Removal of contaminants released from room surfaces by displacement and mixing ventilation: modeling and validation

This paper presents the experimental and numerical modeling of contaminant dispersion in a full-scale environmental chamber with different room air distribution systems. For the experimental modeling, an area source with uniform emissions of a hypothetical contaminant (SF6) from the entire floor surface is designed and constructed. Two different types of ventilation are studied: displacement and mixing ventilation. A computer model for predicting the contaminant dispersion in indoor spaces was validated with experimental data. The validated model is used to study the effects of airflow and the area-source location on contaminant dispersion. Results show that the global airflow pattern has a strong impact on the distribution of the contaminants. In general, the personal exposure could be estimated by analyzing the relative source positions in the airflow pattern. Accordingly, the location of an exhaust diffuser may not greatly affect the airflow pattern, but can significantly affect the exposure level in the room. PRACTICAL IMPLICATIONS: When designing ventilation in addition to bringing fresh air to occupants, it is important to consider the removal of contaminants released in the off-gassing of building materials. Typical indoor off-gassing examples are emissions of volatile organic compounds from building enclosure surfaces such as flooring and painted walls. In this study, we conducted experimental and numerical modeling of different area sources in a mock-up office setup, with displacement or mixing ventilation. Displacement ventilation was as successful as mixing ventilation in removing the contaminant source from the floor area. Actually, the most important consideration in the removal of these contaminants is the relative position of the area source to the main airflow pattern and the occupied zone.     

多孔介质中化学–热–水力–力学耦合过程本构模型和数值模拟

本文发展了一个多孔介质中的化学–热–水力–力学(CTHM)耦合本构模型。该模型基于文献[1]中多孔介质热–水力–力学(THM)本构模型、文献[2,3]中的污染物传输本构模型和文献[4]提出的化学–力学本构模型。在本构模型中引入化学软化函数以模拟孔隙水中有机污染物对多孔介质力学性质的影响。考虑了温度对污染物传输机制的影响。本文CTHM耦合本构模型已在多孔介质的热–水力–力学–污染物传输数学模型中建立。在CTHM本构模拟框架中对计及化学塑性效应、描述在热–水力–污染物传输耦合作用下多孔介质力学行为、在应力–吸力–温度四维空间中包含了五重屈服面的耦合本构模型发展了一致性切线模量矩阵。数值结果突出显示了污染物浓度在多孔介质化学–热–水力–力学(CTHM)耦合系统中的影响。     

Simplified analysis of contaminant rejection during ground- and surface water nanofiltration under the information collection rule

A simple, closed-form analytical expression based on the homogenous solution diffusion model is derived for contaminant removal during nanofiltration (NF) of ground and surface water. Solute permeation and back-diffusion coefficients were used as fitting parameters to model rejection characteristics of four thin-film composite NF membranes under conditions typical of drinking water NF. Nonlinear fits of the model to experimental data suggests that the United States Environmental Protection Agency's (USEPA)'s Information Collection Rule protocol for bench-scale studies could be improved to obtain greater precision of the mass transfer coefficients. The model was found to fit rejection data for several water treatment contaminants including total organic carbon, precursors to total organic halide, four trihalomethanes and nine haloacetic acids containing chlorine and bromine, calcium and total hardness, alkalinity and conductivity. The simplified approach to mass transfer calculations from multisolute systems suggests that feed water recovery has a stronger influence on contaminant rejection than permeate flux. Evidence for coupled transport of divalent inorganic ions is also presented. Even though the model developed does not account for ion coupling and cannot be applied in a purely predictive mode, it can assist in the better design and interpretation of data obtained from site-specific pilot-scale water treatment NF studies conducted in support of plant design.     

重金属离子和LNAPLs在非饱和土中的运移规律研究

以重金属和轻非水相流体(LNAPLs)为例,介绍了清华大学开展非饱和土中污染物运移规律研究的部分工作成果。利用土工离心机对污染物的运移过程模拟,取样得到污染物的浓度分布特征。同时采用数值分析手段对污染物的长期运移规律和其影响因素进行分析。研究表明:离心模型试验可以有效模拟污染物的运移过程,离心模型试验和数值分析相结合,是研究污染物在非饱和土中运移规律的有效手段。     

Prediction of coal mine goaf self-heating with fluid dynamics in porous media

Coal mine goaf self-heating due to exothermic coal oxidation has been recognized as a major threat to coal mine safety. To evaluate the risk of the coal mine goaf self-heating hazard, both gas distribution characteristics in goaf and heat transfer mechanisms in porous media must be studied. However, due to the difficulty of determining goaf permeability and the complexity of the overlaying strata caving characteristics, it is a considerable challenge to determine the thermal-fluid field coupling. Based on the volumetric average method in porous media, this work develops three numerical models for solving fluid dynamics and heat transfer in both longwall face and goaf. The Brinkman-Forchheimer-extended Darcy model is introduced to describe inertia and frictional forces in fluid phase exerted by solid phase. Temperature profile is highly dependent on and affects coal oxidation rate; therefore, governing equations for energy and oxygen mass equilibrium must be coupled. A two-dimensional goaf permeability distribution model is established based on different caving conditions in goafs. Three scenarios are simulated and validated by field and experimental data, and it is observed that these models are capable of predicting gas flow pattern and temperature distribution.     

