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A stream of pure air envelopes a person standing in a room, removes their exhaled air, and creates a barrier against the outside air – this is how the prototype installation designed by AGH UST scientists works, an installation to protect people who might be exposed to COVID-19 infection. The work carried out at the Faculty of Civil Engineering and Resource Management is implemented as part of a university grant.
The COVID-19 pandemic, which broke out towards the end of 2019 in the Chinese city of Wuhan, is still disorganising socio-economic life. To limit its devastating spread, in addition to relentless work towards developing effective treatments and vaccines, we need to take appropriate preventive measures. In this case, proper ventilation of the rooms is crucial. This is because the infectious SARS-CoV-2 virus spreads, for example, through aerosols, that is, a suspension of air, fine solid particles, and liquid droplets that everyone emits when they breathe and speak, not to mention when coughing or sneezing. Such a cloud containing pathogens can move about and linger in air for extended periods of time in crowded and poorly-ventilated rooms.
Engineers from the AGH UST Ventilation and Air-conditioning of Facilities Team at the Faculty of Civil Engineering and Resource Management have decided to face this challenge in an innovative way. They are currently working on an air purification system which, in comparison to those applied in general ventilation and air-conditioning systems, is intended for a single person. The tested solution is made of a ventilation fan, an electrostatic air filter capturing and neutralising microbes, a ventilation installation, and a ceiling diffuser made of an elastic and easily disinfected PET-G material. The diffuser is meant to create a stream of air that will remove the exhaled air from the occupied zone while simultaneously blocking the flow of air from the outside.
Fig. 1. Installation flow diagram; ‘Air purification system used under the conditions of COVID-19 virus exposure on working stations, including mining’ project materials
‘The idea originated as a result of our observations and the needs of industry, where people have to work in close contact or where there is the need to transport people confined to small spaces, as in mining, for example. There, you can’t allow yourself the luxury of transporting individual people one at a time. Therefore, constructing this type of ventilation will facilitate the minimisation of human exposure to the virus in the aforementioned situations’, explains Dr hab. Marek Borowski, AGH UST Associate Professor who coordinates the work.
The solution developed by the AGH UST researchers can be redesigned by using a different type of fan, so that the installation instead of creating a protective barrier around a single person would shield an entire group of people.
The challenge that the designers and producers of HVAC systems (heating, ventilation, and air-conditioning) face is to provide optimal functionality of their devices, while simultaneously bearing in mind the maximum comfort of the user. The latter factor is influenced by the temperature, humidity, purity, and velocity of the air flowing from the system. The last parameter is decisive when it comes to feeling unpleasant draughts in the room. The PMV index (Predicted Mean Vote), which is commonly used to determine the thermal comfort of the user, assumes that the air velocity in the occupied zone should not exceed 0.2 m/s.
One of the important elements of the installation, which affects the profile of the air stream and user comfort, is the plenum box. It is the part to which the diffuser is connected and through which air flows right before it is fanned into the room.
Fig. 2. A diagram of a plenum box with side entry and top entry, Halibart et al., 2021
‘The plenum box decompresses the air stream and causes a decrease in its velocity before it reaches the fan, where it is profiled accordingly by the arrangement of vanes and other elements. Moreover, when the air velocity decreases, the noise level is also reduced. This is why research on the shape of the box, the direction of air entry, or its velocity is so important and has been carried out for many years. We’re still trying to reach the optimal shape, because when you mount the installation, it’s sometimes impossible to apply ideal solutions’, explains Professor Borowski.
During research work, AGH UST scientists have examined the impact of plenum box entry on the velocity profile of the air stream. Their analysis was based on two solutions – the first one included a top entry, the other – a side entry. Their findings are reported in this Energies publication.
To find out which solution is better, the engineers have first analysed the side entry variant. To determine the air velocity field inside the plenum box, they used the PIV method (Particle Image Velocimetry). This is a non-invasive optical research method that allows researchers to trace the movement of particles and their direction within the flow. It rests on the fact that the moving molecules diffuse the laser light, which makes it possible to record their displacement by specialised cameras.
Fig. 3. Air velocity vector field in the plenum box with side entry, Halibart et al., 2021
In the next step, the researchers have looked at a similar situation, but this time they used the CFD method (Computational Fluid Dynamics), using six different numerical models. Such models describe the dynamics of the flow using meshes, which can be given adequate parameters to fit the situation at hand. Such complex systems of equations can be quickly solved on a computer equipped with dedicated software.
The solutions obtained by the scientists during the simulation were compared with previous experimental results, allowing the researchers to indicate a model that delivered solutions that were the most convergent with the actual measurements.
Fig. 4. Comparison of measurements using PIV and CFD methods, Halibart et al., 2021
‘This gives us the grounds for further analyses that we can perform faster now. The measurements are very time-consuming and require concentration. Moreover, a lot of time is also spent describing the results, which is why simulations are a very attractive tool to use. But to be able to infer based on them, first, we have to make the measurements to know whether they converge with reality’, says research work leader.
The model chosen by the AGH UST engineers served them to check the shape of the air stream profile when using the plenum box installation with top and side entries. The CFD analysis showed that the way the stub pipe is connected to the box has a significant impact on the distribution of air from the fan. Although, taking space limits into consideration, the producers of such devices use side-entry plenum boxes, the air stream profile in this configuration is less symmetrical than in the case of a top entry.
Fig. 5. Comparison of air stream profiles using either side- or top-entry plenum boxes, Halibart et al., 2021
The researchers indicate that a partial solution to this problem might be to install perforated panels inside the box. However, this can come at the cost of lowering pressure and increasing noise levels.
Research on this prototype installation has been carried out in the laboratory of the Ventilation and Air-conditioning of Facilities Team, which is equipped with indispensable infrastructure.
Professor Borowski says: ‘Our laboratory is capable of recreating the conditions of an actual room; we have a suspended ceiling which can be placed at various heights and filled with fans in all kinds of configuration. This matters because airflows also depend on the distribution of the side walls and the ceiling. The air, using the so-called Coandă effect, can stay attached to a given surface, which makes it possible to displace such an air stream farther in the room. Air streams generated by neighbouring ventilation fans also affect each other’.
During their work, researchers are focusing on finding the optimal shape of the fan. To do this, they conducted meticulous studies of the velocity of air flowing out of the diffuser with the use of thermoanemometers. During experiments, they rely on a mesh of control points, determined previously by conducting smoke testing.
Fig. 6. Thermoanemometers used to conduct measurements; ‘Air purification system used under the conditions of COVID-19 virus exposure on working stations, including mining’ project materials
Fig. 7. Smoke testing; ‘Air purification system used under the conditions of COVID-19 virus exposure on working stations, including mining’ project materials
In addition to velocity, the scientists analyse other parameters that can influence user thermal comfort, such as humidity and temperature. Their studies also focus on the concentration of particulate matter in the air before and after it passes through the electrostatic filter.
‘The results of the measurements should be ready towards the end of the year; we want to publish them in scientific journals then’, promises the project coordinator. He adds that there are plans to commercialise the solution and that the creators keep in touch with companies interested in the invention.
The project was funded by a university grant within the framework of the “Excellence Initiative – Research University” project (the AGH UST 2020–2022, PRA-2).
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