A building corner model for hygrothermal performance and mould growth risk analyses

Combined multidimensional analysis of heat, air and moisture transport through porous building elements is barely explored in the literature due to many difficulties such as modeling complexity, computer run time, numerical convergence and highly moisture-dependent properties. In this way, a mathematical model considering a combined two-dimensional heat, air and moisture transport through unsaturated building upper corners is presented. In order to improve the discretized model numerical stability, the algebraic equations are simultaneously solved for the three driving potentials: temperature, vapor pressure and moist air pressure gradients. In the results section, the convective effects caused by air stagnation are analyzed in terms of heat flux and mould growth risk for different boundary conditions, showing the importance of a detailed hygrothermal analysis – which is normally disregarded by simulation tools – for accurately predicting building energy consumption, indoor air quality, thermal comfort or mould growth risk.     

Heat and moisture transfer in fibrous thermal insulation with tight boundaries and a dynamical boundary temperature

Prefabricated, lightweight building elements are widely used in the building construction sector. Such elements consist of fibrous thermal insulation encapsulated between two metal sheets. Under various circumstances, moisture can appear in the insulation matrix. Since the temperature of the boundary metal sheets changes dynamically with meteorological conditions, heat and mass transfer between boundaries appear in this case. This paper presents a transient model of the heat and mass transfer, including the sorption and condensation processes. A numerical model considers the dynamical changing of the boundary temperatures. A parametric study considering different amplitudes of temperature change, different moisture masses and different thicknesses of the insulation matrix was made. It was found that a relatively small mass of water in the insulation matrix can result in a significantly increased average heat flux during a periodic cycle. The numerical code was verified with experiments, which showed good agreement with the numerics.     

An experimental data set for benchmarking 1-D,transient heat and moisture transfer models of hygroscopic building materials. Part I: Experimental facility and material property data

As numerical models of heat and moisture transfer in porous building materials advance and numerical investigations increase in the literature, there remains a need for simple accurate and well-documented experimental data for model validation. The aim of this two part paper is to provide such experimental data for two hygroscopic building materials (cellulose insulation and spruce plywood) exposed to 1-D and transient boundary conditions. Part I of this paper describes the transient moisture transfer (TMT) facility used to generate the experimental data as well as the uncertainty and repeatability of the measured data. The measured material properties are also presented to fully document the experimental data set and permit its use by other researchers.     

垂直U型埋管换热器准三维热渗耦合模型及其实验验证

为了探讨热渗耦合对土壤源热泵地埋管换热的影响,基于多孔介质传热传质及地下水渗流理论,通过耦合竖直方向一维流体模型与水平面内土壤二维非稳态热渗耦合模型,建立了考虑地下水渗流影响的准三维U型埋管热渗耦合模型。基于模型的数值求解,探讨了土壤热物性及地下水渗流对埋管换热特性的影响。结果显示:土壤导热系数与比热容的增大均有利于加强埋管的换热,且地下水渗流的存在有利于强化地下埋管与周围土壤间的换热能力,提高土壤源热泵系统的运行效率。实验验证表明,所建模型具有一定的预测精度,可为地下埋管换热器传热特性的研究提供理论基础。     

Transient model for coupled heat,air and moisture transfer through multilayered porous media

Most building materials are porous, composed of solid matrix and pores. The time varying indoor and outdoor climatic conditions result heat, air and moisture (HAM) transfer across building enclosures. In this paper, a transient model that solves the coupled heat, air and moisture transfer through multilayered porous media is developed and benchmarked using internationally published analytical, numerical and experimental test cases. The good agreements obtained with the respective test cases suggest that the model can be used to assess the hygrothermal performance of building envelope components as well as to simulate the dynamic moisture absorption and release of moisture buffering materials.     

建筑墙体热、湿及空气耦合传递

为了研究建筑墙体的热质耦合传递规律，该文考虑了太阳辐射的影响，建立了在第三类边界条件下建筑墙体非稳态热、湿及空气渗透耦合传递的物理及数学模型，开发了相应的数值模拟计算软件。给出了墙体热、湿及空气耦合传递时各部位模拟计算温度随时间的变化曲线及不同使用年限湿容量沿墙体厚度的分布规律，并将墙体的模拟计算结果与现场实测热流及温度进行了对比分析。     

Application of numerical methods for predicting energy transport in earth contact systems

Methods for analysing conductive heat flow with applications to underground earth contact systems are reviewed. A discussion and comparison of both the finite difference and finite element methods are presented. The effect of domain discretisation on accuracy for both methods is presented. One- and two-dimensional models are derived and used to solve selected problems. The results for various discretisation domains are compared and constrasted. The application of both the finite difference and finite element approaches to the analysis of heat transfers in an underground building is given. Recommendations are made to aid in the selection of the numerical techniques that are the most appropriate for analysing conductive heat flow problems. Finally, a statement is made on the analysis of the three-dimensional heat and moisture transport problem associated with underground earth contact systems.     

