Estimation of moisture transport coefficients in porous materials using experimental drying kinetics

From experimental drying kinetics, an inverse technique is used to evaluate the moisture transport coefficients in building hygroscopic porous materials. Based on the macroscopic approach developed by Whitaker, a one-dimensional mathematical model is developed to predict heat and mass transfers in porous material. The parameters identification is made by the minimisation of the square deviation between numerical and experimental values of the surface temperature and the average moisture content. Two parameters of an exponential function describing the liquid phase transfer and one parameter relative to the diffusion of the vapour phase are identified. To ensure the feasibility of the estimation method, it is initially validated with cellular concrete and applied to lime paste.     

Modeling of heat and mass transfer in lateritic building envelopes

The aim of the present work is to investigate the behavior of building envelopes made of local lateritic soil bricks subjected to different climatic conditions. The building envelopes studied in this work consist of lateritic soil bricks with incorporation of natural pozzolan or sawdust in order to obtain small thermal conductivity and low-density materials. In order to describe coupled heat and moisture transfer in wet porous materials, the coupled equations were solved by the introduction of diffusion coefficients. A numerical model HMtrans, developed for prediction of heat and moisture transfer in multi-layered building components, was used to simulate the temperature, water content and relative humidity profiles within the building envelopes. The results allow the prediction of the duration of the exposed building walls to the local weather conditions. They show that the durability of building envelopes made of lateritic soil bricks with incorporation of natural pozzolan or sawdust is not strongly affected by the climatic conditions in tropical and equatorial areas.     

多孔介质在压力梯度作用下的热质耦合数值模拟

多孔介质干燥导致热质耦合传输过程。本文基于连续介质力学的宏观尺度,对多孔介质的热、湿和气三者耦合迁移进行数值模拟,研究压力梯度对热质传输的影响。多孔介质传质机理主要为水汽和空气的对流和扩散传输、吸附水在含湿量梯度作用下的自由扩散和其在温度梯度即Soret效应驱动下的流动。采用Galerkin加权余量的有限元方法,提出了...     

Numerical simulation of coupled heat and mass transfer in wood dried at high temperature

The mutual effect between heat and mass transfer is investigated for wood dried at high temperature. A numerical model of coupled heat and mass transfer under the effect of the pressure gradient is presented. Based on the macroscopic viewpoint of continuum mechanics, the mathematical model with three independent variables (temperature, moisture content and gas pressure) is constructed. Mass transfer in the pores involves a diffusional flow driven by the gradient of moisture content, convectional flow of gaseous mixture governed by the gradient of gas pressure, the Soret effect and phase change of water. Energy gain or loss due to phase change of water is taken as the heat source. Numerical methods, the finite element method and the finite difference method are used to discretize the spatial and time dimension, respectively. A direct iteration method to solve the nonlinear problem without direct evaluation of the tangential matrix is introduced. The local convergence condition based on the contraction–mapping principle is discussed. The mathematical model is applied to a 3-D wood board dried at high temperature with the Neumann boundary conditions for both temperature and moisture content, and the Dirichlet boundary conditions for gas pressure.     

Modeling heat and moisture transport in firefighter protective clothing during flash fire exposure

In this paper, a model of heat and moisture transport in firefighter protective clothing during a flash fire exposure is presented. The aim of this study is to investigate the effect of coupled heat and moisture transport on the protective performance of the garment. Computational results show the distribution of temperature and moisture content in the fabric during the exposure to the flash fire as well as during the cool-down period. Moreover, the duration of the exposure during which the garment protects the firefighter from getting second and third degree burns from the flash fire exposure is numerically predicted. A complete model for the fire-fabric-air gap-skin system is presented.     

