Using large eddy simulation to study particle motions in a room

As people spend most of their time in an indoor environment, it is important to predict indoor pollutant level in order to assess health risks. As particles are an important pollutant indoors, it is of great interest to study the airflow pattern and particle dispersion in buildings. This study uses large eddy simulation (LES) to predict three-dimensional and transient turbulent flows and a Lagrangian model to compute particle trajectories in a room. The motion of three different types of solid particles in a decaying homogeneous isotropic turbulent airflow is calculated. By comparing the computed results with the experimental data from the literature, the computational method used in this investigation is found to be successful in predicting the airflow and particle trajectories in terms of the second-order statistics, such as the mean-square displacement and turbulent intensity. This Lagrangian model is then applied to the study of particles' dispersion in a ventilated cavity with a simplified geometry for two ventilation scenarios. It is shown that light particles follow the airflow in the room and many particles are exhausted, while heavier particles deposit to the floor or/and are exhausted. PRACTICAL IMPLICATIONS: The results of this paper can be used to study dispersion of infectious diseases in enclosed spaces in which virus or bacteria are often attached to particles and transported to different rooms in a building through ventilation systems. In most of studies, the virus or bacteria have been considered to be gaseous phase so there is no slip between virus/bacteria and air. The results in this paper show that heavier particles are submitted to gravity and are sensitive to the ventilation strategy.     

悬浮颗粒数值模拟模型改进研究

分析了目前气粒两相流模拟研究中所使用的方法和模型，针对洁净室内悬浮颗粒极为稀疏的特点，提出了一个改进的颗粒模型，该模型中将颗粒相处理为连续介质，采用当地颗粒湍流动能与浓度梯度来模拟颗粒的扩散。用此模型对全顶棚送风下部两侧回风洁净室内悬浮颗粒的分布进行了模拟计算，与浓度模型模拟所得结果比较发现，改进模型不但在精度方面有明显提高，而且与实验值吻合良好。     

Numerical studies of indoor airflow and particle dispersion by large Eddy simulation

The indoor airflow and contaminant particle concentration in two geometrically different rooms have been investigated using the Large Eddy Simulation (LES) technique based on Renormalization Group (RNG) theory derived by Yakhot, Orszag, Yakhot and Israeli, Journal of Scientific Computing, 1989. The first room is without contaminant particles. Its simulated air phase velocity profiles are validated against the measurements of Posner, Buchanan and Dunn-Rankin, Energy and Buildings, 2003. A good agreement is achieved between the prediction and measured data. The LES model successfully captures the mean flow trends as well as instantaneous flow information, which is required for appropriate design and evaluation of a ventilation system. The second room has contaminant particles, which are simulated with a Lagrangian particle tracking model. In this case, the LES model provides acceptable prediction of the contaminant particle concentration, compared to the particle concentration decay measured by Lu, Howarth, Adam and Riffat, Building and Environment, 1996. The numerical results reveal that the particle-wall impact model has a considerable effect on the Lagrangian concentration prediction. It is proposed that further improvements to the particle-wall impact model are required to correctly predict the contaminant particle concentration through the Lagrangian model.     

Control of Airborne Particle Concentration and Draught Risk in an Operating Room

The influence of location of airborne particle source, ventilation rate, air inlet size, supply air velocity, air outlet location, and heat source on the dkributiuns of airborne particle concentration and draught risk in an operating room is investigated. The investigation is carried out by using a flow program with the k-E mdel of turbulence. Based on a standard case, five cases, each with one changed parameter, are computed, and the detailed field distributions of air velocity, temperature, airborne particle concentration, and draught risk are presented. The parametric study concludes that, for a better air quality and thermal comfort, it is desirable to use a higher inflow rate, a larger inlet area, and a uniform velocity profile of supply air. Outlet location and heat source have little influence on the disrributions of the particle concentration in the room. It has also been found that the distributions of particle concentration in the recirculating zone are very sensitive to the location of the particle sources.     

Concentration and size distribution of particles in abstracted groundwater

Particle number concentrations have been counted and particle size distributions calculated in groundwater derived by abstraction wells. Both concentration and size distribution are governed by the discharge rate: the higher this rate the higher the concentration and the higher the proportion of larger particles. However, the particle concentration in groundwater derived from abstraction wells, with high groundwater flow velocities, is much lower than in groundwater from monitor wells, with minimal flow velocities. This inconsistency points to exhaustion of the particle supply in the aquifer around wells due to groundwater abstraction for many years. The particle size distribution can be described with the help of a power law or Pareto distribution. Comparing the measured particle size distribution with the Pareto distribution shows that particles with a diameter >7 μm are under-represented. As the particle size distribution is dependent on the flow velocity, so is the value of the “Pareto” slope β.     

