Large-eddy simulation of human-induced contaminant transport in room compartments

A large-eddy simulation is used to investigate contaminant transport owing to complex human and door motions and vent-system activity in room compartments where a contaminated and clean room are connected by a vestibule. Human and door motions are simulated with an immersed boundary procedure. We demonstrate the details of contaminant transport owing to human- and door-motion-induced wake development during a short-duration event involving the movement of a person (or persons) from a contaminated room, through a vestibule, into a clean room. Parametric studies that capture the effects of human walking pattern, door operation, over-pressure level, and vestibule size are systematically conducted. A faster walking speed results in less mass transport from the contaminated room into the clean room. The net effect of increasing the volume of the vestibule is to reduce the contaminant transport. The results show that swinging-door motion is the dominant transport mechanism and that human-induced wake motion enhances compartment-to-compartment transport. PRACTICAL IMPLICATIONS: The effect of human activity on contaminant transport may be important in design and operation of clean or isolation rooms in chemical or pharmaceutical industries and intensive care units for airborne infectious disease control in a hospital. The present simulations demonstrate details of contaminant transport in such indoor environments during human motion events and show that simulation-based sensitivity analysis can be utilized for the diagnosis of contaminant infiltration and for better environmental protection.     

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.     

CFD技术在百级洁净手术室设计中的应用

利用CFD技术对手术室设计风量进行模拟计算，并校核了室内洁净度，模拟了人员对手术室洁净度的影响。得到了手术室的设计风量。     

Experimental and numerical investigation of airflow and contaminant transport in an airliner cabin mockup

The study of airflow and contaminant transport in airliner cabins is very important for creating a comfortable and healthy environment. This paper shows the results of such a study by conducting experimental measurements and numerical simulations of airflow and contaminant transport in a section of half occupied, twin-aisle cabin mockup. The air velocity and air temperature were measured by ultrasonic and omni-directional anemometers. A gaseous contaminant was simulated by a tracer gas, sulfur hexafluoride or SF₆, and measured by a photo-acoustic multi-gas analyzer. A particulate contaminant was simulated by 0.7 μm di-ethyl-hexyl-sebacat (DEHS) particles and measured by an optical particle sizer. The numerical simulations used the Reynolds averaged Navier–Stokes equations based on the RNG k–ε model to solve the air velocity, air temperature, and gas contaminant concentration; and employed a Lagrangian method to model the particle transport. The numerical results quantitatively agreed with the experimental data while some remarkable differences exist in airflow distributions. Both the experimental measurements and computer simulations were not free from errors. A complete and accurate validation for a complicated cabin environment is challenging and difficult.     

Performance of ventilation system in a non-standard operating room

The ventilation system of a hospital operating room is to provide a comfortable and healthy environment for the patient and the surgical team. Thermal comfort can be achieved by controlling the temperature, the humidity, and the air movement. A healthy environment can be achieved by minimizing the risk of contamination through appropriate filtration and air distribution scheme. The design and construction of operating rooms in Hong Kong, including the upgrading of the older ones, have been based on the UK Health Building Notes and Health Technical Memoranda. Observations and field measurements in a case study found that the airflow and some design features were not fully complied with the specified requirements. A CFD analysis supported by field measurements was made to simulate the temperature distribution, airflow pattern and the contaminant dispersion. The study placed an emphasis on the health risk of the airborne bacteria released from the surgical team on the patient, and vice versa.     

