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Abstract

Leading edge noise is one of the dominant noise sources from axial fans. This sound source is particularly amplified under disturbed inflow conditions, i.e. high turbulence or inhomogeneous flow fields. In most technical applications, axial fans are operated in disturbed inflow conditions. The increased turbulence intensities and irregularities in the flow field are generated by other upstream system components. For example, heat exchangers and filters in the air-conditioning systems or protection grids and flow redirectors in the field of mobility are used on the suction side of the fan. The flow fields influenced by these components interact with the axial fan, which amplifies the leading edge noise. Since the inflow conditions differ from system to system, it is not possible to integrate this high amount of different flow profiles in the design process of the axial fan. For this reason, the axial fan is designed for undisturbed inflow conditions, which leads to increased sound emissions in real installation situations. In addition to this challenge, the lack of knowledge about the particular effects of turbulence quantities on the sound emissions of axial fans is a challenging task.
In most flows, turbulence characteristics such as turbulence intensity, length scale, anisotropy and inhomogeneity are highly connected. This made it difficult to investigate the influence of individual parameters and to gain a better understanding of the relation to the acoustics. The knowledge about the connection of sound generation mechanisms at the axial fan and turbulence characteristics would offer the possibility to estimate in advance the effects of different disturbed inflow conditions. In order to investigate these two tasks, an active grid was developed, which is used to generate defined inflow conditions. This offers the option to control the turbulence characteristics of the flow and thus to clarify the influence of individual turbulence characteristics. In addition, axial fans can be tested under different inflow turbulence conditions before they are installed in real systems. It was shown that with the active turbulence grid, the flow can be adjusted in a controlled manner and a large number of different flow conditions can be created. Based on the investigations, correlations between the turbulence characteristics and the sound emissions of axial fans were be concluded. In addition, it was shown that the mean flow fields of heat exchangers could be reproduced with this active grid.