On the Fluid Mechanical and Acoustic Mechanisms of Serrated Leading Edges

In most applications, axial fans are operated in disturbed inflow conditions. This is the case, for example, in air conditioning systems when the axial fan is positioned downstream of a heat exchanger. But axial fans are also used in traffic and habitation applications for cooling and fresh air supply. Here, the turbomachines are operated with protection grids or filters, which in turn can generate turbulence on the suction side of the axial fan. All of these induced inflow turbulences cause the sound radiation from the fans to increase and the systems to be recognized as disturbing. Especially due to the current corna-panedemic this disturbing factor increases, due to the fact that air conditioning units are more often installed in indoor areas. In order to reduce the sound emissions of axial fans under disturbed inflow conditions, leading edge modifications of axial fans have been investigated in recent years. For example, it has been attempted to reduce the sound emissions from the fans by means of serrations, slits or porous materials, while at the same time avoiding any losses in the pressure build-up. The basic mechanisms of these sound reduction measures are not yet fully understood and many leading edge modifications are based on internal company experience. In order to understand the physical mechanisms of serrated leading edges, simulations of a flat-plate fan downstream of a turbulence grid were performed. The simulations were validated with experimental measurements. It was shown that the serrations form longitudinal vortices along the fan chord and that this leads to a stabilization of the flow over the blade. In addition, radial flows are reduced by this fluid pattern and backflow over the tip gap are reduced. The improved flow guidance over the blade surface reduces the pressure fluctuations on the blade surface, which in turn leads to a decrease in sound radiation. A transfer of the sound reduction measures from a flat-plate fan model to profiled fans showed that the found fluid mechanical mechanisms have only a smaller impact on profiled fans. This indicates that the possibilities for optimizing axial fans with leading edge modifications are limited and can only result in noise reductions for certain cases.