Particle Image Velocimetry (PIV) application in the measurement of indoor air distribution by an active chilled beam

We study the turbulent air flow behaviours of the attached plane jet discharged from an active chilled beam in a room using Particle Image Velocimetry (PIV). PIV is an innovative technology to study indoor air flow which began in the eighties of the last century for the measurement of whole air flow fields in fractions of a second. Here an experimental PIV system was built to reveal the structure of a turbulent attached plane jet in the entrainment process of the ambient air downstream from the jet slot. For the particle seeding in the PIV experiments, a few different particles were tested with the attached jet PIV application in a room. The results presented in this paper show the clear structure of the turbulent attached plane jet in the entrainment process after issuing from the chilled beam slot. The PIV visualisation results proved that the jet will attach to the ceiling and become fully turbulent a short distance downstream from the slot. The jet velocity vector fields show that the volume flow rate of the attached plane jet increases because of the large vortex mixing mechanism in the outer region of the jet. In three measurement cases, the air jet grows faster at a Reynolds number of 960 than at Reynolds numbers of 1320 and 1680. The calculated spreading angles in the cases with lower Reynolds numbers have similar values compared with the visualisation results.     

Simulation of particle deposition in ventilation duct with a particle–wall impact model

Particle deposition velocities and locations in horizontal ventilation ducts are investigated by incorporating the effect of particle–wall collision. Particle deposition onto two types of surfaces, stainless steel surface and tedlar surface, are simulated and compared. The RNG k–? model is employed to predict the air turbulence, and the Lagrangian particle tracking method integrated with particle–wall impact model is used to reveal particle physical behaviors. Turbulent dispersion of the particles is taken into account by adopting the discrete random walk (DRW) model. Particle deposition velocities and distributions onto the wall, ceiling and floor are simulated and analyzed. For both stainless steel and tedlar ducts, reasonable agreements are achieved between the simulation data and experimental data for particles with larger relaxation time. Particle deposition velocity is related to particle relaxation time and surface materials. The particle–wall impact model affects the prediction of deposition velocity and distribution. As the effects of Brownian diffusion and turbulent fluctuation on particle deposition are not considered, the presented model applies better to the particles with relatively large relaxation time.     

Study on biological contaminant control strategies under different ventilation models in hospital operating room

Transmission of airborne bacteria is the main factor causing surgical site infection (SSI). Previous researches have provided evidence of relationships between cleanness of room air and incidence of SSI, but little work has been done to verify the numerical simulation results of particle dispersion. This paper focuses on the airborne transmission of bacteria in two operating rooms during two surgeries: a surgical stitching of fractured mandible and a joint replacement surgery. Field measurement was carried out in two newly built ISO class 5 (OR.A) and class 6 (OR.B) operating rooms. Bacteria collecting agar dishes were put in different places of the two operating rooms to get the deposited bacteria number during the operation. Then numerical simulation was carried out to calculate the particle trajectories using the Euler–Lagrange approach. Simulation results were compared with field measured data, and acceptable level of consistency was found. Then we changed the supply air velocity and supply vent area in the OR.B numerical model under same room air change rate, to compare bacteria colony deposition onto the “critical area”, which consisted of three connected surfaces around the surgical site on patient body. Result showed that improving air flow pattern can reduce particle deposition on critical surface, but its effect is less evident by increasing the air change rate in a certain amount, and we found that bacteria colony deposition would increase (mainly on upper surface), if air velocity increases beyond a certain velocity.     

Study on dielectrophoretic deposition of airborne particles in a vertical micro channel

Airborne particles significantly contribute to the toxicity of environmental pollution. A mathematical model is developed and analyzed to study the dielectrophoretic deposition of airborne particles in a vertical micro channel. Finite element programs are utilized to calculate the air flow and potential distribution in the vertical micro channel. The electric field in the numerical solution space is obtained by employing a potential gradient in the x and y directions. The dielectrophoretic force and sedimentation motion of a spherical particle are achieved. The simulated results show that the dielectrophoretic force is maximal at the tips of the shorter electrode. The initial position of the particle, initial direction of the particle, radius of the particle, electrode dimension, applied voltage and air flow velocity are the main factors affecting the trajectories of airborne particles in a micro channel. Modification of the particle size distribution using the technology as a dielectrophoretic filter is more feasible in practice. Sedimentation micro channels in series with the shorter electrode on an alternating side will be beneficial to improve particle sedimentation efficiency.     

