Research

35 pages, 7205 KiB

Open AccessArticle

Computational Fluid Dynamics-Based Calculation of Aerosol Transport in a Classroom with Window Ventilation, Mechanical Ventilation and Mobile Air Purifier

by Philipp Ostmann, Dennis Derwein, Kai Rewitz, Martin Kremer and Dirk Müller

Atmosphere 2024, 15(1), 140; https://doi.org/10.3390/atmos15010140 - 22 Jan 2024

Viewed by 913

Abstract

During the SARS-CoV-2 pandemic, the air quality and infection risk in classrooms were the focus of many investigations. Despite general recommendations for sufficient ventilation, quantitative analyses were often lacking due to the large number of combinations of boundary conditions. Hence, in this paper, [...] Read more.

During the SARS-CoV-2 pandemic, the air quality and infection risk in classrooms were the focus of many investigations. Despite general recommendations for sufficient ventilation, quantitative analyses were often lacking due to the large number of combinations of boundary conditions. Hence, in this paper, we describe a computational fluid dynamics model that predicts the time-resolved airflow for a typical 45

\min

classroom scenario. We model 28 students and a teacher, each emitting CO₂ and an individual aerosol. We investigated 13 ventilation setups with window or mechanical ventilation and different positions and operating conditions of an additional air purifier. The ventilation performance is assessed by evaluating the ventilation effectiveness, aerosol removal effectiveness, local air exchange efficiency and overall inhaled aerosol mass of the occupants, which is a measure of the infection risk. If the window is opened according to the “20-5-20” recommendation, the incoming airflow reduces both the CO₂ and aerosol concentration whilst decreasing the thermal comfort at low ambient temperatures. An active air purifier enhances aerosol removal, but, depending on the position, the local air exchange efficiency and individual aerosol inhalation vary. If mechanical ventilation with 700 m³/h is utilised, the CO₂ concentration is kept below 1250 ppm while also effectively removing aerosol from the classroom. Full article

(This article belongs to the Special Issue Research on Indoor Air Cleaners for Particulate, Microbiological and Gaseous Pollutants)

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17 pages, 2307 KiB

Open AccessArticle

Numerical Study on the Impact of Large Air Purifiers, Physical Distancing, and Mask Wearing in Classrooms

by Aman Jain, Finn F. Duill, Florian Schulz, Frank Beyrau and Berend van Wachem

Atmosphere 2023, 14(4), 716; https://doi.org/10.3390/atmos14040716 - 14 Apr 2023

Cited by 4 | Viewed by 1302

Abstract

The risk of COVID-19 infection from virulent aerosols is particularly high indoors. This is especially true for classrooms, which often do not have pre-installed ventilation and are occupied by a large number of students at the same time. It has been found that [...] Read more.

The risk of COVID-19 infection from virulent aerosols is particularly high indoors. This is especially true for classrooms, which often do not have pre-installed ventilation and are occupied by a large number of students at the same time. It has been found that precautionary measures, such as the use of air purifiers (AP), physical distancing, and the wearing of masks, can reduce the risk of infection. To quantify the actual effect of precautions, it is not possible in experimental studies to expose subjects to virulent aerosols. Therefore, in this study, we develop a computational fluid dynamics (CFD) model to evaluate the impact of applying the aforementioned precautions in classrooms on reducing aerosol concentration and potential exposure in the presence of index or infected patients. A CFD-coupled Wells–Riley model is used to quantify the infection probability (IP) in the presence of index patients. Different cases are simulated by varying the occupancy of the room (half/full), the volumetric flow rate of the AP, two different locations of the AP, and the effect of wearing masks. The results suggest that using an AP reduces the spread of virulent aerosols and thereby reduces the risk of infection. However, the risk of the person sitting adjacent to the index patient is only marginally reduced and can be avoided with the half capacity of the class (physical distancing method) or by wearing face masks of high efficiencies. Full article

(This article belongs to the Special Issue Research on Indoor Air Cleaners for Particulate, Microbiological and Gaseous Pollutants)

