On the Application and Limitations of Sound-Absorbing Materials for Axial Fan Blades

The design process of axial fans is based on increasing the aerodynamic efficiency and reducing the sound emissions. Low noise emissions are regarded as a quality feature and further are needed to fulfill increasing legal regulations. In order to achieve the lowest possible noise emission of the fan, a large number of blade modifications have already been investigated. For example, attempts have been made to reduce the turbulent ingestion noise of the fan with serrated, wavy or slotted leading edges. However, these modifications often reduce the aerodynamic efficiency. Therefore, the challenge is to find modifications of the fan blades, which induce low noise radiation at high efficiency. Since it was found that changes in the shape of the blade usually result in a reduced efficiency, the focus is on preserving the blade in its basic form. The reduction of the sound emission will be achieved by changing the materials in the region of the leading edge of the axial fan. Based on previous works on stationary airfoils, this study investigates the impact of porous materials in terms of noise reduction of axial fans. Porous materials with different acoustic impedance and airflow resistivity were selected in order to understand the physical noise reduction mechanisms. The material properties are determined experimentally using a two-port test rig. The axial fans are tested in a standardized axial fan test rig with regard to their acoustic and aerodynamic behavior. Two inflow conditions with different turbulence intensity are considered. In this study two different approaches are taken to identify the noise reduction mechanisms. First, the integration of sound-absorbing materials to increase the acoustic absorption. Second, the integration of a structured porosity, which should influence the momentum exchange in the flow. The first study shows that acoustic absorption is not the main cause of the noise reduction. On the one hand, this is due to the small surface area of the sound absorbers and, on the other hand, to the fact that the insertion of the absorbers generate a disturbed flow over the fan blade. To generate a quiet basic flow, a structured porosity was integrated in the second study. Here, a higher noise reduction could be observed for the axial fans. Further, it could be shown that a connection between the suction and the pressure side of the axial fan is indispensable for sound reduction and that structured porosities create higher noise reduction compared to sound-absorbing materials