Numerical and experimental simulation of pollutants migration in porous media

 Quantitative prediction of pollutant migration is now of great interest in the field of waste management and storage. Numerical modelling of contaminant transport through porous media is an efficient tool to make this prediction possible. Site selection and design of protective liners for landfills and storage facilities are largely related to the behaviour of contaminants in such structures. This behaviour may be well defined by numerical simulations. This paper proposes a two-dimensional finite element model for pollutant migration. The implementation of the advection-dispersion equation in the numerical model and validation tests by comparison with analytical and semi-analytical solutions are presented. Experimental laboratory tests on cadmium and chloride migration through liner samples are also discussed and compared with the results from the numerical model. Received: 5 July 1999 · Accepted: 15 April 2000     

Analysis of coupled gas flow and deformation process with desorption and Klinkenberg effects in coal seams

Coupled gas flow and solid deformation in porous media has received considerable attention because of its importance in pneumatic test analysis, contaminant transport, and gas outbursts during coal mining. Gas flow in porous media is quite different from liquid flow due to the large gas compressibility and pressure-dependent effective permeability. The dependence of gas pressure and gas desorption on gas permeability has a significant effect on gas flow, but has been ignored in most previous studies. Moreover, solid deformation has a direct impact on the porosity, which also leads to desorption or sorption of methane in the coal seam. In this study, a coupled mathematical model for solid deformation and gas flow is proposed and is implemented using a finite element method. The numerical code is used to solve the gas flow equation with Klinkenberg effect, and is validated by comparison with available analytical solutions. Then, it is used to simulate the coupled process during gas migration in a deformable coal seam. The numerical results indicate that the desorption and Klinkenberg effects and mechanical process effect make a significant contribution to gas flow in the coal seam. Without considering the desorption and Klinkenberg effects and the coupling action of mechanical process, the gas pressure in the coal seam would be underestimated.     

Coupled hydro-mechanical effect of a fractured rock mass under high water pressure

To explore the variation of permeability and deformation behaviors of a fractured rock mass in high water pressure,a high pressure permeability test(HPPT),including measuring sensors of pore water pressure and displacement of the rock mass,was designed according to the hydrogeological condition of Heimifeng pumped storage power station.With the assumption of radial water flow pattern in the rock mass during the HPPT,a theoretical formula was presented to estimate the coefficient of permeability of the rock mass using water pressures in injection and measuring boreholes.The variation in permeability of the rock mass with the injected water pressure was studied according to the suggested formula.By fitting the relationship between the coefficient of permeability and the injected water pressure,a mathematical expression was obtained and used in the numerical simulations.For a better understanding of the relationship between the pore water pressure and the displacement of the rock mass,a 3D numerical method based on a coupled hydro-mechanical theory was employed to simulate the response of the rock mass during the test.By comparison of the calculated and measured data of pore water pressure and displacement,the deformation behaviors of the rock mass were analyzed.It is shown that the variation of displacement in the fractured rock mass is caused by water flow passing through it under high water pressure,and the rock deformation during the test could be calculated by using the coupled hydro-mechanical model.     

Modeling and testing of reactive contaminant transport in drinking water pipes: chlorine response and implications for online contaminant detection

A modified one-dimensional Danckwerts convection-dispersion-reaction (CDR) model is numerically simulated to explain the observed chlorine residual loss for a "slug" of reactive contaminants instantaneously introduced into a drinking water pipe of assumed no or negligible wall demand. In response to longitudinal dispersion, a contaminant propagates into the bulk phase where it reacts with disinfectants in the water. This process generates a U-shaped pattern of chlorine residual loss in a time-series concentration plot. Numerical modeling indicates that the residual loss curve geometry (i.e., slope, depth, and width) is a function of several variables such as axial Péclet number, reaction rate constants, molar fraction of the fast- and slow-reacting contaminants, and the quasi-steady-state chlorine decay inside the "slug" which serves as a boundary condition of the CDR model. Longitudinal dispersion becomes dominant for less reactive contaminants. Pilot-scale pipe flow experiments for a non-reactive sodium fluoride tracer and the fast-reacting aldicarb, a pesticide, were conducted under turbulent flow conditions (Re=9020 and 25,000). Both the experimental results and the CDR modeling are in agreement showing a close relationship among the aldicarb contaminant "slug", chlorine residual loss and its variations, and a concentration increase of chloride as the final reaction product. Based on these findings, the residual loss curve and its geometry are useful tools to identify the presence of a contaminant "slug" and infer its reactive properties in adaptive contaminant detections.     

