Investigations on Noise Sources on a Contra-Rotating Axial Fan with Different Modifications

2.3 Design for Low-noise Fans

Axial blowing fans are commonly known in many fields of applications. By using axial fans swirls occur at the trailing edge of the blades due to the working principle. These swirls are unwanted in most cases and may have unfavourable influence on subsequent devices, e.g. higher pressure drop or lower heat transfer coefficient. As the static pressure rise is an evaluation criterion for the fan efficiency there are different possibilities for converting the dynamic pressure of the swirl into a static pressure rise. The most common application for rising the efficiency is the installation of discharge guide vanes.
The installation of contra-rotating axial fans (CRF) is another well-known opportunity to increase the efficiency of a fan. Besides the high power density and the high efficiency it should be noted that these fans are known for problematic noise behaviour. This is induced by the interaction of both wheels with different directions of rotation.
An Acoustic Camera was applied to examine various modifications of fan blade designs regarding their noise emissions. A so-called rotational beamforming algorithm allows for the detection of sources on the rotating blades by using a virtual rotation of the microphones. Depending upon the frequency different sources could be localised.
Both the leading and trailing edge were modified. The leading edge was shaped with a sinusoidal structure and shall lead to a lower leading edge noise, which is mostly induced by impinging vortices. The trailing edge was modified using serrations. These serrations shall lead to vortices at higher frequencies.
This paper shows the performed modifications and tests with the Acoustic Camera. It also presents first results and gives an outlook on future work and usage.