On the numerical study of contaminant particle concentration in indoor airflow

The application of three turbulence models—standard k–ε, re-normalization group (RNG) k–ε and RNG-based large eddy simulation (LES) model—to simulate indoor contaminant particle dispersion and concentration distribution in a model room has been investigated. The measured air phase velocity data obtained by Posner et al. [Energy and Buildings 2003;35:515–26], are used to validate the simulation results. All the three turbulence model predictions have shown to be in good agreement with the experimental data. The RNG-based LES model has shown to yield the best agreement. The flow of contaminant particles (with diameters of 1 and 10 μm) is simulated within the indoor airflow environment of the model room. Comparing the three turbulence models for particle flow predictions, the RNG-based LES model through better accommodating unsteady low-Reynolds-number (LRN) turbulent flow structure has shown to provide more realistic particle dispersion and concentration distribution than the other two conventional turbulence models. As the experimental approach to access indoor contaminant particle concentration can be rather expensive and unable to provide the required detailed information, the LES prediction can be effectively employed to validate the widely used k–ε models that are commonly applied in many building simulation investigations.     

Numerical analysis of particle deposition in ventilation duct

This paper adopts computational fluid dynamics (CFD) to numerically analyze particle deposition in the ventilation duct. A three-dimensional drift-flux model combined with particle deposition boundary conditions for wall surfaces is presented. The numerical method is used to analyze the particle deposition velocity and deposited particle mass flux in the ventilation duct after validation. Twelve groups of particle size, two average air speeds in ducts are investigated to understand the particle deposition in the straight ventilation duct, which ensures a fully developed turbulent duct flow. And then, the particle accumulation by deposition in the ventilation duct is analyzed according to the cleaning code for air duct system in heating, ventilation and air conditioning (HVAC) systems of China. The cases with or without air filter installed are studied by assuming that the duct inlet particle concentration is that of outdoor air in Beijing city, China.     

Buoyant jet in a ventilated room: Velocity field,temperature field and airflow patterns analysed with three different whole-field methods

The instantaneous velocity field and temperature field were measured and the airflow patterns visualised close to a diffuser for displacement ventilation. Since the low-velocity diffuser was located above the floor and the inlet air temperature was below the room temperature, the flow was governed by both momentum and buoyancy forces. The data were recorded with whole-field measuring techniques, particle streak velocimetry (PSV), particle image velocimetry (PIV) and infrared thermography (IR), in conjunction with a low thermal mass screen. The environment is very complex, supply of buoyant air with a commercial supply terminal with 20 nozzles pointing in different directions, which makes it difficult to use point-measuring techniques or computational fluid dynamics (CFD). The aim was twofold: (a) to explore what kind of information can be derived from whole-field measurement techniques in this context and (b) to investigate the trajectory of the flow discharged into the room and the entrainment of the flow.     

Effect of ventilation duct as a particle filter

This paper discusses the effect of ventilation duct as a particle filter by modeling particle deposition in ventilation ducts, which is the reason that ventilation ducts could “filter” particles. An Eulerian model is employed to predict the particle deposition velocity onto the wall and floor from fully developed turbulent flow in ventilation ducts [Zhao B, Wu J. Modeling particle deposition from fully developed turbulent flow in ventilation duct. Atmospheric Environment 2006;40:457–66], while an empirical equation is proposed to predict the particle deposition velocity onto the ceiling combined with experimental data and, another empirical equation by McFarland et al. [Aerosol deposition in bends with turbulent flow. Environmental Science and Technology 1997;31:3371–7] is used for predicting the particle penetration through the bends, which are hard to analyze by theoretical method.     

Numerical evaluation of the particle pollutant homogeneity and mixing time in a ventilated room

With the increase of the computational capability, three-dimensional Lagrangian simulation can now be applied to evaluate the particle pollution evolution in ventilated rooms. The present study deals with the evaluation of 5.0 μm particle dispersion in a prototypical-ventilated room of 10 m³. Two ventilation slots and three different airflow rates have been simulated. Particle trajectories have been calculated using a Lagrangian model. Dispersion of particles has been analyzed for the case of an external pollutant source, a homogeneous particle injection within the room and some puff releases in different locations. Discussion about the best procedure to get useful information about the particle cloud is then presented and illustrated from the simulation results. The definition of two indices based on geometrical and statistical parameters are proposed in order to correctly evaluate the particle cloud homogeneity and mixing time.     

Locating the very early smoke detector apparatus (VESDA) in vertical laminar clean rooms according to the trajectories of smoke particles

Proper locating of a very early smoke detector apparatus (VESDA) in cleanrooms is an important issue for fire safety. The main aim of this study is to analyse the proper locations of a VESDA by determining the trajectories of smoke particles in the early stages from a fire in a vertical laminar flow clean room using computation fluid dynamics (CFD). The CFD model was first verified by experimental data of a reduced-scale clean room. Several influential factors on the trajectories of smoke particles including particle size, fire source temperature and fire location were then studied for a full-scale clean room. Results show that the fire source is the most notable factor influencing the trajectories of smoke particles. When the fire source was located at the centre of the room, the smoke particles travelled to the centre of the return air plenum (RAP). If the fire started close to the return air shaft (RAS), the smoke particles reached the RAP close to the top. If the fire started far away from the RAS, the smoke particles arrived at the RAP close to the bottom.     

