Modeling particle dispersion and deposition in indoor environments

Human exhalation characterization with the aid of schlieren imaging technique

The purpose of this paper is to determine the dispersion and distribution characteristics of exhaled airflow for accurate prediction of disease transmission. The development of airflow dynamics of human exhalation was characterized using nonhazardous schlieren photography technique, providing a visualization and quantification of turbulent exhaled airflow from 18 healthy human subjects whilst standing and lying. The flow shape of each breathing pattern was characterized by two angles and averaged values of 18 subjects. Two exhaled air velocities, u m and u p , were measured and compared. The mean peak centerline velocity, u m was found to decay correspondingly with increasing horizontal distance x in a form of power function. The mean propagation velocity, u p was found to correlate with physiological parameters of human subjects. This was always lower than u m at the mouth/nose opening, due to a vortex like airflow in front of a single exhalation cycle. When examining the talking and breathing process between two persons, the potential infectious risk was found to depend on their breathing patterns and spatial distribution of their exhaled air. Our study when combined with information on generation and distributions of pathogens could provide a prediction method and control strategy to minimize infection risk between persons in indoor environments.

Mathematical modeling and simulation of bacterial distribution in an aerobiology chamber using computational fluid dynamics

BACKGROUND

Computer-aided design and draft, along with computer-aided engineering software, are used widely in different fields to create, modify, analyze, and optimize designs.

METHODS

We used computer-aided design and draft software to create a 3-dimensional model of an aerobiology chamber built in accordance with the specifications of the 2012 guideline from the Environmental Protection Agency for studies on survival and inactivation of microbial pathogens in indoor air. The model was used to optimize the chamber's airflow design and the distribution of aerosolized bacteria inside it.

RESULTS

The findings led to the identification of an appropriate fan and its location inside the chamber for uniform distribution of microbes introduced into the air, suitability of air sample collection from the center of the chamber alone as representative of its bacterial content, and determination of the influence of room furnishings on airflow patterns inside the chamber.

CONCLUSIONS

The incorporation of this modeling study's findings could further improve the design of the chamber and the predictive value of the experimental data using it. Further, it could make data generation faster and more economical by eliminating the need for collecting air samples from multiple sites in the chamber.

Modeling of occupational exposure to accidentally released manufactured nanomaterials in a production facility and calculation of internal doses by inhalation

BACKGROUND

Occupational exposure to manufactured nanomaterials (MNMs) and its potential health impacts are of scientific and practical interest, as previous epidemiological studies associate exposure to nanoparticles with health effects, including increased morbidity of the respiratory and the circulatory system.

OBJECTIVES

To estimate the occupational exposure and effective internal doses in a real production facility of TiO2 MNMs during hypothetical scenarios of accidental release.

METHODS

Commercial software for geometry and mesh generation, as well as fluid flow and particle dispersion calculation, were used to estimate occupational exposure to MNMs. The results were introduced to in-house software to calculate internal doses in the human respiratory tract by inhalation.

RESULTS

Depending on the accidental scenario, different areas of the production facility were affected by the released MNMs, with a higher dose exposure among individuals closer to the particles source.

CONCLUSIONS

Granted that the study of the accidental release of particles can only be performed by chance, this numerical approach provides valuable information regarding occupational exposure and contributes to better protection of personnel. The methodology can be used to identify occupational settings where the exposure to MNMs would be high during accidents, providing insight to health and safety officials.

Large eddy simulation of wind-induced interunit dispersion around multistory buildings

Previous studies regarding interunit dispersion used Reynolds-averaged Navier-Stokes (RANS) models and thus obtained only mean dispersion routes and re-entry ratios. Given that the envelope flow around a building is highly fluctuating, mean values could be insufficient to describe interunit dispersion. This study investigates the wind-induced interunit dispersion around multistory buildings using the large eddy simulation (LES) method. This is the first time interunit dispersion has been investigated transiently using a LES model. The quality of the selected LES model is seriously assured through both experimental validation and sensitivity analyses. Two aspects are paid special attention: (i) comparison of dispersion routes with those provided by previous RANS simulations and (ii) comparison of timescales with those of natural ventilation and the survival times of pathogens. The LES results reveal larger dispersion scopes than the RANS results. Such larger scopes could be caused by the fluctuating and stochastic nature of envelope flows, which, however, is canceled out by the inherent Reynolds-averaged treatment of RANS models. The timescales of interunit dispersion are comparable with those of natural ventilation. They are much shorter than the survival time of most pathogens under ordinary physical environments, indicating that interunit dispersion is a valid route for disease transmission.

