With the ever-increasing issues caused by global warming, the emission of carbon dioxide has to be minimized. With the transport section being responsible for 24% of global emissions, the demand for electric alternatives is on the rise.
The electrification of passenger vehicles comes with benefits like a smoother driving experience, requiring less maintenance, and has led to an overall noise decrease. However, annoying noises that were previously masked by loud engines are now audible. Automotive cooling fan noise is such an example. The tonal character of the fan is annoying to pedestrians and residents, especially during car charging. The batteries reach high temperatures while charging, increasing the cooling demand from the fan module compared to conventional cars. In this situation, no inflow is generated from driving and the fan is operating at high rotational velocities and pressure requirements.

When the (axial) inflow to the fan is non-uniform, (the velocity changes over the circumference), a varying force is generated on the fan blade, when it rotates. When this non-uniformity is stationary (constant in time), these force fluctuations are periodic, and this is the main cause of the tonal noise. The non-uniformities in fan modules arise from inflow distortions, like for example heat exchanger, duct bends, or the shape of the shroud.

In current fan modules, unequal blade spacing is used to reduce the annoyance of these tones. This reduces the noise at the Blade Pass Frequency, but introduces tones at other harmonics of the rotational frequency, overall mainly leading to a reduction in noise annoyance.

Flow obstructions have been shown an effective means of reducing noise in equally spaced axial fans with low Mach number, while minimizing aerodynamic losses. They introduce a secondary non-uniformity in the inflow, which creates a secondary aeroacoustic source on the fan blade. When placed at the correct circumferential angle, cancellation between the primary and secondary source is possible. This research investigates if such flow obstructions can still yield adequate noise reduction on unequally spaced fans, and if (multiple) upstream heat exchangers prohibit the working mechanism of flow obstructions.

It was found that flow obstructions can indeed significantly reduce tonal noise at the fan BPF, by up to 13 dB on the fan axis, and 6dB at an angle 45\degree from the axis. The upstream radiator and condenser did not compromise this reduction, and the obstruction still reduces noise when the fan operates at a lower rotational velocity. Using a superposition of obstructions (creating a multi-modal obstruction) could also control multiple tones. However, with the large number of tones to be controlled in an unequally spaced fan, the overall sound pressure attenuation and annoyance reduction are insignificant.

Furthermore, the low-fidelity analytical tool developed in the thesis was an effective means of rapid obstruction design and optimization. Although information on the fan's primary noise is needed, as well as some parameters to define the obstruction wake (which can be obtained with cheaper, RANS simulations), the model correlated well with simulation data obtained in PowerFLOW.