Experimental Studies on the Aeroacoustics of Low-Speed Axial Fans

Licentiate thesis, 2023

Low-speed axial fans are used in thermal management systems of internal
combustion and electric vehicles. These systems are compact, cost-effective, all
while addressing safety concerns. On the downside, fan operation leads to flow
induced noise. Fan noise generation is strongly linked to installation effects,
because they impact inflow and outflow conditions. This thesis focuses on the
aeroacoustic performance of low-speed axial fans including installation effects.
An experimental study on the interplay between the aeroacoustic performance
of a multi-fan system and installation conditions, is carried out.
Specifically, two electric axial fans with rotating rings are installed in parallel.
Inlet shroud length and distance between the fan centers are varied. It is found
that the system’s aerodynamic and acoustic performance is marginally affected
for different fan distances. On the contrary, a longer inlet shroud improves the
aerodynamic performance, while significantly amplifying the sound power over
a wide frequency range. The amplification is linked to the change of radiation
impedance for the fan-shroud system.
Another aspect of this work concerns the validity and improvement of
acoustic measurements in a bespoke fan test facility. Initially, a round robin
test of a low-pressure axial fan is conducted. Measurement repeatability of
aerodynamic and acoustic performance is achieved. However, it is observed
that the estimating of the fan’s sound power and directivity are affected by
environment effects. Consequently, a transfer function method is presented
for sound power estimation. Furthermore, an approach based on spherical
harmonic decomposition is outlined. This approach is poised towards source
directivity studies in non-ideal acoustic environments.