The moisture performance of framed structures—A mathematical model

A mathematical model is developed for the moisture performance of a framed structure (e.g., a flat roof or a wall), containing a hygroscopic framing material and a cavity filled with air or insulation. A formula is developed that connects the enclosed and unenclosed drying time constants for the framing material. The enclosed drying time constant alone describes the longer term moisture behaviour of the structure (much greater than one day) under any driving forces given the linearity assumptions used. The model allows for anisotropic framing materials with initial moisture contents above or below fibre saturation.     

Sensitivity analysis of CFD coupled non-isothermal heat and moisture modelling

CFD (Computational Fluid Dynamics) is a useful tool to study air flow patterns in a room. Current CFD models are able to simulate air flow combined with temperature distributions and species distributions. In this paper a coupled CFD–HAM model is discussed. This model combines CFD with a HAM model (Heat, Air and Moisture) for hygroscopic materials. This coupled model is able to simulate air flow around a porous material and combines this with heat and moisture transport in the porous material. Validation with a small scale experiment in which gypsum board was used as a hygroscopic material showed good results. In this paper a further validation of the model is discussed based on a sensitivity analysis of some model parameters. Especially hygrothermal parameters like sorption isotherm and water vapour permeability proved to have a non negligible influence on the modelling outcome. Adding a hysteresis model showed improvement of the model during desorption. The model was also used to compare two modelling strategies. In one strategy the gypsum board was modelled as a uniform material, in a second approach the material was modelled as being layered. The difference between the two approaches showed to be negligible.     

Transfer function method to calculate moisture absorption and desorption in buildings

A transfer function approach has been described to calculate dynamic moisture absorption and desorption by building materials in the hygroscopic range, and the dynamic moisture absorption and desorption processes have been theoretically modeled. Their surface vapor pressure and moisture absorption and desorption flux can be evaluated by this approach. It has turned out mathematically that the material surface attains instantaneous moisture equilibrium with the surroundings at high Biot number (Bi → ∞), and that the material moisture behavior can be described through a lumped-parameter modeling at low Biot number (Bi → 0). For building materials at intermediate Biot number, comparisons with experimental results and numerical solutions have shown satisfactory agreement with the proposed approach. Using this approach, the dynamic response of moisture absorption and desorption in interior building materials to the variation of indoor air humidity and the dynamic effect of moisture absorption and desorption by building interior materials on indoor air humidity and space latent cooling load can be simultaneously calculated.     

Model for simulation of freezing and thawing processes in building materials

This paper presents a mathematical model for simultaneous heat and moisture transfer under freezing–thawing in porous hygroscopic material. Even below 0°C, unfrozen liquid water exists in the material which plays an important role in the processes. The model takes into account the existence of unfrozen liquid water. The system is treated as a three-(gas, liquid and solid) phase system of water. Moisture chemical potential is used as a moisture potential. Under freezing, moisture chemical potential is a unique function of local temperature. So, during freezing, two unknown variables to be solved are solid water content and moisture potential or temperature. A simple example of simulation for freezing processes is shown.     

The effect of structures on indoor humidity--possibility to improve comfort and perceived air quality

The research presented in this paper shows that moisture transfer between indoor air and hygroscopic building structures can generally improve indoor humidity conditions. This is important because the literature shows that indoor humidity has a significant effect on occupant comfort, perceived air quality (PAQ), occupant health, building durability, material emissions, and energy consumption. Therefore, it appears possible to improve the quality of life of occupants when appropriately applying hygroscopic wood-based materials. The paper concentrates on the numerical investigation of a bedroom in a wooden building located in four European countries (Finland, Belgium, Germany, and Italy). The results show that moisture transfer between indoor air and the hygroscopic structure significantly reduces the peak indoor humidity. Based on correlations from the literature, which quantify the effect of temperature and humidity on comfort and PAQ for sedentary adults, hygroscopic structures can improve indoor comfort and air quality. In all the investigated climates, it is possible to improve the indoor conditions such that, as many as 10 more people of 100 are satisfied with the thermal comfort conditions (warm respiratory comfort) at the end of occupation. Similarly, the percent dissatisfied with PAQ can be 25% lower in the morning when permeable and hygroscopic structures are applied.     