北方某水厂各流程常规水质指标的检测分析

为研究水厂各工艺流程对有机污染物的去除效能及其分布特征,对我国北方采用常规处理工艺的某水厂进行了为期1年的调查分析。结果表明：该水厂除出水总氮略高外,全年出水中其他各项指标基本满足国家饮用水标准。通过气相色谱-质谱联用分析可知,常规水处理工艺对有机污染物的去除能力有限,出水中仍含有大量有机物,传统水处理工艺难以有效保障水质安全性。     

How groundwater seepage and transport modelling software can be useful for studying gaseous transport in an unsaturated soil

The aim of this paper is to show how standard hydrogeologic software, usually used to model contaminant transport in groundwater under unsaturated conditions, can also be used to model gas transport in unsaturated porous media. Physical processes involved in the interaction between the atmosphere and the unsaturated soils are considered: transport by diffusion through the air and the groundwater, exchange between the liquid and gas phases and consumption. These physical processes are incorporated into the governing equations of a groundwater numerical code; by considering air, contained in the unsaturated soil, as water in the seepage numerical model, the air effectively becomes fluid within the numerical code. Then, the investigated gas is defined as the contaminant in the transport model, which is transported by -the air for the modeller-, and -water for the numerical code-. The over-riding assumption is that the air profiles and, therefore, water profiles of volume contents remain constant. The approach is illustrated using two examples, which consider the transport of oxygen. The first deals with oxygen distribution through a laboratory-cell diffusion containing reactive mining tailings. The second deals with the oxygen fluxes through the vadose zone, between the atmosphere and an unconfined aquifer's water table. Both examples consider different cases of oxygen consumption.     

Contaminant flows in urban residential water systems

A contaminant balance for urban residential water systems has been performed for 12 pollutants. The characteristics of the residential site were based on Ellenbrook, a suburb of Perth, Australia. The sources of contaminants were identified and characterised using published literature values. A water balance was used to estimated flow data, and these results were used in conjunction with the contaminant source characteristics to calculate contaminant loads. The contaminants examined were total nitrogen, ammonia, total phosphorus, copper, zinc, lead, cadmium, suspended solids, dissolved solids, chemical oxygen demand, polycyclic aromatic hydrocarbons and oil and grease. Diagrams of contaminant flows through the water, wastewater and stormwater systems are presented. By identifying the sources of contaminants, the diagrams are a useful reference when considering the fate of contaminants in alternative urban water system configurations or how to better handle or reduce these contaminants.     

污染物离子在粘土介质中扩散系数和分配系数的测定(英文)

介绍了利用一种自制的特殊装置,测定污染物离子在粘土介质中迁移的扩散参数D和分配系数Kd的方法。通过一系列污染物在上海粘土中的纯扩散迁移试验,系统中污染物的质量不变,源浓度随试验的进行是下降的,运用一个简单的理论模型,通过拉普拉撕(Laplace)变换方法,对一个单独的试验,可同时推断出扩散系数和分配系数。通过拉普拉撕转换程序求出了阴离子(Cl-)和阳离子(K+,Ca2+,Zn2+,Cu2+)的扩散系数D和分配系数Kd。试验结果发现:阴离子在粘性土壤中的显扩散系数比阳离子的大;一价阳离子K+的显扩散系数比二价阳离子的大。     

CO2-dispersion studies in an operation theatre under transient conditions

Maintaining air quality and thermal comfort inside an operation theatre equipped with horizontal jet flow type air-conditioning units, has been a challenge to engineers. The objective of this study is to analyze the airflow pattern in such operation theatres and the influence of location of the air-conditioners. The outcome of this study is expected to reduce the post-operation problems faced due to excess concentration of contaminants. Experimental studies were conducted in 10 different hospital operation theatres. Parameters such as air temperature and carbondioxide concentration were measured at discrete points chosen in the theatre. A 3D time-dependent numerical model was developed to simulate the airflow in terms of parameters such as velocity, temperature and CO₂ distributions in an operation theatre under transient conditions. The Eulerian approach using the volume fraction of the mixture of air and CO₂ was used to solve the numerical model. Finite volume approach was attempted in this work with PISO (pressure implicit with split operators) algorithm for the pressure correction equations. The simulated results were compared with the experimental results for validity. The locations of the air-conditioners were changed in the numerical model to analyze the airflow patterns and the contaminant distribution.     