Solving 2-D nonlinear coupled heat and moisture transfer problems via a self-adaptive precise algorithm in time domain

Introduction Thestudyonnonlineartransienttransferproblemsissignificantpracticallyand theoretically[1,2].Insolvingtheseproblemsdiscretelyinthetimedomain,eitherbyiterative techniques,orbylinearizingapproachesbasedonsomeadditionalassumptions,the adaptabilityofcomputingaccuracytothechangeofthesizeoftimestepneedtobetakeninto account[3].Yang[3]presentedaprecisealgorithminthetimedomaintosolvetransfer problems,themajoradvantagesofthisalgorithmtosolvenonlinearproblemsisthatno additionalassumptionandite…     

大气压力对织物热湿特性影响的数值研究

为了模拟多孔织物内复杂的热湿传递过程，为不同大气压力条件下服装的热湿舒适性设计提供理论基础，从织物内热湿传输机理角度出发，建立了考虑大气压力影响的织物热湿传输耦合模型，比较了常压下的理论预测和实验结果，通过数值算例考察了大气压力对织物热湿特性的影响。     

建筑结构中的湿迁移

湿迁移; 多孔建筑材料; 内部毛细冷凝;      

Heat and moisture transfer in gaps between sweating imitation skin and nonwoven cloth: effect of gap space and alignment of skin and clothing on the moisture transfer

This study investigates heat and moisture transfer between a sweating film and a nonwoven sheet both experimentally and numerically. A mathematical model based on heat conduction and moisture diffusion in both the air gap and cloth is presented. The evaporation rate and surface temperature of the sweating film are well predicted under various conditions such as air gap height, heating conditions, and sweating film orientation by evaluating the effective thermal conductivity and diffusion coefficient from the empirical equations of the Nusselt number for a fluid layer, even though the air gap height is sufficiently large to cause natural convections.     

Elasto-plastic approach for paper cockling phenomenon: On the importance of moisture gradient

Cockling of paper is a common problem occurring in the production, storage and end-use of paper. It is usually induced by a moisture content change. In many cases, cockling is an irreversible phenomenon; i.e. the initial shape is not obtained although the initial moisture content is restored. This kind of moisture content change occurs in copying machines and in the printing process, for example. In this paper, we present a continuum mechanical model, which is used to study the irreversible cockling of paper. In the model, paper is treated as an orthotropic elasto-plastic material and the model takes into account the small-scale variation of fibre orientation. The model is used to show the importance of the through-thickness moisture gradient on the cockling phenomenon during a cyclic moisture content change. The results suggest that the moisture gradient is a crucial factor for the irreversible cockling.     

Self-similar solution of a problem of freezing of finely dispersed soils with allowance for moisture migration in thawed and frozen zones

Heat transfer in freezing and thawing soils is accompanied by various processes among which phase transition of moisture and mass transfer should be distinguished in both the thawed and the frozen zones. Their consequence is the formation of ice schlieren and the swelling associated with this. In developing the methods of calculation of moisture migration it was assumed (see, for example, [1, 2]) that the mass transfer occurs only in the thawed part of the soil and is realized predominantly in the liquid phase through the diffusion-film mechanism. It was assumed that the phase conversion of water into ice occurs wholly on the phase interface (the Stefan formulation) and at the same time supplementary conditions for the moisture function are specified on it. Not all these assumptions are valid. In particular, the marked redistribution of moisture in the frozen zone is an important factor in the freezing of moist rocks [3, 4]. This last is also observed in the thawing of dispersed rocks and in frozen samples which are under the influence of a temperature gradient. These phenomena were modeled in [5, 7] on the basis of a single mathematical model which describes the conductive heat transfer, the moisture transfer in thawed and frozen zones, the phase transition of moisture in the temperature range, and the kinetic relaxation effects of moisture crystallization and ice melting. Analysis of the solutions obtained by means of a finite difference method showed that the proposed method of calculation gives results near the experiment. The present paper is devoted to a further study of the model indicated.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 2, pp. 113–120, March–April, 1986.     

Hysteretic Behaviour and Moisture Buffering of Hemp Concrete

Moisture transfer in hygroscopic building materials affects the indoor air quality by exchanging moisture and buffering the ambient relative humidity variations. The paper deals with experimental and numerical study on hysteretic sorption behaviour of the hemp concrete sorption process. Experimental intermediate scanning curves of hemp concrete are measured and used to compare two hysteresis models, Huang’s model and Carmeliet’s model. An original method is achieved to fit the numerical results on the experimental ones leading to the identification of the main desorption curve. The most relevant model, Huang’s model, is implemented in a heat and moisture transfer model based on Künzel formalism. The transient hydric response of hemp concrete submitted to cyclic hydric loadings is investigated and compared to experimental results issued from the literature. These investigations show the relevance to consider the hysteresis phenomenon into the model. Then, the influence of initial conditions is discussed. The results point out that transient response of hemp concrete strongly depends on the initial hydric state (initial moisture content as well as initial relative humidity).     