Numerical simulation on a horizontal airflow for airborne particles control in hospital operating room

Provision of downward unidirectional clean air has been prevalent for decades in modern hospital operating rooms (ORs) to protect patients and surgeons from infectious airborne particles and has been found to be effective in reducing Surgical Site Infection (SSI), however, its shortcomings are inevitable. In this study we investigated an alternative of horizontal airflow pattern and the airflow performance in an OR with a dimension of 300 cm long, 296 cm wide and 240 cm high. We also evaluated the effectiveness of the horizontal unidirectional airflow to control infectious airborne particles through onsite test and computational fluid dynamics (CFD) simulation method. The investigation was focused mainly on the influence of the medical lamps and the thermal plume with different airflow patterns around the critical zone under the horizontal air supply system. Ultraclean air was supplied from a fan-filter unit. The patient and surgeon were assumed to be releasing 200 and 400 particles per minute, respectively. The results show that when the air supply and return facilities are installed on the same lateral wall to keep a state of horizontal flow ventilation in the OR, medical lamps and the thermal plume have no obvious influence on the horizontal airflow patterns around the critical zone in the OR, and performance of the air supply system is highly related to the relative position of the source to the wound.     

Carbon monoxide transport in an enclosed room with sources from a water heater in the adjacent balcony

This paper has presented a computational analysis of carbon monoxide (CO) concentration inside a typical enclosed room of a residential building in Taiwan. CO is produced from a house-used natural gas water heater installed in the balcony. It is then diffused into the adjacent bedroom, which often causes serious poisoning accidences. A general-purpose computational fluid dynamics (CFD) code is employed to predict the CO concentration and airflow fields inside a three-dimensional (3D) modeled house. The variation of CO concentration was simulated under different scenarios of vent air flow rates and exit openings. It was found that under the ventilation conditions of V>0.0003 m/s, the levels of CO concentration in the bedroom is significantly decreased due to the entrainment of fresh air into the bedroom from the inside door. The present results could be used as a base for ventilation design for enclosed rooms, aiming at a proper ventilation system selection for avoiding the CO poisoning.     

Vertical profiles of temperature and contaminant concentration in rooms ventilated by displacement with heat loss through room envelopes

The purpose of this study is to examine the effect of heat loss through walls upon the gradients of temperature and contaminant concentration in room with displacement ventilation. It is known that conduction heat loss is governed by outside temperature, heat load inside the room, supply air temperature and overall heat transfer coefficient of walls. Experiments were conducted to measure the temperature gradient and the ventilation efficiency in the room ventilated by displacement ventilation with various combinations of heat load and temperature difference between supply air and outside air. In order to simulate the change of seasons, the supply air temperature was changed instead of the outside air temperature. The effect of supply air temperature and heat generation inside the room on the temperature gradient and the concentration of tracer gas were investigated through the experiments. As a result, it turned out that the higher the heat generation rate and the lower the supply temperature, the stronger the temperature stratification and the lower the concentration in the lower zone. Additionally, ventilation heat loss turned out to be a good index for assessing the concentration in the lower zone. Temperature differences of around 3 degrees C between supply air temperature and exhaust temperature are at least needed for displacement ventilation under the conditions of the experiment presented in this paper.     

Influence of river discharge patterns on the hydrodynamics and potential contaminant dispersion in the Douro estuary (Portugal)

Freshwater input to estuaries is a fundamental feature of these ecosystems, which may be profoundly altered by river damming as human needs for water consumption, irrigation or energy production increase. The Douro estuary is limited upstream by a dam since 1985, which reduced its length by ca. 60%. Freshwater inputs to the estuary are now irregular and greatly dependent on hydroelectric power demand; values ranging from zero to over 1000 m³ s⁻¹, in a matter of hours, especially in summer are common. In the present study, a three-dimensional hydrodynamic model was applied to the Douro estuary. The model was calibrated and validated against water elevation, current velocity, salinity and temperature data. Thereafter, it was used to analyse the effects of different flow regimes and magnitudes on estuarine hydrodynamics and contaminant dispersion. Results obtained suggest that the highly variable flow regimes, currently observed in the Douro, tend to reduce water column stratification and to enhance seawater intrusion, when compared with flow discharges of similar average magnitude, but lower variability. Stable flows seem to be the most effective in dispersing contaminants eventually introduced into the estuary through its small river tributaries. Overall results suggest that flow management may have important effects on estuarine hydrodynamics through non-linear interactions between flow magnitude and variability.     