Transient transport model of particles resulting from high momentum respiratory activities: Inter-personal exposure

In this work, a transient mathematical multi-region zonal transport model of particle behavior resulting from high momentum respiratory activities (HMRA) is developed focusing on the transient inter-personal exposure (IPE) in indoor spaces ventilated by displacement ventilation (DV) systems. The developed model was validated by experimentation and by published empirical data. Three stages are identified with respect to time for the variation of the IPE: a first stage dominated by the propagation and decay of the exhaled jet, a particles' redistribution stage, and a particles' removal stage. The inhaled dose is affected by the DV flow rate, cough velocity, particle diameter and distance between the occupants. The DV system with a flow rate of 100 L/s reduced significantly the inhaled dose during particle redistribution and removal stages decreasing the total inhaled dose by 83% compared to a flow rate of 50 L/s. IPE is higher when particle diameter is increased from 1 to 20 μm due to the opposition of particle removal by the upward DV. A comparison between steady and transient modeling of the IPE showed that steady modeling captures the physics affecting particle spread due to HMRA but it over-predicts the inhaled dose. It is found that for a DV flow rate of 100 L/s and a cough velocity of 22 m/s during 1 s, and 1 μm particles, the minimum required distance between the occupants for a threshold inhaled dose of 10^-5 kg is nearly 0.5 m by transient modeling while it is 2.15 m by steady state modeling.

Evaluation of a Moments-Based Formulation for the Transport and Deposition of Small Inertia Aerosols

This paper introduces and evaluates a formulation for the modeling of transport and wall deposition of aerosols, written in terms of moments of the particle size distribution (PSD). This formulation allows coupling the moment methods with computational fluid dynamics (CFD) to track the space and time evolution of the PSD of an aerosol undergoing transport, deposition and coagulation. It consists in applying the quadrature method of moments (QMOM) to the diffusion-inertia model of Zaichik et al. [6], associated with the dynamic boundary layer (DBL) approach of Simonin [8] for wall deposition. After presenting the QMOM formulation of the transport equation and of the DBL wall function, the paper presents several test cases in which the method is compared to existing experimental and numerical results. It is shown that the moment formulation of the model does not introduce particular bias compared to its concentration-based formulation. This extension of the diffusion-inertia/DBL approach to the QMOM method hence allows modeling with a good numerical efficiency and at building scale the dynamics of aerosols undergoing transport and modification of their PSD through coagulation and deposition.

Vortex dynamics and scalar transport in the wake of a bluff body driven through a steady recirculating flow

The air ventilation system in wide-body aircraft cabins provides passengers with a healthy breathing environment. In recent years, the increase in global air traffic has amplified contamination risks by airborne flu-like diseases and terrorist threats involving the onboard release of noxious materials. In particular, passengers moving through a ventilated cabin may transport infectious pathogens in their wake. This paper presents an experimental investigation of the wake produced by a bluff body driven through a steady recirculating flow. Data were obtained in a water facility using particle image velocimetry and planar laser induced fluorescence. Ventilation attenuated the downward convection of counter-rotating vortices produced near the free-end corners of the body and decoupled the downwash mechanism from forward entrainment, creating stagnant contaminant regions.

A numerical study of bend-induced particle deposition in and behind duct bends

This paper investigated the microparticle deposition and distribution due to the presence of duct bends by employing the Eulerian approach with Reynolds stress turbulent model and a Lagrangian trajectory method. The air velocity, particle velocity and particle deposition velocity were validated with available experimental data. Several particle deposition ratios were proposed to describe the particle accumulation due to bends. Particle deposition velocities in and behind bends were analyzed numerically. It is found that bend walls with surfaces of higher capture velocity tend to accumulate more contaminant particles as seen with an increased factor of 1.2 times on particle deposition velocity. Particle deposition reaches a maximum value near bend outlet, e.g. 15.2 times deposition ratio for particles of d _p  = 23 μm, and decay exponentially to a status of fully developed deposition in approximately 10D length. Compared to traditional consideration of sole deposition in bends, a new general concept of total deposition including that in bends and behind bends is proposed to better describe the particle deposition induced by bends since the enhancement deposition ratios behind bends compose 42-99% in the total ratios for particles of d _p  = 3-23 μm. Furthermore, models of fast power and exponential decay trend are demonstrated to uncover the relationship among enhancement factor of deposition velocity behind bend, dimensionless distance behind bends and particle Stokes number. The present study can contribute to the understanding and controlling of contaminant aerosol flow behavior in ducts, e.g. particle sampling, removal and associated epidemiologic study between particle and human health.

