Test The Limits

Quiet Comfort: Acoustic Simulation to Resolve Fan Coil Cassette Unit Noise

By jnewberg

October 03 2017

Quiet Comfort: Acoustic Simulation to Resolve Fan Coil Cassette Unit Noise
By Mohammed Meskine


Look up and you might be able to spot a fan coil cassette unit: an active comfort unit integrated into the suspended ceilings of offices, open spaces, meeting rooms, shop units and entrance halls. If you’re warm – or cool – and comfortable right now, you can thank this unit, which enables the room temperature to be adjusted to suit occupants’ preferences. And if you’ve ever noticed this unit before – or maybe if you haven’t – you’ll know that the aerodynamic and noise performance of the fan is crucial for achieving satisfying efficiency and ensuring the building occupants’ comfort.


Typically, during the design and development of fan coil cassette units, physical prototypes are used to assess aerodynamic and aeroacoustic performance. But the complexity of these units and the limitations of physical measurement techniques make it impossible to identify the source of the noise coming from them, which significantly limits the development of quieter, more efficient units.

CIAT, a European leader in heating, cooling, and indoor air quality, publishing jointly with Exa Corporation, figured there had to be a better way. Presenting at the Internoise 2015 Conference (“Aerodynamic Noise Prediction of a Fan Coil Cassette Unit Using LBM”, http://internoise2015.com/), CIAT and Exa showed how a digital approach to simulating fan coil cassette unit performance can help to assess and identify the main noise generation mechanisms, leading to improved designs and quieter units.
 
The study investigated two configurations simultaneously: the fan only (configuration 1) and the fan integrated into the real comfort unit (configuration 2), in which the fan blades are connected to a ring on the underside of the unit as shown in Figure 1.
 
Figure 1
Figure 1
(Left 2 pictures) Experimental and numerical Configuration 1; (right 3 pictures) Experimental and numerical Configuration 2 with a zoom on the fan.
 
To validate the ability of digital simulation in Exa to accurately represent the real-world conditions of the fan coil cassette unit, the project compared the two physical configurations with their digital counterparts in Exa. The experiment concluded that aerodynamic performance was re-created in Exa simulations to within 5% accuracy, and that the noise radiating from both configurations was accurately predicted, as well: with Blade Passing Frequency (BPF) emerging on configuration 2 but not configuration 1, and accurate Over All Sound Pressure Level (OASPL) predictions made within 2 dB for configuration 1 and 3 dB for configuration 2 when compared with their physical counterparts.
 
With an accurate digital simulation for the aerodynamic and acoustic performance of the fan coil cassette units validated with real-world results, it was time to explore the power of digital simulation to identify the noise sources for further analysis and control. During this step, the team leveraged the Exa FIND (Flow Induced Noise Detection) post-processing module to deeply analyze the flow field for each configuration and identify acoustic sources.

FIND helped the team pinpoint the critical noise areas of the unit. The integration of acoustic power of the noise sources within different boxes showed that the noise sources are specifically present in local areas around the fan (see Fig. 3). FIND highlighted Box 4 as the main noise source area, which corresponds to the Leading Edge vortex location. This finding means that design optimization efforts need to be focused only on the connection between the blades and the ring to reduce the noise sources.   
 
Figure 2
Figure 2
Power Spectral Density (PSD) at 1.2 m from fan center for Configuration 1 (Left) and for Configuration 2 (Middle); (Right) microphone position.
 
Figure 3
Figure 3
Noise sources Acoustic Power: (left) integration boxes location; (right) Acoustic Power levels

While FIND was shown here to be a powerful tool to help engineers target the critical areas of a unit for noise optimization, it was also used to compare the noise sources generated by the two different configurations. The radiated Acoustic Power generated by these sources is shown in Figure 4, where areas with a high density of acoustic sources are shown in red. These are the regions that need to be optimized to make the system quieter overall. The Acoustic Power with the six boxes around the unit clearly shows stronger noise sources in Configuration 1, especially between boxes 3 and 6 where the fan is located. The difference between the acoustics sources is well highlighted by the radiated noise (see Fig. 5) and matches the ranking predicted by the experiments.

The accuracy of the predictions and the insight into the flow behavior provided by analyzing the digital simulations with Exa FIND confirms that this computational approach can be used to assess and make design decisions with confidence during the comfort unit development process. Think about that the next time you look up.
 
Figure 4
Figure 4
Acoustic sources visualization from FIND: (left) Configuration 1; (middle) Configuration 2; (right) acoustic power per integration box
 
Figure 5
Figure 5
PSD at 1.2 m from fan center for Configuration 1 (blue) and Configuration 2 (black); (left) Experiment; (right) Simulation

Reference: “Aerodynamic noise prediction of a fan coil cassette unit using LBM”, M. Meskine, M.S. Kim, F. Pérot, F. Polidoro, Y. Portier, F. Fournier, P. J. Vialle, Internoise 2015, San Francisco, USA