URANS Simulations and Experimental Investigations on Unsteady Aerodynamic Effects in the Blade Tip Region of a Shrouded Fan Configuration

Abstract

The acoustical characteristics of cooling fans are an essential criterion of product quality in the automotive industry. Fan modules have to suffice growing customer expectations which are reflected in the comfort requirements set by car manufacturers around the world. In order to locate dominant acoustic sources and to reduce the noise emission generated by a shrouded fan configuration, numerical simulations and experimental investigations 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 post-processing 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 Duesseldorf University of Applied Sciences. The aerodynamic performance is measured in a fan test rig with an inlet chamber setup in accordance to ISO 5801. The measured acoustical and aerodynamic performances are 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 innovative geometry modification is presented which can be implemented into shrouded fan configurations with backward-skewed blades. 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. While the design was registered for patent approval cooperatively by the industrial partner and Duesseldorf University of Applied Sciences, further investigations regarding variations of design parameters are performed and presented in this paper. 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.

Experimental Study of the Leakage Flow in an Axial-Flow Fan at Variable Loading

“…However, sealing the gap, Canepa et al [3] could inhibit the leakage flow; this showed that not only the leakage flow noise may constitute the major contribution to the low-frequency noise but to the overall radiation also, e.g., see Canepa et al [4]. Although the main features of the leakage flow are known, very few works related to it are available in the literature, e.g., see Fukano et al [5], Fukano and Jang [6], Piellard et al [7], Magne et al [8], Moreau and Sanjose [9], Zenger et al [10], Na et al [11], Canepa et al [3,4,12,13], and Zhu et al [14]. In order to find effective solutions for noise reduction, a deeper insight in the leakage flow features and in the effect of relevant parameters is necessary.…”

Leakage Noise and Related Flow Pattern in a Low-Speed Axial Fan with Rotating Shroud

“…Their interaction with the rotor blades results in noise in the low-to medium-frequency range, with the appearance of narrowband humps at subharmonic frequencies and a general increase of the broadband noise. The leakage flow noise often constitutes the major contribution to the overall radiation and its main features are known, but very few works related to it are available in the literature, e.g., see Fukano et al [2], Fukano and Jang [3], Piellard et al [4], Magne et al [5], Moreau and Sanjosé [6], Zenger et al [7], Na et al [8], Canepa et al [9][10][11][12], Zhu et al [13] and Sanjosé and Moreau [14]. In order to find effective solutions for noise reduction, a deeper insight in the leakage flow features and in the effect of relevant parameters is necessary.…”

Characterization of Unsteady Leakage Flow in an Axial Fan

“…Such a noise is often predominant in the lowto medium-frequency range and may also constitute the major contribution to the radiated power, e.g., see Canepa et al [2,3]. The qualitative features of the leakage flow are known, e.g., see Fukano et al [4], Fukano and Jang [5], Piellard et al [6], Magne et al [7], Moreau and Sanjose [8], Zenger et al [9], Na et al [10], Canepa et al [11][12][13], and Zhu et al [14]. However, coherent flow structures generally constitute a rather complicated fluid dynamic problem and, in the present case, their properties are not completely known: the fact that noise humps at sub-harmonics of the blade passing frequency appear indicates that they are co-rotating, but the relation with gap geometry and operating point is not completely clear.…”