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11 pages, 3223 KiB

Open AccessEditor’s ChoiceArticle

Assessment of Mobile Air Cleaners to Reduce the Concentration of Infectious Aerosol Particles Indoors

by Christian J. Kähler, Rainer Hain and Thomas Fuchs

Atmosphere 2023, 14(4), 698; https://doi.org/10.3390/atmos14040698 - 08 Apr 2023

Cited by 3 | Viewed by 1549

Abstract

Airborne transmission via aerosol particles without close human contact is a possible source of infection with airborne viruses such as SARS-CoV-2 or influenza. Reducing this indirect infection risk, which is mostly present indoors, requires wearing adequate respiratory masks, the inactivation of the viruses [...] Read more.

Airborne transmission via aerosol particles without close human contact is a possible source of infection with airborne viruses such as SARS-CoV-2 or influenza. Reducing this indirect infection risk, which is mostly present indoors, requires wearing adequate respiratory masks, the inactivation of the viruses with radiation or electric charges, filtering of the room air, or supplying ambient air by means of ventilation systems or open windows. For rooms without heating, ventilation, and air conditioning (HVAC) systems, mobile air cleaners are a possibility for filtering out aerosol particles and therefore lowering the probability of indirect infections. The main questions are as follows: (1) How effectively do mobile air cleaners filter the air in a room? (2) What are the parameters that influence this efficiency? (3) Are there room situations that completely prevent the air cleaner from filtering the air? (4) Does the air cleaner flow make the stay in the room uncomfortable? To answer these questions, particle imaging methods were employed. Particle image velocimetry (PIV) was used to determine the flow field in the proximity of the air cleaner inlet and outlet to assess regions of unpleasant air movements. The filtering efficiency was quantified by means of particle image counting as a measure for the particle concentration at multiple locations in the room simultaneously. Moreover, different room occupancies and room geometries were investigated. Our results confirm that mobile air cleaners are suitable devices for reducing the viral load indoors. Elongated room geometries, e.g., hallways, lead to a reduced filtering efficiency, which needs to be compensated by increasing the volume flow rate of the device or by deploying multiple smaller devices. As compared to an empty room, a room occupied with desks, desk separation walls, and people does not change the filtering efficiency significantly, i.e., the change was less than 10%. Finally, the flow induced by the investigated mobile air cleaner does not reach uncomfortable levels, as by defined room comfort standards under these conditions, while at the same time reaching air exchange rates above 6, a value which is recommended for potentially infectious environments. Full article

(This article belongs to the Special Issue Research on Indoor Air Cleaners for Particulate, Microbiological and Gaseous Pollutants)

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17 pages, 9874 KiB

Open AccessEditor’s ChoiceArticle

Reducing Particle Exposure and SARS-CoV-2 Risk in Built Environments through Accurate Virtual Twins and Computational Fluid Dynamics

by Fabian Quintero, Vijaisri Nagarajan, Stefan Schumacher, Ana Maria Todea, Jörg Lindermann, Christof Asbach, Charles M. A. Luzzato and Jonathan Jilesen

Atmosphere 2022, 13(12), 2032; https://doi.org/10.3390/atmos13122032 - 03 Dec 2022

Cited by 3 | Viewed by 1217

Abstract

The World Health Organization has pointed out that airborne transmission via aerosol particles can be a strong vector for the spread of SARS-CoV-2. Protecting occupants from infectious diseases or harmful particulate matter (PM) in general can be challenging. While experimentally outlining the detailed [...] Read more.

The World Health Organization has pointed out that airborne transmission via aerosol particles can be a strong vector for the spread of SARS-CoV-2. Protecting occupants from infectious diseases or harmful particulate matter (PM) in general can be challenging. While experimentally outlining the detailed flow of PM in rooms may require complex setups, computational fluid dynamics (CFD) simulations can provide insights into improving the safety of the built environment and the most effective positioning of air-purifying devices. While previous studies have typically leveraged Reynolds-averaged Navier–Stokes (RANS) approaches for predicting particle propagation, the turbulence length scales accurately captured in these simulations may not be sufficient to provide a realistic spread and the mixing of particles under the effects of forced convection. In this paper, we experimentally validate a Lattice Boltzmann very large eddy simulation (VLES) approach including particle modeling. We also demonstrate how this simulation approach can be used to improve the effectiveness of air filtration devices in realistic office environments. Full article