Thermal and moisture parameters of a dry coarse-grained fill or drainage layer

The thermal conductivity, water vapour permeability, hygroscopic and capillary equilibrium water contents of dry coarse-grained fill and drainage materials are determined by a series of laboratory tests. Test results show that there is a significant difference between the hygroscopic equilibrium moisture contents of the materials and the volumes of water the same material can transport by capillary action. Yet the relative humidity (RH %) is the same in materials either in hygroscopic region or almost fully saturated by capillary action. Therefore the water content is the best indication variable when determining the moisture behaviour of drainage or fill layers next to the foundation or slab structures. In dry conditions the thermal conductivity of coarse-grained drainage materials is significantly lower than the conductivity of moist or wet subsoil layer. In structures where the slab-on-ground structure is built on top of a relatively thick fill or drainage layer, this difference in thermal conductivities between the subsoil and the drainage materials should be taken into consideration when determining the temperature distribution at the fill–slab interface or the heat loss via the ground slab structures.     

新建节能建筑墙体材料层及排布对干燥过程的影响

建筑围护结构内的热湿耦合传递是一个非常复杂的过程,其研究是降低建筑能耗、评估和预防湿害、提高室内热舒适性、室内卫生及优化围护结构性能的基础。新建节能建筑墙体具有初始含湿量大的特点,若墙体湿积累过大,则容易出现墙体表面剥蚀、渗漏、发霉甚至结构出现损坏的现象。墙体干燥时,传热传质过程同时发生且相互耦合。目前相关热物性仿真软件、理论研究和设计规范主要建立在热传递的基础上,忽略了湿传递的影响,对新建建筑墙体干燥不适用。WUFI~? Pro热湿仿真软件充分考虑了材料本身含湿量、风驱雨、太阳辐射、长波辐射、毛细传输和夏季结露等典型气候的影响,实现了对自然气候条件下建筑构件非稳态热湿性能的真实计算。节能墙体多在外墙添加内外保温层来增加围护结构的传热热阻,且在保温层内外两侧分别添加隔汽层和空气层的措施来防止保温层受潮,最终提高围护结构的保温性能。为墙体美观,多在围护结构的内外两侧分别黏贴墙纸和釉面砖。采用WUFI~? Pro对北京地区2种典型的建筑墙体进行热湿耦合传递模拟,分析新建建筑墙体在不同保温层材料和位置时的干燥过程,以及保温层两侧的隔汽层和空气层、墙体两侧的墙纸和釉面砖对墙体干燥过程的影响。模拟用室外条件为北京典型气象年小时室外气象参数,室内条件设定室内冬季供暖温度T_1=20℃,夏季室内温度设计值T_2=25℃,全年平均相对湿度为50%。模拟外围护结构属于西向,墙体温湿度初始条件为:相对湿度为100%,温度为15℃。模拟结果表明:内保温层的设置非常不利于围护结构的干燥,容易在内保温层和砌块之形成湿积累,降低围护结构的耐久性;EPS、PU和XPS都能降低围护结构含湿量,但EPS更有利于墙体干燥;隔汽层和空气层的添加可一定程度上阻止保温层受潮,避免造成湿积累,进而提高围护结构的保温性能;釉面砖和墙纸的黏贴将严重延缓围护结构的干燥过程,降低围护结构的保温性能,缩减建筑构件的使用寿命。     