Analytical model for coupled consolidation and diffusion of organic contaminant transport in triple landfill liners

A triple-layer composite liner consisting of a geomembrane liner (GMB), a geosynthetic clay liner (GCL) and a compacted clay liner (CCL) is commonly used at the landfill bottom liner system to isolate the contaminated leachates. In this paper, one-dimensional quasi-steady-state small deformation model (SDSS) was developed to investigate the behavior of organic chemicals transport in landfill composite liner system considering coupled effect of consolidation, diffusion and degradation. The first and second type bottom boundary conditions are used to derive the analytical solutions. The generalized integral transform technique (GITT) is adopted to derive the analytical solutions. The effect of consolidation on the performance of GMB/GCL/CCL with intact or leaking GMB is investigated. The triple liner under double drainage boundary condition (DDBC) has better performance compared to the case under single drainage boundary condition (SDBC). This is because the velocity induced by consolidation under DDBC is lower than that under SDBC. The effect of GCL consolidation shows an opposite trend compared to CCL consolidation. Considering GCL consolidation can increase the breakthrough time. The effective diffusion coefficient of GCL can be two magnitude orders smaller after consolidation, which provides a better diffusion barrier for the chemical transport. The effects of adsorption and degradation have been analyzed as well. Increasing the adsorption capacity of a deforming composite liner can increase the steady-state bottom flux, which shows the opposite tendency compared to the case without considering consolidation. This is due to the fact that for the case of a deforming composite liner, the advection induced by consolidation includes a new term due to the solid velocity. This velocity will result in the increase the mass of chemical migration through the composite liner.     

Study on two operating conditions of a full-scale oxidation ditch for optimization of energy consumption and effluent quality by using CFD model

The operating condition of an oxidation ditch (OD) has significant impact on energy consumption and effluent quality of wastewater treatment plants (WWTPs). An experimentally validated numerical tool, based on computational fluid dynamics (CFD) model, was proposed to optimize the operating condition by considering two important factors: flow field and dissolved oxygen (DO) concentration profiles. The model is capable of predicting flow pattern and oxygen mass transfer characteristics in ODs equipped with surface aerators and submerged impellers. Performance demonstration and comparison of two operating conditions (existing and improved) were carried out in two full-scale Carrousel ODs at the Ping Dingshan WWTP in Henan, China. A moving wall model and a fan model were designed to simulate surface aerators and submerged impellers, respectively. Oxygen mass transfer in the ditch was predicted by using a unit analysis method. In aeration zones, the mass inlets representing the surface aerators were set as one source of DO. In the whole straight channel, the oxygen consumption was modeled by using modified BOD-DO model. The following results were obtained: (1) the CFD model characterized flow pattern and DO concentration profiles in the full-scale OD. The predicted flow field values were within 1.98 ± 4.28% difference from the actual measured values while the predicted DO concentration values were within −4.71 ± 4.15% of the measured ones, (2) a surface aerator should be relocated to around 15 m from the curve bend entrance to reduce energy loss caused by fierce collisions at the wall of the curve bend, and (3) DO concentration gradients in the OD under the improved operating condition were more favorable for occurrence of simultaneous nitrification and denitrification (SND).     

Influence of water chemistry and travel distance on bacteriophage PRD-1 transport in a sandy aquifer