(This article belongs to the Special Issue Research on Indoor Air Cleaners for Particulate, Microbiological and Gaseous Pollutants)

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17 pages, 3692 KiB

Open AccessArticle

Incremental Evaluation Model for the Analysis of Indoor Air Measurements

by Andreas Schmohl, Michael Buschhaus, Victor Norrefeldt, Sabine Johann, Andrea Burdack-Freitag, Christian R. Scherer, Pablo A. Vega Garcia and Christoph Schwitalla

Atmosphere 2022, 13(10), 1655; https://doi.org/10.3390/atmos13101655 - 11 Oct 2022

Cited by 3 | Viewed by 1183

Abstract

The investigation of the cleaning effectiveness of air cleaners under realistic conditions is challenging. Mathematical models are needed to extract characteristic properties of the air cleaning system from experimental data. An incremental evaluation model based on a source term and a total first-order [...] Read more.

The investigation of the cleaning effectiveness of air cleaners under realistic conditions is challenging. Mathematical models are needed to extract characteristic properties of the air cleaning system from experimental data. An incremental evaluation model based on a source term and a total first-order loss coefficient in each segment was developed to analyze indoor particle measurements. The application of the model is demonstrated using two scenarios, one in a well-mixed testing room and another in a fully equipped aircraft cabin at 750 hPa with a typical aircraft ventilation system. In the first scenario, a normalized version of the model is used to eliminate the source’s influence. For the investigation in the aircraft cabin, the model served to extract temporal and spatial resolved source terms and first-order loss coefficients. The incremental evaluation model is applicable to enhance the certification of air cleaners. The application of the model is not only limited to particles; measurements of gaseous compounds like ozone, carbon dioxide, or volatile organic compounds can be evaluated analogously. The model’s utility for the data analysis of experiments with complex flow conditions should be studied in further investigations. Full article

(This article belongs to the Special Issue Research on Indoor Air Cleaners for Particulate, Microbiological and Gaseous Pollutants)

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20 pages, 3718 KiB

Open AccessArticle

Indoor Air Quality Intervention in Schools: Effectiveness of a Portable HEPA Filter Deployment in Five Schools Impacted by Roadway and Aircraft Pollution Sources

by Nancy Carmona, Edmund Seto, Timothy R. Gould, Everetta Rasyid, Jeffry H. Shirai, BJ Cummings, Lisa Hayward, Timothy V. Larson and Elena Austin

Atmosphere 2022, 13(10), 1623; https://doi.org/10.3390/atmos13101623 - 05 Oct 2022

Cited by 4 | Viewed by 1997

Abstract

The Healthy Air, Healthy Schools Study was established to better understand the impact of ultrafine particles (UFPs) on indoor air quality in communities surrounding Seattle-Tacoma (Sea-Tac) International Airport. The study team took multipollutant measurements of indoor and outdoor air pollution at five participating [...] Read more.

The Healthy Air, Healthy Schools Study was established to better understand the impact of ultrafine particles (UFPs) on indoor air quality in communities surrounding Seattle-Tacoma (Sea-Tac) International Airport. The study team took multipollutant measurements of indoor and outdoor air pollution at five participating school locations to estimate infiltration indoors. The schools participating in this project were located within a 7-mile radius of Sea-Tac International Airport and within 0.5 mile of an active flight path. Based on experimental measures in an unoccupied classroom, infiltration rates of (a) UFPs of aircraft origin, (b) UFPs of traffic origin, and (c) wildfire smoke or other outdoor pollutants were characterized before and after the introduction of a portable high-efficiency particulate air (HEPA) filter intervention. The portable HEPA cleaners were an effective short-term intervention to improve the air quality in classroom environments, reducing the UFP count concentration from one-half to approximately one-tenth of that measured outside. This study is unique in focusing on UFPs in schools and demonstrating that UFPs measured in classroom spaces are primarily of outdoor origin. Although existing research suggests that reducing particulate matter in homes can significantly improve asthma outcomes, further investigation is necessary to establish the benefits to student health and academic performance of reducing UFP exposures in schools. Full article