Ein Trocknungskoeffizient für Baustoffe

Ein wesentliches Element der hygrothermischen Charakterisierung von Baustoffen ist der Trocknungsversuch. Im Gegensatz zu anderen Feuchtetransportexperimenten wie dem Diffusionsversuch oder dem Wasseraufnahmeexperiment ist es bislang nicht möglich, aus der Trocknung einen einfachen Kennwert abzuleiten. In vielen Fällen, beispielsweise in der Interaktion von Forschung und Industrie, aber auch beim praktischen Vergleich bzw. der Auswahl geeigneter Baustoffe wäre ein solcher Kennwert jedoch wünschenswert. Im vorliegenden Artikel wird zunächst die Bedeutung des Trocknungsversuches für die hygrische Charakterisierung von Baustoffen herausgestellt, aus der sich das Bestreben ableitet, das Trocknungsverhalten zu standardisieren und einen Einzahlen‐Materialkennwert zu definieren. Nach einer die verschiedenen Einflussfaktoren der Trocknung differenzierenden Einleitung werden bestehende Ansätze für die Standardisierung des Trocknungsverlaufes bzw. die Ableitung eines Trocknungskoeffizienten vorgestellt. Die einhergehenden Probleme werden diskutiert und weitere Möglichkeiten evaluiert. Ein einfacher Trocknungskoeffizient, der sich aus dem Trocknungsverlauf ableiten lässt, wird definiert. Die Korrelation dieses Koeffizienten mit dem Wasseraufnahmekoeffizienten und dem Dampfdiffusionswiderstand wird analysiert. Sein zusätzlicher Informationsgehalt wird in diesem Zusammenhang kritisch hinterfragt. Im Ergebnis steht die Definition des Trocknungskoeffizienten als ein neuer, unabhängiger Materialkennwert, der die Feuchtetransporteigenschaften im Übergang zwischen hygroskopischem und gesättigtem Transport beschreibt. Mit diesem Kennwert ist es möglich, Baustoffe einfach und schnell hinsichtlich ihres Trocknungsverhaltens zu unterscheiden und zu beurteilen, was insbesondere bei feuchtesensitiven Materialien von Bedeutung ist. A drying coefficient for building materials. The drying experiment is an important element of the hygrothermal characterisation of building materials. Contrary to other moisture transport experiments as the vapour diffusion and the water absorption test, it is until now not possible to derive a simple coefficient for the drying. However, in many cases such a coefficient would be highly appreciated, e.g. in interaction of industry and research or for the distinction and selection of suitable building materials throughout design and practise. This article first highlights the importance of drying experiments for hygrothermal characterisation of building materials on which the attempt is based to standardize the drying experiment as well as to derive a single number material coefficient. The drying itself is briefly reviewed and existing approaches are discussed. On this basis, possible definitions are evaluated. Finally, a drying coefficient is defined which can be determined based on measured drying data. The correlation of this coefficient with the water absorption and the vapour diffusion coefficient is analyzed and its additional information content is critically challenged. As result, a drying coefficient has been derived and defined as a new and independent material parameter. It contains information about the moisture transport properties throughout the wide range of moisture contents from hygroscopic up to saturation. With this new and valuable coefficient, it is now possible to distinguish and select building materials quickly and easily by means of their drying behaviour. This is particularly important for moisture sensitive materials.     

非饱和土孔隙气、水、汽、热耦合运动之模拟(英文)

旨在介绍一个描述非饱和土孔隙气、水、汽、热耦合运动的理论模型。该模型假定孔隙气和孔隙水运动分别遵循达西定律 ,而影响水蒸汽运动的两种主要因素分别是分子扩散和孔隙气运动 ,其中受分子扩散影响的孔隙水蒸汽运动可用Fick定律描述。热转移则主要包括了三种形式 ,即传导、对流和汽化潜热。根据有限单元法 ,编制了一个三维的计算机程序用以模拟非饱和土孔隙气、水、汽、热的耦合运动。通过数值分析与干沙试验结果之比较 ,验证了文中之理论模型和计算机程序的可靠性     

调湿材料的研究

调湿材料是能自动调节室内湿度的一种新型多功能材料.它是通过对空气中水分的吸附与解吸来调节密闭空间内的湿度;因此,调湿材料首先是亲水性材料.而吸附与解吸又与材料内部的孔的体积含量、孔的分布及孔的形状等有关.天然沸石具有多孔微晶结构、内表面积大,具有优良的吸湿与排湿的功能.本课题研制了以天然沸石为基材的调湿材料,以找出调温材料的合理组成.     