Particle transport characteristics in indoor environment with an air cleaner: The effect of nonuniform particle distributions

Air cleaners are expected to improve the indoor air quality by removing the gaseous contaminants and fine particles. In our former work, the effects of the air cleaner on removing the uniformly distributed particles were numerically investigated. Based on those results, this work further explores the performances of the air cleaner in the reduction of two nonuniform particle distributions generated by smoking and coughing. The Lagrangian discrete trajectory model combined with the Eulerian fluid method is employed to simulate the airflow pattern and particle transport in a room. In general, the results show that the particle fates have been resulted from the interaction between the emitting source and the air cleaner. And the position of the air cleaner is a key parameter affecting the particle concentration, for which a shorter distance between the air cleaner and the human body corresponds to a lower concentration. Besides, the air velocity emitted from the human mouth and the orientation of the air cleaner can also influence the transport of particles.     

Non-buoyant pollutant sources and particles in displacement ventilation

Particle transportation and ventilation efficiency, with non-buoyant pollutant sources, in a displacement-ventilated room were evaluated. A resuspension of floor deposited particles caused by the influence of the supply air or people moving around may increase the number of particles in the convection flows. Particle concentrations at different positions under steady state and transient conditions were measured. The results indicate that there seem to be little risk of resuspension of particles, in the measured size interval, by the influence of the supply air. With a forced resuspension the particle concentrations in the convection flows differ from the concentration outside the convection flow. The contaminant removal effectiveness was much dependent on the position of the pollutant sources.     

Three-dimensional non-isothermal numerical model for predicting semi-volatile organic compound transport process in a room

In this paper, a three-dimensional non-isothermal computational model for predicting indoor SVOC distribution is proposed, considering the effects of turbulence diffusion and suspended particles. The realizable k-ε model is introduced for turbulent flow simulation in a room. The Euler-Euler method is adopted to deal with the gas-particle two-phase flow coupled problem. Inertia slip velocity and irreversible first-order absorption boundary are employed for more accurate prediction of particle motion. The simulated curve of outlet gas-phase di-2-ethylhexyl phthalate (DEHP) concentration with emission time is verified by available experimental data. The emission process of DEHP in a 15 m² room in Beijing during 100 days with or without air cleaner is simulated by the developed model considering air leak through window and door gaps. It is found that if the air cleaner keeps on all the time during 100 days the gas-phase DEHP concentration in the room will tend to be uniform, while the emission process is far from equilibrium without an air cleaner even the emission lasts 100 days. Results also suggest that floor heating, decrease of particle concentration, weaken of heat transfer, enhancement of mass transfer, and air infiltration in window gap contribute to decrease DEHP concentration.     

Particle dispersion and deposition in ventilated rooms: Testing and evaluation of different Eulerian and Lagrangian models

Indoor particle dispersion in a three-dimensional ventilated room is simulated by a Lagrangian discrete random walk (DRW) model and two Eulerian models: drift flux model and mixture model. The simulated results are compared with the published measured data to check the performance of the three models for indoor particle dispersion simulation. The deposition velocity of the particles is also computed and compared with published data. The turbulent airflow is modeled with the renormalization group (RNG) k−ε and a zero equation turbulence model. Comparison of the calculated air velocities with measurement shows that both the two turbulence models can simulate the airflow well for the presented case. For the Lagrangian DRW model, a post-process program is used to state the particle trajectories and transfer the results to particle concentration distribution. For Eulerian models, the effect of particle deposition towards wall surfaces is incorporated with a semi-empirical particle deposition model. The comparison shows that both the Lagrangian DRW model and drift flux model yield satisfactory predictions, while the predicted results by the mixture model are not satisfied. The deposition velocity obtained by the three models match the experimental data well.     

Classification for the room air conditioning strategies

This paper introduces a new strategy approach for the room air conditioning including classification and terminology. The basis of the classification is different aims or ideas of the temperature, gas, particle, humidity distributions and room air flow patterns that can be created within a room. A certain strategy can be applied by using different system combinations of room air distribution, exhaust, heating and cooling methods and their control. The realization of an ideal strategy is also dependent on the operating parameters and internal sources. Separating the ideal strategies from the practical room air conditioning solutions will help the evaluation of the present room air distribution methods in different operating conditions. It also creates a solid base for the development and promotion of new innovations in the field.