煤微观结构三维表征及其孔–渗时空演化模式数值分析

 探讨煤岩微观孔隙结构统计意义上的等效构建方法,并于孔隙尺度下模拟流体运移行为的时空演化过程。首先,结合煤岩孔隙结构的各向异性及非线性特征,采用修正的四参数随机生长(QSGS)算法构建煤岩多孔介质模型。随后,采用格子Boltzmann方法(LBM)模拟孔隙中气体单相流的运移过程,并基于数值试验结果,分析煤岩介质中影响渗透率的主要因素及流体运移达到平衡态前的演化模式。分析结果表明：(1) 多孔介质的渗透性能受控于少量、连通性好的大孔所形成的通道,而小孔及微孔中气体的行为基本属于浓度扩散过程;(2) 孔隙属性空间变异条件相同情况下,孔隙度同渗透率之间满足乘幂关系,但幂率系数随孔隙属性变异因子的降低而增加;(3) 渗透率随时间呈减少趋势,自运移开始达到动态平衡的耗时量同孔隙度之间是一种负相关的幂率关系。     

Numerical Simulation of Contaminant Transport in One-Dimensional Infinite Media

Abstract: In this paper, seawater or contaminant intrusion into fresh groundwater in lowland areas is treated as a mass transfer problem in infinite media. For numerical simulation of such problem, a transient infinite element model has been presented to simulate the mass transfer process in infinite media. The main merit of using the present transient finite and infinite element coupled model is that the numerical analysis of contaminant transport problems in infinite media can be carried out using microcomputers. The comparison of the present numerical results with the previous analytical results indicates the accuracy, efficiency and effectiveness of the present infinite element.     

Adsorption and desorption of trace organic contaminants from granular activated carbon adsorbers after intermittent loading and throughout backwash cycles

A granular activated carbon (GAC) adsorption simulation methodology using the observed trace organic contaminant mid-point breakthrough and the pore diffusion model is presented, validated, and used to model adsorption and concentration gradient driven desorption. Trace organic contaminant adsorption was well-simulated by this approach; however, desorption from GAC adsorbers was found to occur at lower concentrations than predicted by either pore or surface diffusion model calculations. The observed concentration profiles during desorption yielded a lower peak concentration and more elongated attenuation of contaminants after intermittent loading conditions than predicted by the models. Hindered back diffusion caused by irreversibly adsorbed dissolved organic matter on the GAC surface is hypothesized to be responsible for slowing the desorption kinetics. In addition, laboratory test results indicate a negligible impact of simulated backwashing the GAC media on trace organic contaminant breakthrough.     

Surfactant enhanced electrokinetic remediation of DDT from soils

Electrokinetic remediation has been investigated extensively as one of the noble technologies in remediation of metal contaminated soils. However, its applications in remediation of organic contaminants have been limited due to low solubilities of organics in water. In addition, most organic contaminants are non-ionic and therefore, they are not mobile under electrical field. The use of surfactants may increase the remediation efficiency by increasing the solubility of organics. Significant fraction of organics associated with soil, can be transferred to micellar phase, which then can be transported toward either cathode or anode, depending on the ionic group of surfactants. In this study, the removal of hydrophobic organic contaminants from a soil using electrokinetic method was investigated in the presence of surfactants. A nonionic surfactant, Tween 80, and an anionic surfactant, SDBS, were used in the experiments. DDT was chosen as the model organic contaminant. Phase distribution studies and column experiments were conducted. It was found that both Tween 80 and SDBS had similar solubilization potentials for DDT. It was also shown that the aqueous DDT mass could reach from 0.01 to 13% of the total mass in the presence of 7500 mg/L of SDBS. No significant movement of DDT was observed when Tween 80 was used in the column experiments. This was attributed to low rates of electroosmotic flows and strong interaction of Tween 80 with the soil. The amount of surfactant was not enough to mobilize DDT significantly in the column studies. On the other hand, electrokinetic transport with SDBS yielded much better results. DDT transport toward the anode within the negatively charged micelles overcame the opposite electrosmotic flow. This was attributed to the lower degree of interaction between the soil and SDBS, and the electrokinetic transport of negatively charged micelles.     

非饱和带水–气二相流数值模拟研究

为了真实反映非饱和带中流体(水和气)的运移规律和渗透特性,将非饱和带中的孔隙水和孔隙气作为研究对象,基于水-气二相流理论,建立了水-气二相流数学模型。该模型采用积分形式的有限差分法对控制方程进行离散,在应用Newton-Raphson迭代方法得到相应的线性方程组的基础上,编制了三维计算程序SEEP2P。通过对土坝稳定渗流和降雨入渗情况的计算分析,得出稳定状态的渗流可以近似采用单相流模型来模拟,降雨入渗下的非稳定渗流问题采用水-气二相流模型来模拟更加符合实际。采用水-气二相流模型模拟得到水相、气相的流动状态,为定量研究孔隙气压力对入渗水流的影响提供了有效途径。