The building volume with hygroscopic materials—an analytical study of a classical building physics problem

A global analytical solution covering all cases of a building volume with hygroscopic materials is given. The mathematical and physical simplifications and assumptions are quite modest. Isothermality is not assumed. Examples are rooms, attics, subfloor spaces and building cavities. All share the same physics describing the vapour pressure in the building volume and the moisture content in the hygroscopic materials as a function of building volume temperature and moisture emission rates, external vapour pressure and building volume ventilation levels, heat and mass transfer between the building volume and the hygroscopic materials, and heat and mass storage and transfer within the hygroscopic materials.     

水泥基材料热-湿-碳化耦合模型数值分析及试验研究

基于水泥基材料热湿耦合模型和水泥基材料碳化机理,结合水泥水化模型,得了到碳化初始状态下可碳化物质的含量、孔隙率、饱和度等参数,建立了水泥基材料热-湿-碳化耦合模型.该模型通过控制干空气、液态水及气态水的质量守恒关系,气、液态相二氧化碳和钙离子质量和能量守恒关系,结合水泥基材料的本构方程,建立了碳化数学模型的控制方程组.运用COMSOL多场耦合软件求解分析,对比验证了不同水灰比、不同碳化龄期下净浆碳化试验结果与数值计算结果,证明该模型是合理可靠的.     

毛细管辐射空调的湿环境控制与防结露措施

毛细管辐射空调系统具有优良的热舒适性能、无室内噪声影响、高效节能等特点,但受系统庞大复杂、造价较高、夏季易结露等因素影响,限制了辐射空调在夏热冬冷及高湿度地区进一步推广应用。在毛细管辐射空调系统小型化和防止结露的探索实践中,采取温湿度独立控制及新风冷凝除湿方式能使结构简化,加大在别墅及高档住宅中应用的灵活性,采取新风除湿和被动式吸湿材料共同承担室内湿负荷的技术方案,以及减少维护结构水蒸汽渗透量和利用"空气湖"效应,有助于避免维护结构表面结露现象。     

Moisture buffering capacity of hygroscopic building materials: Experimental facilities and energy impact

Research into dynamic moisture storage in hygroscopic building materials has renewed interest in the moisture buffering capacity of building materials and shown the potential for these materials to improve indoor humidity, thermal comfort and indoor air quality in buildings. This paper complements previous research by estimating the effect of hygroscopic materials on energy consumptions in buildings. The results show that it may be possible to reduce heating and cooling energy consumption by up to 5% and 30%, respectively, when applying hygroscopic materials with well-controlled HVAC systems. The paper also describes two different experimental facilities that can be used to measure accurately the moisture buffering capacity of hygroscopic building materials. These facilities provide different convective transfer coefficients between the hygroscopic material and ambient air, ranging from natural convection in small, sealed jars to fully developed laminar and turbulent forced convection. The paper presents a numerical model and property data for spruce plywood which will be used in a companion paper [O.F. Osanyintola, P. Talukdar, C.J. Simonson, Effect of initial conditions, boundary conditions and thickness on the moisture buffering capacity of spruce plywood, Energy and Buildings (2006), doi:10.1016/j.enbuild.2006.03.024.] to provide additional insight into the design of an experiment to measure the moisture buffering capacity of hygroscopic materials.     

Simulation of coupled heat and moisture transfer in air-conditioned buildings

The simultaneous heat and moisture transfer in the building envelope has an important influence on the indoor environment and the overall performance of buildings. In this paper, a model for predicting whole building heat and moisture transfer was presented. Both heat and moisture transfer in the building envelope and indoor air were simultaneously considered; their interactions were modeled. The coupled model takes into account most of the main hygrothermal effects in buildings. The coupled system model was implemented in MATLAB-Simulink, and validated by using a series of published testing tools. The new program was applied to investigate the moisture transfer effect on indoor air humidity and building energy consumption under different climates. The results show that the use of more detailed simulation routines can result in improvements to the building's design for energy optimisation through the choice of proper hygroscopic materials, which would not be indicated by simpler calculation techniques.     

