Moisture Absorption in Capillary Active Materials: Analytical Solution for a Multiple Step Diffusivity Function

The absorption of moisture in a homogeneous, capillary active material obeys a diffusion law in which the driving potential is the volumetric water content, and the material behavior is characterized by the diffusivity function. In this study, an analytical solution for the one-dimensional case is proposed. The transient moisture profile is found by assuming the diffusivity as a multiple step function of the local water content and by solving the corresponding free boundary problem. Finally, a method for inverse determination of the diffusivity function is proposed. The case of a diffusivity function with three steps is investigated and discussed as an example.(VLID)4690992Version of recor

Pore scale modelling of moisture transfer in building materials with the phase field method

This study explores the applicability of the phase field method for modelling moisture storage and transport in porous materials. Accordingly, the system is treated as a continuum where the phases (liquid and humid air) are separated through a diffuse interface, which evolves in the pores until the equilibrium state is reached. The interface thickness is related to the surface tension, while the contact angle is defined as a boundary condition. The mass transfer in the porous matrix is driven by the Cahn-Hilliard equation and the phase transition is controlled by an equation of state. The above method is tested for a simple geometry (infinitely extended parallel plates), by comparing the numerical outcomes against available measured data and analytical solutions. The challenges arising for a further application to complex pore structures and real building materials are discussed

Impact of the drying rate on the moisture retention curve of porous building materials

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Characterization of the diffusivity function through water-uptake tests

A method is proposed to determine the moisture diffusivity of capillary active materials by means of water-uptake tests performed with different initial water contents. The method is based on an analytical approach, in which the diffusivity is approximated as a multiple step function of the water content. Contrary to other well-established techniques, the method proposed here requires neither the knowledge of the water content distribution in the absorbing sample, nor the application of numerical simulations. Experiments are carried out on calcium silicate samples

Steady state and transient simulation of a radiant heating system

Radiant heating generally addresses all heat emission systems that have a share of radiant heat emission greater than 50 %, compared to a convector or fan coil where the heat is transferred mainly by means of convection. Recently, so-called infrared-heating systems are increasingly discussed as a cost-effective heating system. Relative small areas with high surface temperatures of typically up to 120 \ub0C are used. In order to investigate in detail radiant heating systems, building models able to reproduce accurately the occurring physics phenomena are required. Physics-detailed steady state and transient room models have been developed in Matlab\uae. The required view factors for the radiative exchange between all surfaces and between each surface and a sphere representing a person are calculated using COMSOL\uae. Moreover, the thermal comfort in different positions of the room has been evaluated. 1. Introduction The implementation of the concept of NZEB (Kurnitski et al. (2013) will lead to a further reduction of the heating demand of new buildings. Also the heating demand of the building stock will decrease by applying deep renovation. The technology to achieve very low energy demands is available since about 25 years, when the first Passive House was built in Darmstadt, Germany Feist (2016). Technology and products have been further improved since then and cost-effectiveness has been significantly improved. However, in order to improve the economic feasibility of these very efficient buildings, cost-effective heating systems are required. In parallel the share of renewable energies (such as PV or wind) in the electric grid will further increase. Both these developments make electric heating interesting again in spite of the fact that, because of thermodynamic principles, electricity should not be used for heating

Determination of the water retention curve from drying experiments using infrared thermography: A preliminary study

A method is proposed for experimental determination of the water retention curve from drying tests performed on capillary active materials. Non-destructive techniques (gravimetric analysis, infrared thermography) are employed for measuring water content, drying rate and surface temperature during time for a set of material samples. The surface relative humidity is calculated from the measured data through analytical procedure. Hence, assuming uniform water content distribution inside the samples, which applies if the mass transfer Biot number is small enough, the relation between relative humidity and water content is derived. This relation represents the water retention curve for the considered transient desorption behavior and, as shown in previous studies, it may deviate from the trend measured through steady state experiments. The proposed method differs substantially from other ones commonly employed to the same aim. It is useful for investigation of the so called dynamic effects, recently observed by other authors in the hygrothermal behavior of construction materials. For a first test, calcium silicate specimens are employed and the results are compared with those reported in the literature. An error propagation analysis is also included

Effect of evaporation cooling on drying capillary active building materials

The relevance of evaporation cooling on drying capillary active building materials is investigated through numerical simulation and non-destructive measurements. The drying rate results to be strongly related to the so-called wet bulb temperature, i.e. the temperature reached inside the sample during the early drying phase. It is shown that the faster the process occurs, the lower is the wet bulb temperature. The experiments are carried out inside a climatic chamber under controlled atmospheric conditions (temperature and relative humidity), using calcium silicate samples. The drying rates are determined by weighting the samples during time, while the surface temperature is measured via infrared thermography. A mathematical model describing transient heat and moisture transfer is implemented with the software COMSOL for 3D-simulation, and afterward validated by comparison with the measured data. The numerical solution presents a satisfactory agreement with the experimental results. A sensitivity analysis is also performed for different input parameters including convective heat transfer coefficient and uncertainties in material properties. The validated model is then used for simulation of a set of drying cases by varying the sample thickness and boundary conditions. Hence, the water content distribution inside the samples is investigated by determining boundary conditions and sample dimensions, in which nearly uniform water content can be obtained. In fact, uniform distribution is a prerequisite for an experimental method, recently studied by the authors, that aims at determining the water retention curve of capillary active materials by means of drying tests