Two-dimensional heat and moisture transfer analysis of a cylindrical moist object subjected to drying: A finite-difference approach

In this paper a two-dimensional numerical analysis of heat and moisture transfer during drying of a cylindrical object is presented. Drying is a process of simultaneous heat and moisture transfer whereby moisture is vaporized by means of a drying fluid (e.g., air), as it passes over a moist object. The two-dimensional analysis of heat and moisture transfer during drying of a cylindrical object is carried out using an explicit finite-difference approach. Temperature and moisture distributions inside the moist objects are obtained for different time periods and the results predicted from the present analysis are compared with two sets of experimental data available in the literature. A considerably high agreement is found between the predicted and measured values.     

Heat,air and moisture transfer through hollow porous blocks

The combined heat, air and moisture transfer in building hollow elements is of paramount importance in the construction area for accurate energy consumption prediction, thermal comfort evaluation, moisture growth risk assessment and material deterioration analysis. In this way, a mathematical model considering the combined two-dimensional heat, air and moisture transport through unsaturated building hollow bricks is presented. In the brick porous domain, the differential governing equations are based on driving potentials of temperature, moist air pressure and water vapor pressure gradients, while, in the air domain, a lumped approach is considered for modeling the heat and mass transfer through the brick cavity. The discretized algebraic equations are solved using the MTDMA (MultiTriDiagonal-Matrix Algorithm) for the three driving potentials. Comparisons in terms of heat and vapor fluxes at the internal boundary are presented for hollow, massive and insulating brick blocks. Despite most of building energy simulation codes disregard the moisture effect and the transport multidimensional nature, results show those hypotheses may cause great discrepancy on the prediction of hygrothermal building performance.     

A 3D transient nonlinear modelling of coupled heat,mass and deformation fields in anisotropic material

This paper deals with the numerical solution of a three-dimensional problem of non-isothermal moisture transfer in the anisotropic structure of wood and corresponding wood deformations in convective drying.The simulation is based on the unsteady-state nonlinear diffusion of moisture and heat with respect to the orthotropic nature of wood. The moisture gradient and the temperature gradient are set as driving forces for mass and heat transfer. The model respects the dependence of material coefficients on temperature and moisture, the Soret effect, the Duffour effect and the anisotropic nature of wood. A matrix form of the multiphysics model for FEM solver was derived and numerical simulations were performed.     

NUMERICAL SIMULATION OF TWO-DIMENSIONAL HEAT AND MOISTURE TRANSFER DURING DRYING OF A RECTANGULAR OBJECT

The present article deals with the numerical modeling of heat and moisture transfer during the drying process of a two-dimensional (2-D) rectangular object subjected to convective boundary conditions. As is common in solids drying, it is assumed that drying takes place as a simultaneous heat and moisture transfer whereby moisture is vaporized by means of a drying fluid (e.g., air), which passes over a moist object. The governing equations representing the drying process in a 2-D rectangular object are discretized using an explicit finite-difference approach, and a computer code is developed to predict the temperature and moisture distributions inside the object. Moreover, the results obtained from the present model are compared with the experimental data available in the literature, and considerably high agreement is found.     

An analytical method to calculate the coupled heat and moisture transfer in building materials

A dynamic model for evaluating the transient thermal and moisture transfer behavior in porous building materials was presented. Both heat and moisture transfer were simultaneously considered and their interactions were modeled. An analytical method has been proposed to calculate the coupled heat and moisture transfer process in building materials. The coupled system was first subjected to Laplace transformation, and then the equations were solved by introducing the Transfer Function Method. The transient temperature and moisture content distribution across the material can thus be easily obtained form the solutions. The results were compared with the experimental data and other analytical solutions available in the literature; a good agreement was obtained.     

Two-Dimensional Numerical Analysis of Membrane-Based Heat and Mass Cross-Flow Exchanger

ABSTRACT

A two-dimensional numerical simulation model for a membrane-based heat and mass exchanger was developed. The system model equations were used to determine the coupled heat and moisture transfer from the humid air to the high concentrated liquid desiccant solution (LiCl, lithium chloride) by means of a parallel stack hydrophobic permeable membrane. The two streams of air and liquid desiccant solution were arranged in cross-flow directions. The fourth-order Runge–Kutta method was employed to solve these system model equations in a steady-state condition. This model enables one to predict the latent effectiveness of a membrane-based parallel cross-flow exchanger for dehumidification purpose in response to air to liquid mass flow ratio and the mass transfer unit number.     

Experimental and numerical validation of hygrothermal transfer in brick wall

In this paper, a mathematical model dealing with a coupled heat, air, and moisture transfer in a building envelope was developed. Based on the three-following driving potential: vapor pressure, dry air pressure, and temperature, an application on a hygrothermal behavior of a real wall was carried out for different climatic conditions. For this purpose, a characterization of the heat and moisture properties of the materials constituting the wall made with red brick and cement mortar was carried out in the laboratory. This was used to evaluate experimentally the input parameters of the model as a function of relative humidity. To validate the numerical model, an experimental platform was improved. The wall was set up in a double-climatic chamber with different boundary conditions, and then the temperature and humidity evolutions were recorded using several sensors within the wall thickness. The results have highlighted a good agreement between numerical simulation results and experimental ones.     