调湿建材调节室内湿度的可行性分析

探讨了调湿建材作为一种室内湿度调节方式的可行性，计算了北京地区气候条件下某办公室全年的湿负荷，估算了所需调湿建材的蓄湿量，介绍了一种以高分子树脂凝胶为主要材料的调湿建材。     

Transient analysis of the thermal and moisture physical behaviour of building constructions

For the transient analysis of the thermal and moisture conditions in multilayer constructions a numerical algorithm and a computer program based on the Crank-Nicholson method and quasi-linearization are are formulated. Constitutive equations for simultaneous heat and mass transfer in porous material are derived from the integrated mass, momentum and energy balance equations using the volume averaging technique. The temperature and moisture content are used as transport potentials due to general practice. In energy balance equations the conduction, convection and accumulation of heat and heat sources due to interstitial phase changes are considered. In moisture balance equations the accumulation of moisture, the diffusion flow of water vapour, the capillary and surface diffusion flow of liquid water and the viscous flow of humid air and water are considered. The boundary layer and interfacial balance equations are derived, too. The accuracy of the numerical algorithm is compared with an analytical solution for thermally semi-infinite body. The validity of the simulation method is verified by two experiments.     

有机、无机内养护剂在混凝土中的应用研究

混凝土内养护剂为在混凝土绝湿、绝热条件下在其内部缓慢释放水分的储水材料,维持混凝土内部相对湿度,有效缓解混凝土的收缩。主要介绍了混凝土内养护剂的分类及其在混凝土中应用。内养护剂主要分为无机内养护剂(轻集料、多孔超细粉末)、有机内养护剂(高吸水性树脂)和有机-无机复合内养护剂,总结其影响吸水性能的主要因素,包括它们的组成及结构特征。此外,还系统讨论了不同材料内养护剂对混凝土自收缩、干燥收缩及强度的影响。最后,对内养护剂研究中还存在的问题和将来一些可能的研究方向进行了总结和展望。     

Modeling the effect of hygroscopic curtains on relative humidity for spaces air conditioned by DX split air conditioning system

The use of hygroscopic materials for moisture buffering is a passive way to moderate the variation of indoor humidity. Through absorption and desorption, surface materials in the indoor environment, such as curtains, carpets and wall paper, are able to dampen the moisture variations. The moisture buffering capacity of these materials may be used to improve the relative humidity of the indoor environment at reduced energy costs.The objectives of this paper are threefold. The first objective is to derive a theoretical model for the transient moisture transfer between a curtain system and the indoor air for the case where the curtain is placed in front of a wall. The second objective is to conduct experiments inside environmental chambers to validate the theoretical model and to test the ability of curtains to moderate indoor humidity. It is shown that the experimental results for the curtain moisture uptake and the relative humidity inside the chamber compared well with the model simulation results. The third and final objective is to test and evaluate the model under “real environment conditions” for a case study of a hygroscopic cotton curtain, placed in a “typical” office space in the city of Beirut with an area of 25 m² that uses direct expansion (DX) air conditioning system. It is found that hygroscopic curtains maintain humidity of less than 65% during part load operation compared to the upper limit of 70% relative humidity when no curtain is used. On the other hand, it is found that the energy use, as determined by the daily electrical power consumption of the DX system, is almost the same for the two cases, (with and without a curtain), where approximately 20 kWh of energy input is required 13 kWh of sensible energy and 7 kWh of latent energy.     

炭化工艺参数对竹炭调湿性能的影响

以平衡吸放湿量、吸放湿效率和调湿稳定性来评价竹炭调湿性能,研究了炭化温度、升温速率以及保温时间这3个炭化工艺参数与竹炭调湿性能之间的关系.结果表明:较低的炭化温度和较长的保温时间有利于竹炭平衡吸放湿量的提升,在较低的升温速率下竹炭的平衡吸放湿量较高;炭化工艺参数对竹炭的吸放湿效率影响不大;在升温速率低、保温时间短的情况下竹炭的调湿稳定性更好.