Comparison of Multiple Fan System Assemblies Using an Acoustically Transparent Duct

2.2 Experimental Methods for Localizing / Characterizing Sources

Fans are widely used for industrial as well as household applications. In many of these cases, for instance air conditioning or ventilation systems, the fans are built into ducted environments. The number of such applications surrounding us in our daily lives has increased with the advancement of technology, and therefore an eye needs to be kept on noise pollution levels, which need to be reduced further in order to meet the requirements of strict laws and the demands of consumers. In order to do so, the noise generation mechanisms of fans have to be investigated, understood, and inevitably eliminated. Combining an array of microphones with beamforming technology, one can localize noise sources and determine noise generation mechanisms.
In the case of ducted fans, such as the one investigated herein, acoustic investigations should be carried out in a ducted environment in order to guarantee proper flow conditions. One approach could be to mount microphones on the wall of the duct. Such measurements run with many complications, including the disturbing effects of duct modes and flow induced microphone self-noise. Another approach would be to place the array of microphones outside of the flow (outside of the duct) and to use an Acoustically Transparent Duct (ATD) in order to gain direct access to the noise radiating from the source (eliminating the effect of duct modes). Such an ATD was designed by our research team, and has been shown to allow through acoustic signal which are sufficient for investigating low speed fans with beamforming technology for a wide frequency range while providing a hydrodynamically impenetrable duct surface for the flow. In our design, a perforated duct section provides the ducts shape and connects to other components (for example the fan), while a layer of stretch film, characterized by low acoustic impedance, provides hydrodynamic impenetrability. This article is one in a series regarding the ATD designed by our research team. While the first articles investigated the acoustic characteristics of the ATD and its applicability [1, 2], this is the first test case, where the focus of the investigation is on studying fan noise sources.
In this investigation the most commonly occurring configurations, which can be built from a fan, the ATD, an inlet cone and spiral wound duct sections (ex. free-inlet free-outlet, ducted-inlet ducted-outlet, etc.) are looked at. The various components of the configurations have differing effects on the flow, and therefore also on the acoustic characteristics. The examination and comparison of the various cases helps to define the effect of each component on the noise generation mechanisms of the fan. In our research we examined the effects of each component on the classical fan noise generation mechanisms found in the literature, such as turbulence ingestion noise, boundary layer noise, trailing edge noise, vortex shedding noise, separation noise and tip-leakage flow noise.

[1.] Tokaji K., Horváth C., Design of acoustically transparent duct, 25th International Conference of Mechanical Engineering, Kolozsvár, Romania, 2017. April 27-30, Kolozsvár: Erdélyi Magyar Műszaki Tudományos Társaság (EMT), pp. 416-419.
[2.] Tokaji K., Horváth C., Acoustically Transparent Duct, International Journal of Aeroacoustics, (Submitted).