On the Impact of Operating Conditions and Testing Environment on the Noise Sources in an Industrial Engine Cooling Fan

In the evolving automotive landscape, the shift from conventional thermal engines to electric ones has made unconventional noise sources more relevant, such as the engine cooling fan, especially from the human listener's point of view. The engine cooling module has therefore become of primary concern, leading to design studies for quieter and efficient fans. However, it has been shown that different testing conditions can widely alter the noise footprint of these fans; thus, the assessment of the installation's effect on different testing cases rapidly gained importance. This paper, therefore, investigates an industrial engine cooling fan's main noise sources and generation mechanisms. Installation effects in the semi-anechoic room and at two different operating points (free blowing and maximum efficiency) are numerically assessed. First, the fan is tested at free blowing conditions in a testing environment that resembles a semi-anechoic room. At free blowing conditions there's no pressure difference across the fan, with a high mass flow rate through it. As a consequence, being off-design condition, the flow over the blades is largely separated, locally recovering when the blade passes through a blockage region given by the motor cable strut. The noise footprint is therefore highly tonal, with a clear peak on the blade passing frequency tone. If the same fan is tested in a free field environment, it is found that there is a difference on the acoustic pressure at higher harmonics of the blade passing frequency. Then, the fan is tested at maximum efficiency. In this condition is found that the blockage mentioned above is no longer present, since a pressure difference is set, yielding a lower mass flow rate through the fan. The main noise generation mechanism can be instead found in the tip leakage vortex which arises because of the pressure difference across the fan and interacts with the blade tip leading edge, also resulting in a more broadband noise footprint. This highlights the importance of assessing the effect of the testing point on the fan's acoustics, since in the industry is a standard practice to test with zero pressure difference.