An advanced numerical model for the assessment of airborne transmission of influenza in bus microenvironments

A CFD-based numerical model was integrated with the Wells-Riley equation to numerically assess the risk of airborne influenza infection in a popular means of public transportation, e.g. the bus microenvironment. Three mixing ventilation methods, which are widely used in current bus configurations, and an alternative displacement ventilation method were numerically assessed in terms of their ability to limit the risk of airborne influenza infection. Furthermore, both the non air-recirculation and air-recirculation with filtration ventilation modes were investigated in terms of the influenza infection probability. According to the simulation results, air-recirculation mode with high efficiency filtration was found to cause almost the same infection risk as non air-recirculation mode (100% outdoor air supply), which indicated a potential benefit of filtration in reducing the infection risk. Additionally, for the commonly used mixing ventilation methods, air distribution method, location of return/exhaust opening and seat arrangement affected the airborne transmission of influenza between passengers. The displacement ventilation method was found to be more effective in limiting the risk of airborne infection. Overall, the developed numerical model can provide insights into how the micro-environmental conditions affect airborne infection transmission in buses. This numerical model can assist in developing effective control strategies related to airborne transmitted diseases for other frequently used public transportation systems, such as trains and airplanes.

Use of risk assessment and likelihood estimation to analyze spatial distribution pattern of respiratory infection cases

Obvious spatial infection patterns are often observed in cases associated with airborne transmissible diseases. Existing quantitative infection risk assessment models analyze the observed cases by assuming a homogeneous infectious particle concentration and ignore the spatial infection pattern, which may cause errors. This study aims at developing an approach to analyze spatial infection patterns associated with infectious respiratory diseases or other airborne transmissible diseases using infection risk assessment and likelihood estimation. Mathematical likelihood, based on binomial probability, was used to formulate the retrospective component with some additional mathematical treatments. Together with an infection risk assessment model that can address spatial heterogeneity, the method can be used to analyze the spatial infection pattern and retrospectively estimate the influencing parameters causing the cases, such as the infectious source strength of the pathogen. A Varicella outbreak was selected to demonstrate the use of the new approach. The infectious source strength estimated by the Wells-Riley concept using the likelihood estimation was compared with the estimation using the existing method. It was found that the maximum likelihood estimation matches the epidemiological observation of the outbreak case much better than the estimation under the assumption of homogeneous infectious particle concentration. Influencing parameters retrospectively estimated using the new approach can be used as input parameters in quantitative infection risk assessment of the disease under other scenarios. The approach developed in this study can also serve as an epidemiological tool in outbreak investigation. Limitations and further developments are also discussed.

CFD study of exhaled droplet transmission between occupants under different ventilation strategies in a typical office room

This paper investigated the transmission of respiratory droplets between two seated occupants equipped with one type of personalized ventilation (PV) device using round movable panel (RMP) in an office room. The office was ventilated by three different total volume (TV) ventilation strategies, i.e. mixing ventilation (MV), displacement ventilation (DV), and under-floor air distribution (UFAD) system respectively as background ventilation methods. Concentrations of particles with aerodynamic diameters of 0.8 μm, 5 μm, and 16 μm as well as tracer gas were numerically studied in the Eulerian frame. Two indexes, i.e. intake fraction (IF) and concentration uniformity index R _C were introduced to evaluate the performance of ventilation systems. It was found that without PV, DV performed best concern protecting the exposed manikin from the pollutants exhaled by the polluting manikin. In MV when the exposed manikin opened RMP the inhaled air quality could always be improved. In DV and UFAD application of RMP might sometimes, depending on the personalized airflow rate, increase the exposure of the others to the exhaled droplets of tracer gas, 0.8 μm particles, and 5 μm particles from the infected occupants. Application of PV could reduce R _C for all the three TV systems of 0.8 μm and 5 μm particles. PV enhanced mixing degree of particles under DV and UFAD based conditions much stronger than under MV based ones. PV could increase the average concentration in the occupied zone of the exposed manikin as well as provide clean personalized airflow. Whether inhaled air quality could be improved depended on the balance of pros and cons of PV.

