Alvin C.K. Lai

MODELING INDOOR PARTICLE DEPOSITION FROM TURBULENT FLOW ONTO SMOOTH SURFACES

Abstract Particle deposition to indoor surfaces is frequently modeled by assuming that indoor air flow is homogeneously and isotropically turbulent. Existing formulations of such models, based on the seminal work of Corner and Pendlebury (1951, Proc. Phys. Soc. Lond. B 64, 645), lack a thorough physical foundation. We apply the results of recent studies of near-surface turbulence to produce an analogous model for particle deposition onto indoor surfaces that remains practical to use yet has a stronger physical basis. The model accounts for the effects of Brownian and turbulent diffusion and gravitational settling. It predicts deposition to smooth surfaces as a function of particle size and density. The only required input parameters are enclosure geometry and friction velocity. Model equations are presented for enclosures with vertical and horizontal surfaces, and for spherical cavities. The model helps account for a previously unexplained experimental observation regarding the functional dependence of deposition velocity on particle size. Model predictions agree well with recently published experimental data for a spherical cavity (Cheng, Y. S., Aerosol Sci. Technol. 27, 131–146, (1997)).

Inhalation Transfer Factors for Air Pollution Health Risk Assessment

ABSTRACT To facilitate routine health risk assessments, we develop the concept of an inhalation transfer factor (ITF). The ITF is defined as the pollutant mass inhaled by an exposed individual per unit pollutant mass emitted from an air pollution source. A cumulative population inhalation transfer factor (PITF) is also defined to describe the total fraction of an emitted pollutant inhaled by all members of the exposed population. In this paper, ITFs and PITFs are calculated for outdoor releases from area, point, and line sources, indoor releases in single zone and multizone indoor environments, and releases within motor vehicles. Typical PITFs for an urban area from emissions outdoors are ~10-6–10-3. PITFs associated with emissions in buildings or in moving vehicles are typically much higher, ~10-3–10-1.

Comparison of a new Eulerian model with a modified Lagrangian approach for particle distribution and deposition indoors

Abstract Understanding of aerosol dispersion characteristics has many scientific and engineering applications. It is recognized that Eulerian or Lagrangian approach has its own merits and limitations. A new Eulerian model has been developed and it adopts a simplified drift–flux methodology in which external forces can be incorporated straightforwardly. A new near-wall treatment is applied to take into account the anisotropic turbulence for the modified Lagrangian model. In the present work, we present and compare both Eulerian and Lagrangian models to simulate particle dispersion in a small chamber. Results reveal that the standard k–ε Lagrangian model over-predicts particle deposition compared to the present turbulence-corrected Lagrangian approach. Prediction by the Eulerian model agrees well with the modified Lagrangian model.

Experimental Studies of the Effect of Rough Surfaces and Air Speed on Aerosol Deposition in a Test Chamber

Understanding the fate of particles indoors is important for human health assessment because deposited particles, unless resuspended, cannot be inhaled. To complement studies in real buildings, where control of variables is often difficult, an experimental test chamber facility (8 m 3 ) was designed to study particle deposition under well-stirred conditions using monodisperse tracer aerosol particles in the range of 0.7 to 5.4 w m. The use of neutron-activatable tracers facilitated simultaneous surface sampling and aerosol concentration decay measurements. Aerosol deposition on both smooth surfaces and regular arrays of three-dimensional roughness elements under 3 different airflow speeds was investigated in the test chamber.It was expected that the texture of the chamber surface would significantly influence particle deposition, but some counterintuitive results were observed: under the lowest airflow condition and for the smallest particle size, particle deposition onto rough samples was found to be less than on the corresponding smooth surfaces. The ratio of particle deposition on rough surfaces relative to smooth surfaces increased with particle size and magnitude of airflow. For the largest particle size and airflow speed, particle deposition on the rough surfaces exceeded that on the smooth surfaces by a factor of 3.

Numerical modeling of exhaled droplet nuclei dispersion and mixing in indoor environments

Abstract The increasing incidence of indoor airborne infections has prompted attention upon the investigation of expiratory droplet dispersion and transport in built environments. In this study, a source (i.e. a patient who generates droplets) and a receiver (i.e. a susceptible object other than the source) are modeled in a mechanically ventilated room. The receivers exposure to the droplet nuclei is analyzed under two orientations relative to the source. Two droplet nuclei, 0.1 and 10μm, with different emission velocities, are selected to represent large expiratory droplets which can still be inhaled into the human respiratory tracts. The droplet dispersion and mixing characteristics under well-mixed and displacement ventilation schemes are evaluated and compared numerically. Results show that the droplet dispersion and mixing under displacement ventilation is consistently poorer. Very low concentration regions are also observed in the displacement scheme. For both ventilation schemes, the intake dose will be reduced substantially if the droplets are emitted under the face-to-wall orientation rather than the face-to-face orientation. Implications of using engineering strategies for reducing exposure are briefly discussed.