(This article belongs to the Special Issue Research on Indoor Air Cleaners for Particulate, Microbiological and Gaseous Pollutants)

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16 pages, 3922 KiB

Open AccessArticle

Particulate Matter versus Airborne Viruses—Distinctive Differences between Filtering and Inactivating Air Cleaning Technologies

by Andrea Burdack-Freitag, Michael Buschhaus, Gunnar Grün, Wolfgang Karl Hofbauer, Sabine Johann, Anna Maria Nagele-Renzl, Andreas Schmohl and Christian Rudolf Scherer

Atmosphere 2022, 13(10), 1575; https://doi.org/10.3390/atmos13101575 - 27 Sep 2022

Cited by 3 | Viewed by 1648

Abstract

The current pandemic of the SARS-CoV-2 virus requires measures to reduce the risk of infection. In addition to the usual hygiene measures, air cleaners are a recommended solution to decrease the viral load in rooms. Suitable technologies range from pure filters to inactivating [...] Read more.

The current pandemic of the SARS-CoV-2 virus requires measures to reduce the risk of infection. In addition to the usual hygiene measures, air cleaners are a recommended solution to decrease the viral load in rooms. Suitable technologies range from pure filters to inactivating units, such as cold plasma or UVC irradiation. Such inactivating air cleaners, partly combined with filter technology, are available on the market in various designs, dimensions and technical specifications. Since it is not always clear whether they may produce undesirable by-products, and the suitability for particular applications cannot be assessed on the basis of the principle of operation, the effectivity of six inactivating devices was investigated in a near-real environment. The investigations were based on a standard method published by the VDI. The procedure was extended in such a way that a permanent virus source was simulated, which corresponds to the presence of a person suffering from COVID-19 in a room. The study addresses the difference of the mere presence of viruses to the determination of the virulence. As a result, a deep understanding is provided between the behavior of a virus as a pure aerosolized particle and its real infectivity in order to enable the assessment of suitable air cleaners. Full article

(This article belongs to the Special Issue Research on Indoor Air Cleaners for Particulate, Microbiological and Gaseous Pollutants)

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17 pages, 1713 KiB

Open AccessArticle

Testing Filter-Based Air Cleaners with Surrogate Particles for Viruses and Exhaled Droplets

by Stefan Schumacher, Arantxa Banda Sanchez, Anna Caspari, Katharina Staack and Christof Asbach

Atmosphere 2022, 13(10), 1538; https://doi.org/10.3390/atmos13101538 - 20 Sep 2022

Cited by 7 | Viewed by 1849

Abstract

Indoor air cleaners can contribute to reducing infection risks by the filtration of virus-carrying droplets. There are various national standards to test indoor air cleaners that determine the clean air delivery rate (CADR), but typically only as a size-integrated value for particles > [...] Read more.

Indoor air cleaners can contribute to reducing infection risks by the filtration of virus-carrying droplets. There are various national standards to test indoor air cleaners that determine the clean air delivery rate (CADR), but typically only as a size-integrated value for particles > 0.3 μm. Thus, a test method using potassium chloride (KCl) and paraffin as surrogate particles in the size range of viruses and exhaled droplets was developed. We show that air cleaners with fibrous and electrostatic filters are generally capable of reducing the airborne particle concentrations. However, for electret filters, the performance can strongly degrade over time by being loaded with particles. By comparing filters with different efficiencies in the same air cleaner, we demonstrate that the use of high-efficiency filters can be even at the expense of the cleaning efficacy. We developed a mathematical model to estimate the inhaled dose of viruses and show that the combination of natural venting and an air cleaner can lead to a substantial reduction of the infection risk. Full article

(This article belongs to the Special Issue Research on Indoor Air Cleaners for Particulate, Microbiological and Gaseous Pollutants)

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