Numerical simulation of coupled heat-fluid transport in freezing soils using finite volume method

A solution procedure using finite volume method has been established for the coupled heat-fluid transport model of freezing soils, and details about determination of the time step interval and discretization at special nodes have been introduced. Comparison between the simulation and the freezing experiments of silica flour and Zhangye loam has been conducted, and the calculated results are in general agreement with the experimental data. The research indicates that the moisture migration in the frozen zone is insignificant, and water mainly migrates from the frozen zone to the vicinity of the moving freezing front; the moving velocity of the freezing front has a great effect on the extent of moisture accumulation to the freezing front, and high extent of accumulation occurs when the freezing front advances slowly. Finally, an apparent heat capacity model has been suggested for the temperature calculation of the soil freezing process in low water content conditions; however, when the moisture migration is significant, water redistribution during the freezing process should be considered.     

On the Solution of Coupled Heat and Moisture Transport in Porous Material

Comparisons of experimental observation of heat and moisture transfer through porous building materials with numerical results have been presented in numerous studies reported in the literature. However, some discrepancies have been observed, highlighting underestimation of sorption process and overestimation of desorption process. Some studies intend to explain the discrepancies by analyzing the importance of hysteresis effects as well as carrying out sensitivity analyses on the input parameters as convective transfer coefficients. This article intends to investigate the accuracy and efficiency of the coupled solution by adding advective transfer of both heat and moisture in the physical model. In addition, the efficient Scharfetter and Gummel numerical scheme is proposed to solve the system of advection–diffusion equations, which has the advantages of being well-balanced and asymptotically preserving. Moreover, the scheme is particularly efficient in terms of accuracy and reduction of computational time when using large spatial discretization parameters. Several linear and nonlinear cases are studied to validate the method and highlight its specific features. At the end, an experimental benchmark from the literature is considered. The numerical results are compared to the experimental data for a pure diffusive model and also for the proposed model. The latter presents better agreement with the experimental data. The influence of the hysteresis effects on the moisture capacity is also studied, by adding a third differential equation.     

The Use of Chemical Potential to Describe Water Transfer in Complex Media with Strong Solid–Liquid Bonding

We consider a complex medium composed of finely intertwined micro-skeletons and micro-compartments where water transfer can occur. In these media, at low moisture content, water pressure measurement is not longer possible. Mass transfer is then expressed in terms of chemical potential gradient. The assumption of local thermodynamic equilibrium, resulting in the uniformity of water chemical potential in all microstructures, is essential to define a sorption isotherm reflecting the relationship between water activity and average moisture content. In this case, it is also possible to describe water transfer by using the chemical potential gradient. Radial water transfer in wood is examined using a destructive method for calculating water flux and chemical potential gradient at the same position and at the same time. We deduce the variation of transport coefficient as a function of moisture content.     

Application of hybrid and moment methods to the measurement of moisture diffusion coefficients of building materials

This work proposes two simple dynamic methods that provide an accurate method for measurement of diffusion coefficients in building materials. Experimental measurements of moisture diffusion coefficients covered three commonly used building materials and they were carried out for a range of the relevant parameters, as temperature and relative humidity. The diffusion coefficients obtained by the two dynamic methods show a deviation comparatively to the steady-sate cup method; however, this variance is in accordance with the results presented in literature.     

Modeling of vacuum desorption of multicomponent moisture in freeze drying

A mathematical model of multicomponent vacuum desorption, which occurs in the vacuum freeze drying process has been developed. Drying with conductive heating and constant contact surface temperature was considered. Pressure drop in the layer of the material to be dried was taken into account in the model formulation and process simulation. Equilibrium moisture content for pure water, toluene, and m-xylene and their two- and three-component mixtures on zeolite DAY 20F were described by means of the multitemperature extended Langmuir isotherm equation. Model equations were solved by the numerical method of lines. Moisture content and temperature distributions within the drying material were predicted from the model as a function of drying time.     