Study of expiratory droplet dispersion and transport using a new Eulerian modeling approach

Abstract Understanding of droplet nuclei dispersion and transport characteristics can provide more engineering strategies to control transmission of airborne diseases. Droplet dispersion in a room under the conventional well-mixed and displacement ventilation is simulated. Two droplet nuclei sizes, 0.01 and 10μm, are selected as they represent very fine and coarse droplets. The flow field is modeled using k–ε RNG model. A new Eulerian drift-flux methodology is employed to model droplet phase. Under the conventional ventilation scheme, both fine and coarse droplets are homogeneously dispersed within approximately 50s. Droplet nuclei exhibit distinctive dispersion behavior, particularly for low airflow microenvironment. After 270s of droplet emission, gravitational settling influences the dispersion for 10μm droplets, and concentration gradient can still be observed for displacement ventilation.

AEROSOL DEPOSITION IN TURBULENT CHANNEL FLOW ON A REGULAR ARRAY OF THREE-DIMENSIONAL ROUGHNESS ELEMENTS

Abstract Understanding particle deposition onto rough surfaces is important for many engineering and environmental applications. An experimental system was designed for the study of aerosol deposition on regular arrays of uniform elements (in the form of discrete protrusions) in a turbulent ventilation duct flow using monodisperse tracer small particles, in the range 0.7– 7.1 μ m . The Reynolds number for the test conditions was 44,000 in the 150 mm square duct. The roughness elements were arranged at two different orientations with respect to the airflow direction and the aerosol deposition velocity and pressure drop were measured for both orientations. Compared to earlier measurements in the same duct system involving smooth or ribbed surfaces, a significant increase in deposition velocity onto the regular roughness elements is observed. In addition, the associated pressure loss penalty is lower than in the presence of the roughness elements than in the presence of the ribbed surfaces. This may be attributable to the small dimensionless roughness height of the elements, which results only in a moderate distortion of the flow structure near the surfaces.

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

Abstract 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 10μm 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.

An eulerian model for particle deposition under electrostatic and turbulent conditions

Particle deposition under the influence of electrostatic force is modeled theoretically based on the semi-empirical (three-layer) model proposed previously (J. Aerosol. Sci. 31 (2000) 463). The electrostatic forces considered are the Coulombic force and the image force. Both Boltzmann and combined diffusion and field charging mechanisms are investigated. A modified Ficks law equation accounting for Brownian and turbulent diffusion, spatially-independent external forces, i.e. gravitational and Coulombic forces, and spatially-dependent external force, i.e. image force, is presented based on the simplified three-layer model. The results show that the concentration boundary layer thickness is very thin and the previous three-layer model can be further simplified. The Coulombic force influences the particle deposition significantly while the image force is important for high charge level particles in the presence of a weak electric field. The model predictions agree very well with the literature DNS results.

Experimental and Numerical Investigation of Interpersonal Exposure of Sneezing in a Full-Scale Chamber

It is well known that person-to-person spreading of infectious pathogens occurs mostly because of sneezing or coughing in indoor environments. This study analyzes the spatial droplet concentration generated by human sneezing and its potential impact on the adjacent person, experimentally and numerically. Experiments were carried out in a controlled chamber with dimensions of 2.3 × 2.3 × 2.3 m3 with two manikins (source and susceptible) standing opposite to each other at a fixed distance. Two kinds of ventilation schemes were employed in this study: well-mixed ventilation (WMV) and displacement ventilation (DV). For each ventilation scheme, the manikins were placed at two different locations: one at the center and the other near the wall on the right side of the room. Numerical simulations of sneezing droplets distribution in the room were conducted with a drift-flux model to account for the influence of the thermal plume around the human body. The results show good agreement between experimental measurements and computational fluid dynamics (CFD) predictions for vertical temperature profiles in the room and reasonably good agreement between normalized peak aerosol concentrations and the time at which peak concentrations occur. Results for measured points indicated that droplet concentrations around the susceptible manikin were higher when the manikins were placed near the wall, rather than at the center. Copyright 2012 American Association for Aerosol Research