Blade Skewing Impact on Engine Cooling Fan-system Acoustic Performances at Various Operating Points

2.6 Case Studies: Acoustics

Design parameters for axial fan, especially those driving the blade geometry, are key factors impacting acoustic performance of automotive cooling Fan-Systems (FS). Most common requirements for OEM are specified for pedestrian annoyance measured in front of the fan, usually in the axis at 1 m. Frequency distribution and overall noise levels are the main criteria set for acoustic selection, with other considerations on both tonal and broadband noises.
It is now widely admitted that blade skew is a major contributor to the noise production, and the aim of the current paper is to assess its actual effect on broadband noise, for fan working in cooling modules.
Some investigations have been conducted for comparison between 3 fans designed for the same operating point with different blade skew, i.e. forward, backward-forward and forward. Designs have been obtained through equivalent optimization processes, keeping the same characteristics for blade profile and fan system parameters.
Some care was also taken to keep the tonal noise as low as possible by removing close proximity of support and arms from the fan. Both aerodynamic and acoustic performances were measured on cooling modules with a stacking of heat exchanger, with reduced or increased pressure losses for off-design conditions.
Acoustic spectra will be presented for the 3 configurations, leading to results showing the fan ranking regarding acoustics and flow rates. At high flow rate, the backward design has the best acoustic behavior, whereas it becomes the worst at low flow rate. The mixed design backward-forward has shown the most stable performance with quite small noise level variations with flow coefficient. In contradiction with other results found in literature on blade stacking effect, the forward blade does not provide good acoustic performance: these results will be discussed in regards to the high pressure coefficient and the moderated curvature used for the forward design.
Human annoyance from the fan noise is further investigated using loudness criterion, which is best correlated with a panel of listeners than A-weighting SPL for usual FS levels. Using this method, the backward stacking is still the best design for high flow rate and has shown some improvement at low flow rate, even if acoustic ranking is not changed.
Further studies to complete conclusions will be discussed as a perspective for the understanding of fan noise mechanism in actual configuration (i.e. cooling module).