Numerical Simulations of Unsteady Aerodynamic Effects in the Blade Tip Region of Cooling Fans

Abstract

The acoustical characteristics of fans are an essential criterion of product quality and are continually growing in importance as for example cooling fan modules in the automotive industry have to suffice high comfort requirements. In order to locate dominant acoustic sources and to reduce the noise emission generated by a shrouded fan configuration, numerical simulations are performed. The working approach considers variously modified fan geometries and their evaluation regarding arising vortex flow phenomena and their effect on a decreased sound pressure level (SPL) in consideration of an improvement or the constancy of aerodynamic fan performance. Particular emphasis lies on the analysis of secondary flows in the blade tip region by postprocessing CFD-results. Due to the large number of geometrical modifications investigated and the importance of highly resolved eddy structures, a hybrid approach is chosen by applying the SAS-SST turbulence model in URANS simulations. The SAS (Scale Adaptive Simulation) delivers LES (Large Eddy Simulation) content in unsteady regions of a RANS-simulation and exhibits not nearly the high computational effort needed to perform a full scale LES. An assessment of the actual propagation of noise emission into the far-field is made by performing experimental investigations on the most promising modifications. The acoustic measurements are carried out in a fan test stand in the anechoic chamber of the University of Applied Sciences Duesseldorf. The aerodynamic performance is measured in a fan test rig with an inlet chamber setup in accordance with ISO 5801. The measured acoustical and aerodynamic performance is validated by the industrial partner. The results of the acoustic measurements are in turn utilized to determine indicators of noise radiation in the numerical simulation. Within this work an approach is presented where the analyses of secondary flows in the blade tip region provide the basis for an innovative noise reduction design in a shrouded fan configuration. The new design exhibits a reduced SPL (A-weighted) of approx. 4 dB over the entire operating range while showing no significant deterioration on the aerodynamic performance. The design was registered for patent approval cooperatively by the industrial partner and the University of Applied Sciences Duesseldorf, while minor design parameters are still subject to further improvements. All numerical simulations are performed with Ansys CFX, a commercial solver widely spread in the industry. Methods similar to those shown in this work can be implemented in the design phase of axial fans in order to develop acoustically optimized fan geometries.