Aero-Acoustic Analysis of an Axial Fan

New fan concepts must achieve high efficiency and low noise levels in order to offer relevant alternatives in an increasingly competitive market. Analytical tools based on simplified theoretical approaches allow to quickly propose interesting concepts. Numerical simulation in fluid mechanics is used to complete these approaches because it offers a three-dimensional understanding of the flow by considering, to some extent, the effects of secondary flows. In this study, various numerical simulations are performed to analyze the tip clearance impact on a low-speed axial fan performance and noise emission. The studied configuration is the USI7 installed in ducts and experimentally tested at the University of Siegen. Initially, the work focuses on calibrating the mesh (on the casing, the blades, the clearance) and the calculation parameters in order to reproduce the aerodynamic performance of the fan at its nominal speed (3000 rpm) with a clearance corresponding to 1% of the nominal impeller diameter. This configuration is then used to perform simulations on the same fan with a thinner clearance of 0.1% of diameter. Afterwards, systematic analyses of the flow are carried out on the two configurations in order to quantify the performance losses associated to the increased clearance and to evaluate if acoustic models based on steady-state RANS calculations can accurately evaluate the noise emission. The results show that the accuracy of RANS simulations is limited for large tip clearance due to unsteady flow mechanisms. The noise predictions suffer from missing modeling for these complex flow features. Still the aeroacoustic analytical noise models allow to predict the noise increase associated with tip clearance increase.