the strut perforation rate(SP).There are two ways to change the strut perforation rate:(1)making a slot on the strut surface and(2)distributing uniformly holes on the strut surface.There are also two ways to control the noise of the axial flow cooling fan: one is to change the windward width H of the downstream strut;the other is to change the strut perforation rate which is similar to the simplified airfoil-strut model.The results show that these control strategies can effectively suppress the far-field noise emitted by the fan through reasonable layout.2.For the simplified airfoil-strut model composed of a single fan blade and downstream strut,the acoustic emission caused by aerodynamic interaction between the airfoil and the strut is studied by using a hybrid numerical simulation method combined with CFD+FW-H equations.The effects of spacing L between the airfoil and the strut,and the perforation rate SP on the acoustic propagation characteristics are analyzed quantitatively,and the mechanism of these effects is analyzed in combination with the flow field and aerodynamic characteristics.The results show that the noise emission characteristics of the flow configuration are closely related to the unsteady lift/drag near the strut surface.By increasing the separation distance L and the SP,the vortex-body interaction noise between the airfoil and the strut can be reduced significantly.3.For the axial flow cooling fan,a hybrid numerical simulation method combining large eddy simulation LES and FW-H equation is firstly used to simulate the flow field and the far-field noise,and the experimental verification of the axial flow fan noise is carried out in the anechoic chamber.Secondly,different fan noise suppression strategies are studied by using the same numerical simulation and experiment methods.The results show that the aerodynamic noise of the fan is closely related to the fan speed,the windward width H of the downstream strut and the perforation rate SP of the strut.Reducing the width of the downstream struts or increasing the perforation rate of the strut can significantly reduce the blade passing frequency noise(BPF). |
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