A parametric analysis of moisture migration in an unsaturated porous slab caused by convective heat and mass transfer

An analysis is performed which describes the approach to a quasisteady state of heat and moisture migration in an unsaturated porous slab where one surface is impermeable to heat and mass transfer whereas convective heat and mass transfer occur at the other surface. The initial temperature and moisture content are uniform. The surface atx=0 is suddenly exposed to a gas stream at different temperature and with relative humidity. The conservation equations describing the heat and moisture migration are developed and then simplified with the assumption that the properties can be considered to be constant. The difference between the initial and the wet bulb temperature is used to make the equations dimensionless. In this way, the dimensionless temperature profiles are a function of only one parameter - a modified Biot number, and the dimensionless moisture profiles are functions of four parameters. The numerical results are presented in the form of temperature and moisture profiles as well as temperature and moisture gradients at the surface as a function of those parameters. The heat and moisture transfer at the surface as well as the time for the approach to the quasisteady state can be obtained from these results.

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Eine parametrische Analyse der Feuchtigkeitswanderung in einer nichtsaturierten porösen Wand erzeugt durch konvektiven Wärme- und Stoffübergang

Zusammenfassung Eine analytische Untersuchung beschreibt den Übergang vom Anfangszum quasistationären Endzustand (im ersten Trocknungsabschnitt) der Wärme- und Stoffwanderung in einer ungesättigten porösen Wand, deren eine Oberfläche für Wärme und Stoff undurchlässig ist, während an der anderen Oberfläche konvektiver Wärme- und Stoffaustausch herrscht. Die Temperatur und der Feuchtigkeitsgehalt in der Wand sind ursprünglich konstant. Die Oberfläche beix = 0 ist plötzlich einem Gasstrom von verschiedener Temperatur und einer relativen Feuchtigkeit ausgesetzt.Die Erhaltungsgleichungen, die die Wärme- und Feuchtigkeitswanderung beschreiben, werden dadurch vereinfacht, daß die Stoffwerte als konstant angesehen werden. Der Unterschied zwischen der ursprünglichen und der Kühlgrenztemperatur wird benutzt um die Gleichungen dimensionslos zu machen. Die dimensionslosen Temperaturprofile sind dann eine Funktion eines einzigen Parameters - einer zweckmäßig definierten Biotzahl. Die Feuchtigkeitsprofile sind durch vier Parameter bestimmt. Numerische Ergebnisse sind als Profile und als Gradienten der Temperatur und der Feuchtigkeit an der Oberflächex = 0 dargestellt. Der Wärme- und Feuchtigkeitsübergang an der Oberfläche sowie die zur Erreichung des quasistationären Endzustandes benötigte Zeit können damit bestimmt werden.

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Nomenclature Bi ^* modified Biot number, Eq. (30) - c _c composite specific heat (solid + fluid) - c _pa specific heat of air at constant pressure - C _t defined by Eq. (15) - D ^* moisture diffusion coefficient due to temperature gradient - D _c vapor diffusion coefficient through porous medium - h convective heat transfer coefficient - h _m convective mass transfer coefficient - i enthalpy - i _vl heat of evaporation - j diffusive mean flux - k thermal conductivity - k _c composite thermal conductivity (including vapor diffusion) - K moisture diffusion coefficient due to moisture content gradient - K moisture diffusion coefficient due to suction potential gradient - m _a mass flux into air space - P_l,P ₂ dimensionless parameters, Eqs. (46), (47) - q heat flux - t temperature - t _wb wet bulb temperature - w vapor mass fraction (mass of vapor/mass of air) - W moisture content (mass of moisture/mass; of dry medium) - x coordinate Greek symbols _c composite thermal diffusivity (including vapor diffusion) - suction potential - relative humidity - density - time Indices a air - c composite (solid + fluid) - d dry - i initial - l liquid - matrix potential - 0 atx = 0 - s saturated - v vapor