Three dimensional numerical modeling of simultaneous heat and moisture transfer in a moist object subjected to convective drying

The drying behavior of a moist object subjected to convective drying is analyzed numerically by solving heat and moisture transfer equations. A 3-D numerical model is developed for the prediction of transient temperature and moisture distribution in a rectangular shaped moist object during the convective drying process. The heat transfer coefficients at the surfaces of the moist object are calculated with an in-house computational fluid dynamics (CFD) code. The mass transfer coefficients are then obtained from the analogy between the thermal and concentration boundary layer. Both these transfer coefficients are used for the convective boundary conditions while solving the simultaneous heat and mass transfer governing equations for the moist object. The finite volume method (FVM) with fully implicit scheme is used for discretization of the transient heat and moisture transfer governing equations. The coupling between the CFD and simultaneous heat and moisture transfer model is assumed to be one way. The effect of velocity and temperature of the drying air on the moist object are analyzed. The optimized drying time is predicted for different air inlet velocity, temperature and moisture content. The drying rate can be increased by increasing the air flow velocity. Approximately, 40% of drying time is saved while increasing the air temperature from 313 to 353 K. The importance of the inclusion of variable surface transfer coefficients with the heat and mass transfer model is justified.     

Evaluation of the interfacial conduction heat transfer coefficient in two-temperature macroscopic models of homogenous porous media using a fully developed unsteady microscopic model in periodic unit cells

A method is proposed for the evaluation of the interfacial conduction heat transfer coefficient in two-temperature macroscopic models of homogeneous fluid-saturated porous media. It is based on the numerical solutions of a microscopic model of unsteady conduction heat transfer in periodic unit cells, with different uniform initial temperatures of the fluid and solid. A novel formulation of the microscopic model in the fully developed regime is also proposed. Results for the variation of interfacial conduction Nusselt number with porosity, fluid–solid thermal conductivity ratio, and fluid–solid thermal diffusivity ratio are presented and discussed for four two-dimensional and two three-dimensional cases.     

Integrated analysis of whole building heat,air and moisture transfer

There is a continuous dynamic heat, air and moisture (HAM) interaction between the indoor environment, building envelope and mechanical systems. In spite of these interdependences, the current indoor, building envelope and energy analysis tools are used independently. In this paper a holistic HAM model that integrates building envelope enclosures, indoor environment, HVAC systems, and indoor heat and moisture generation mechanisms, and solves simultaneously for the respective design parameters is developed. The model is benchmarked with internationally published test cases that require simultaneous prediction of indoor environmental conditions, building envelope moisture performance and energy efficiency of a building.     

Modeling heat and moisture transfer through fibrous insulation with phase change and mobile condensates

This paper reports on a transient model of coupled heat and moisture transfer through fibrous insulation, which for the first time takes into account of evaporation and mobile condensates. The model successfully explained the experimental observations of Farnworth [Tex. Res. J. 56 (1986) 653], and the numerical results of the model were found to be in good agreement with the experimental results of a drying test. Based on this model, numerical simulation was carried out to better understand the effect of various material and environmental parameters on the heat and moisture transfer. It was found that the initial water content and thickness of the fibrous insulation together with the environmental temperature are the three most important factors influencing the heat flux.     

Mathematical models based on heat transfer and coupled heat and mass transfers for rapid high temperature treatment in fluidized bed: Application for grain heat disinfestation

This study sought to develop a mathematical model of rapid, high temperature heat treatment of stored grains in a fluidized bed. The model was intended to evaluate dynamic changes in temperature distributions inside grain kernel, and grain and exit air temperatures. No differences in temperature profiles within individual paddy kernels obtained from either analytical or numerical solutions for one- and two-dimensional heat diffusion models were found. Cylindrical coordinates gave clearer pictures of temperature profiles than spherical coordinates, and the former was chosen for the model. Thin-layer heat diffusion alone is inadequate for explaining transport phenomena in a fluidized bed; it must be incorporated into a deep bed model. The loss of as little as 1.0% dry basis moisture content from the grain surface during heating significantly affected the predictiveness of the model. Therefore, a model coupling heat and mass transfer performs much better in predicting grain and exit air temperatures than one that neglects the effect of moisture loss, when compared with the experimental results. The results showed agreement between the measured and predicted results, although the predicted results tended toward over-estimation. The results indicate that the model is a powerful tool for disinfestation applications, to predict the exposure time required to obtain lethal temperatures throughout grain kernel, so ensuring the total mortality of insects within it.     

自然环境下湿分分层土壤中热湿迁移规律的研究

建立描述存在干饱和层时的土壤热湿传递的数学模型并进行数值模拟，获得自然环境下土壤中温度、湿分分布以及水分蒸发的动态特性，分析干饱和土壤层对土壤热湿迁移及水分蒸发的影响。数值模拟获得实验支持。