Experiments were conducted to evaluate the impact of groundwater chemistry and travel distance on the transport and fate behavior of PRD-1, a bacteriophage employed as a surrogate tracer for pathogenic enteric viruses. The experiments were conducted in the unconfined aquifer at the United States Geological Survey Cape Cod Toxic-Substances Hydrology Research Site in Falmouth, Massachusetts. The transport behavior of bromide (Br(-)) and PRD-1 were evaluated in a sewage-effluent contaminated zone and a shallower uncontaminated zone at this site. Several multilevel sampling devices located along a 13-m transect were used to collect vertically discrete samples to examine longitudinal and vertical variability of PRD-1 retardation and attenuation. The concentration of viable bacteriophage in the aqueous phase decreased greatly during the first few meters of transport. This decrease is attributed to a combination of colloid filtration (attachment) and inactivation. The removal was greater (10(-12) relative recovery) and occurred within the first meter for the uncontaminated zone, whereas it was lesser (10(-9) relative recovery) and occurred over 4m in the contaminated zone. The lesser removal observed for the contaminated zone is attributed to the influence of sorbed and dissolved organic matter, phosphate, and other anions, which are present in higher concentrations in the contaminated zone, on PRD-1 attachment. After the initial decrease, the aqueous PRD-1 concentrations remained essentially constant in both zones for the remainder of the tests (total travel distances of 13 m), irrespective of variations in geochemical properties within and between the two zones. The viable, mobile PRD-1 particles traveled at nearly the rate of bromide, which was used as a non-reactive tracer. The results of this study indicate that a small fraction of viable virus particles may persist in the aqueous phase and travel significant distances in the subsurface environment.     

CFD仿真技术在空调房间温度场研究中的应用

利用流场仿真软件对四面出风和传统水平出风空调器温度场、速度场进行模拟,从舒适性角度作出评价,认为前者效果优于后者。实验研究为产品开发和改进提供了依据。     

Behaviour of well-mixed zones (WMZs) in an imperfectly mixed ventilated room

In this paper the behaviour of 3D well-mixed zones (WMZs) of temperature is studied using numerically generated data. A WMZ of temperature is a 3D zone of improved mixing whereby an acceptably low spatial temperature difference occurs. Here it was anticipated to set a criterion to define a WMZ that takes into account the comfort and health of building occupants. It was found that WMZs are irregular in shape and unpredictable in size in imperfectly mixed ventilated rooms. Furthermore airflow exchange between the considered WMZ and its surrounding was quantified. The results of the analysis show the potential of computational fluid dynamics (CFD) in quantifying the shape, size and volume of WMZs that may not be possible to perform using experimental methods because of the practical limitation of placing thousands of sensors inside a ventilated room.     

The effect of source motion on contaminant distribution in the cleanrooms

In the recent decades, cleanrooms have found growing applications in broad range of industries such as pharmacy and microelectronics. Concerns about negative effects of the contaminant exposure on the human health and product quality motivate many researchers towards understanding of the airflow and contaminant distribution though these environments. With an improvement in computational capacity of the computers, computational fluid dynamics (CFD) technique has become a powerful tool to study the engineering problems including indoor air quality (IAQ). In this research, indoor airflow in a full-scale cleanroom is investigated numerically using Eulerian-Eulerian approach. To evaluate the ventilation system effectiveness, a new index, called final efficiency, is introduced which takes all aspects of the problem into account. The results show that the contaminant source motion and its path have a great influence on the contaminant dispersion through the room. Based on the results, the contaminant distribution indexes, e.g. final efficiency and spreading radius, are improved when the source motion path is in the dominant direction of the ventilation airflow. Consequently, the efficiency of an air distribution system which provides a directional airflow pattern shows the least source path dependency. This study and its results may be useful to gain better understanding of the source motion effects on the indoor air quality (IAQ) and to design more effective ventilation systems.     

Influence of biofilms on the movement of colloids in porous media. Implications for colloid facilitated transport in subsurface environments

Colloid transport through porous media can be influenced by the presence of biofilms. Sterile and non-sterile sand columns were investigated using Laponite RD as model colloid and a highly mucoid strain of Pseudomonas aeruginosa as model biofilm former. Laponite RD was marked specifically by fluorescent complexes with rhodamine 6G. Breakthrough curves (BTCs) were used as parameters for determination of colloid transport characteristics. In the sterile columns, the colloid was mobile (collision efficiencies from 0.05 to 0.08) both after the presence of Na(+) and Ca(2+) ions followed by deionised water influent. In the biofilm-grown column, the same treatment did not result in colloid retention in the case of Na(+) exposure, but in altered or enhanced colloid transport. In the case of Ca(2+) ions exposure, colloid retention increased with biofilm age. After 3 weeks, almost complete retention was observed. Similar observations were made in columns packed with material from slow sand filtration units. These data reveal the complex interactions between biofilms, cations and colloid transport. Changes in the electrolyte composition of water percolating the subsurface can frequently occur and will result in different colloid transport characteristics with regard to the dominating species of ions and the relative abundance of microbial biofilms. This has to be considered when modelling colloid transport through the subsurface.     