Removal of exhaled particles by ventilation and deposition in a multibed airborne infection isolation room

Removal of airborne particles in airborne infection isolation rooms is important for infection control of airborne diseases. Previous studies showed that the downward ventilation recommended by Centers for Disease Control and Prevention (CDC) could not produce the expected 'laminar' flow for pushing down respiratory gaseous contaminants and removing them via floor-level exhausts. Instead, upper-level exhausts were more efficient in removing gaseous contaminants because of upward body plumes. The conventional wisdom in the current CDC-recommended design is that floor-level exhausts may efficiently remove large droplets/particles, but such a hypothesis has not been proven. We investigated the fate of respiratory particles in a full-scale six-bed isolation room with exhausts at different locations by both experimental and computational studies. Breathing thermal manikins were used to simulate patients, and both gaseous and large particles were used to simulate the expelled fine droplet nuclei and large droplets. Gaseous and fine particles were found to be removed more efficiently by ceiling-level exhausts than by floor-level exhausts. Large particles were mainly removed by deposition rather than by ventilation. Our results show that the existing isolation room ventilation design is not effective in removing both fine and large respiratory particles. An improved ventilation design is hence recommended.

PRACTICAL IMPLICATIONS

Our findings of the relatively poor performance of fine-particle removal by the existing CDC design of isolation room ventilation suggests a need for improvement, and the findings of the removal of large particles by deposition, not by ventilation, suggest that floor-level exhausts are unnecessary, and that regular surface cleaning and disinfection is necessary, thus providing evidence for maintaining isolation room surface hygiene.

The airborne transmission of infection between flats in high-rise residential buildings: Particle simulation

Several case clusters occurred in high-rise residential buildings in Hong Kong in the 2003 SARS (the severe acute respiratory syndrome) epidemic, which motivated a series of engineering investigations into the possible airborne transport routes. It is suspected that, driven by buoyancy force, the polluted air that exits the window of the lower floor may re-enter the immediate upper floor through the window on the same side. This cascade effect has been quantified and reported in a previous paper, and it is found that, by tracer gas concentration analysis, the room in the adjacent upstairs may contain up to 7% of the air directly from the downstairs room. In this study, after validation against the experimental data from literatures, Eulerian and Lagrangian approaches are both adopted to numerically investigate the dispersion of expiratory aerosols between two vertically adjacent flats. It is found that the particle concentration in the upper floor is two to three orders of magnitude lower than in the source floor. 1.0 μm particles disperse like gaseous pollutants. For coarse particles larger than 20.0 μm, strong deposition on solid surfaces and gravitational settling effect greatly limit their upward transport.

Experimental and numerical study on particle distribution in a two-zone chamber

Better understanding of aerosol dynamics is an important step for improving personal exposure assessments in indoor environments. Although the limitation of the assumptions in a well-mixed model is well known, there has been very little research reported in the published literature on the discrepancy of exposure assessments between numerical models which take account of gravitational effects and the well-mixed model. A new Eulerian-type drift-flux model has been developed to simulate particle dispersion and personal exposure in a two-zone geometry, which accounts for the drift velocity resulting from gravitational settling and diffusion. To validate the numerical model, a small-scale chamber was fabricated. The airflow characteristics and particle concentrations were measured by a phase Doppler Anemometer. Both simulated airflow and concentration profiles agree well with the experimental results. A strong inhomogeneous concentration was observed experimentally for 10 μm aerosols. The computational model was further applied to study a simple hypothetical, yet more realistic scenario. The aim was to explore different levels of exposure predicted by the new model and the well-mixed model. Aerosols are initially uniformly distributed in one zone and subsequently transported and dispersed to an adjacent zone through an opening. Owing to the significant difference in the rates of transport and dispersion between aerosols and gases, inferred from the results, the well-mixed model tends to overpredict the concentration in the source zone, and under-predict the concentration in the exposed zone. The results are very useful to illustrate that the well-mixed assumption must be applied cautiously for exposure assessments as such an ideal condition may not be applied for coarse particles.