Influence of solution ionic strength on the collision efficiency distribution and predicted transport distance of a Sphingomonas sp. flowing through porous media

The effects of solution ionic strength on the collision efficiency (alpha) distribution of a Sphingomonas sp. were investigated using multiple sand columns of varying lengths and analyzing the bacteria clean-bed breakthrough concentrations using a distributed colloid filtration theory (D-CFT). Five different probability density functions (PDFs) were investigated and all accurately replicated the lab-scale experimental data, whereas a single alpha value could not. The alpha distribution shifted toward smaller values with decreasing ionic strength and the PDF parameters were strongly correlated to the Debye length, indicating that electrostatic interactions had a direct impact on the alpha distribution. The results indicate that while ionic strength has a large impact on bacterial transport distances for a concentration reduction of a few orders of magnitude, as occurs at the laboratory scale, due to the distributed nature of the collision efficiency, it has a minor effect on predicted transport distances required to achieve concentration reductions on the order of 10(6), which occurs at the field scale. Because of this, bacterial inactivation (e.g., death), rather than physically removing the bacteria from solution via filtration, is likely the key process impacting the transport of viable bacteria at the field scale. Overall, for systems with a distributed alpha, the results indicate that ionic strength has a strong influence on the transport of bacteria at the lab-scale (centimeters to one meter), both ionic strength and bacterial inactivation are important at the meso-scale (tens of meters), and inactivation becomes the dominant mechanism for reducing the transport of viable bacteria at the field scale (hundreds of meters).     

地下水中三氮污染物研究综述与展望

地下水中三氮（氨氮、亚硝酸盐氮和硝酸盐氮）污染正日益受到国内外研究者的关注,相关研究也取得了大量的研究成果,推动了三氮污染物防治研究工作的进展。本文在已有研究的基础上首先介绍了三氮在地下水中的运移转化机理,然后分析了当前研究常用的定性实验方法和定量数值模型方法,最后提出了研究中存在的问题和三氮污染防治建议。     

Influence of different make-up air configurations on the fire-induced conditions in an atrium

In this paper, the influence of the make-up air velocity as well as the position and area of the vents in an atrium is assessed both experimentally and numerically. In the experiments, the effect of different make-up air supply positions and inlet area on the fire-induced inner conditions and smoke-layer descent was studied by means of three full-scale fire tests conducted in a 20 m cubic atrium. Detailed transient measurements of gas and wall temperatures, as well as pressure drop through the exhaust fans and airflow at the inlets were recorded. These data could be used as benchmark for future numerical validation studies. Later computational fluid dynamics (CFD) simulations of these tests were performed with the code Fire Dynamics Simulator (FDSv4). In the experiments, the lack of symmetry in make-up air vents and the large inlet area turn the flame and plume more sensitive to outer effects. However, no significant difference has been observed between the make-up air topologies assessed. Even make-up velocities higher than 1 m/s, with symmetric venting topology, have not induced important flame or plume perturbations. In the numerical simulations, the predictions agree well with the experiments for the cases with larger make-up air openings. Poor agreement has been found for the case with the smallest inlet openings.     

香港某医院手术室内通风效果的数值研究

医院手术室对室内的速度场、温度场及有害气体和细菌的分布有严格的要求。应用商用软件对某手术室的通风效果作了数值模拟。计算采用RNG K-ε紊流模型,使用现场测量数值作为边界条件,求解了三维速度场、温度场及空气龄,研究表明,使用数值方法可以得出通风空间内每一点的详细情况,可对整体通风效果进行全面评价,对此类有严格要求的通风系统的设计及验收具有一定的意义。