Hygroscopic Porous Materials Research Articles

Investigation of coupled vapor and heat transport in hygroscopic material during adsorption and desorption

Vapor sorption in hygroscopic porous materials is accompanied by latent heat release/storage, which can influence indoor thermal comfort and building heating and cooling energy consumption. There is a need to better understand the coupled vapor and heat transport during adsorption and desorption. In this study, longitudinal spruce samples are exposed to adsorption and desorption experiments. Neutron radiography provides accurate measurement of moisture content variations spatially and temporally. Wireless thermocouples provide accurate measurements of temperature at different locations. Large changes in moisture content and temperature are observed during both adsorption and desorption experiments. Both moisture content and temperature variations seen in experiments are well simulated with hygrothermal modeling. The latent heat associated with vapor sorption is found to be the source of the large variations in temperature. It is found that vapor permeability influences both vapor and thermal transport while thermal conductivity influences only thermal transport. The vapor transfer coefficient has a small influence on vapor transport while the convective heat transfer coefficient has an influence on heat transport. The validated hygrothermal model is further used to simulate the coupled vapor and heat transport occurring in moisture buffering tests. It is found that moisture buffering values are different by up to 14% depending on the presence or absence of thermal insulation around the samples. For more hygroscopic materials, the difference can be even much larger. It is recommended not only to seal and but also to insulate samples for moisture buffering tests.

Study of hygroscopic behaviour of spruce during adsorption and desorption

Sorption of water vapour in hygroscopic porous materials is associated with latent heat release and absorption. This phenomenon should be taken into account to achieve a better understanding of the coupled transfer of vapour and heat in hygroscopic porous materials. In this paper, water vapour adsorption and desorption in the longitudinal direction of spruce samples are studied. Neutron radiography is used to measure changes in moisture content and wireless thermocouples are used to measure temperature changes. During the adsorption and desorption experiments, large changes in moisture content and temperature are observed. A hygrothermal model is developed to simulate vapour and heat transfer during adsorption and desorption experiments. Generally, the numerical model predicts well the measured moisture and temperature changes. The large moisture change is due to the low vapour resistance factor in the longitudinal direction of the spruce samples. The latent heat associated with vapour adsorption is the cause of the large temperature changes. It was found that vapour permeability affects both vapour and heat transfer, while thermal conductivity only affects heat transfer.

Model-based prediction of the moisture sorption kinetics and humidity-induced collapse for freeze–dried cakes

Freeze–drying produces highly hygroscopic porous materials derived from ice microstructures. Because of this feature, moisture sorption over time during storage in contact with moist air, in the worst case, induces collapse. In this study, a mathematical model of the moisture sorption kinetics applicable to glassy freeze–dried matrices is developed. The model is based on the glass transition properties and sorption equilibrium data with a sorption rate constant, which includes the specific surface area of the freeze–dried cake. The glass transition lines and moisture sorption isotherms of dextrins are experimentally obtained and applied to predict the durations before the humidity-induced collapse in the range of 0–24 h. The impacts of the combinations of the temperature and relative humidity (20–40 °C, 40–100%) are successfully visualized in stability maps. The proposed mathematical model can be a robust tool for guiding favorable formulations to produce stable freeze–dried products against storage.

Impact of humidity on formaldehyde and moisture buffering capacity of porous building material

The aim of this article is to present a validated model to study the influence of humidity on formaldehyde sorption of building material and the effect of formaldehyde and moisture buffering capacity of hygroscopic porous material on indoor air environment. New empirical formulas proposed to describe the diffusion and partition coefficients as functions of humidity based on previous experimental data are incorporated in a coupled moisture and pollutants transport simulation model. The numerical model that takes into account the effect of RH (or moisture) in building materials on VOC diffusion can be used to simulate VOC emissions/sorption from building materials. The developed model is implemented in the environment SPARK (Simulation Problem Analysis and Research Kernel) using finite difference technique. The model is then applied to study the hygric and formaldehyde (FOR, a water soluble VOC) behavior of calcium silicate (CS) subjected to different dynamic conditions of RH and formaldehyde. The results obtained in this paper help to emphasize the importance of moisture and formaldehyde buffering capacity as a new key parameter when selecting clean and hygroscopic building materials in building design because they can contribute to dampen indoor RH and formaldehyde variations. In addition, the impact of RH variation is significant and needed to be taken into account in the simulation to correctly predict the indoor formaldehyde concentration. The model developed in this paper can also be used to optimize the hygric performance and IAQ in building.

Evaluation of the reliability of a heat and mass transfer model in hygroscopic material

The reliability of a model is its accuracy in predicting the physical phenomena. In this paper, the robustness of a model of heat and mass transfer in a porous material is evaluated by comparing the numerical predictions with experimental observations. An experimental facility composed of an enclosure made with spruce CLT panels is used. An increase of temperature is applied in the inside air volume to force the heat transfer from the inner to the outer surfaces. Sensors inside the material enables to have experimental observations of the physical phenomena. Before bench-marking the numerical model, a first set of experimental data is used to reduce the two major source of uncertainties in the model. Indeed, the first source arises from surface heat and mass transfer coefficients, usually determined by empirical correlations. The second comes the thermal conductivity of the material which is defined through standard methods as invariant for the three layers of the spruce panels. To overcome this issue, a set of seven uncertain parameters are estimated using an hybrid optimizer after demonstrating their theoretical and practical identifiability. Then, the reliability of the numerical model, based on the DuFort–Frankel explicit scheme, is evaluated by comparison to a second set of experimental data obtained in another wall of the enclosure. A very satisfactory agreement is remarked showing the accuracy of the model to predict the physical phenomena in this hygroscopic porous material.

Modelling convective drying of foods: A multiphase porous media model considering heat of sorption

This study presents a theoretical and numerical analysis to derive a multiphase model of heat and mass transfer for low intensity/temperature convective drying of hygroscopic porous materials, with focus on food products. Some commonly used simplifications in modelling drying and similar processes are evaluated and a reduced model is obtained consequently. Such model is close in complexity to single-phase or diffusion-based models, but presents a multiphase and multiphysics nature, which is important to understand the underlying physics of the process. In addition, this multiphase model considers the heat of sorption in order to account for energy aspect of bound water transport, an essential feature for dealing with hygroscopic materials. The proposed formulation was implemented in commercial software to contribute to the developing of more user-friendly modelling tools. Finally, the model is used to discuss some relevant aspects of multiphase transport modelling such as treatment of evaporation rate term or closure of system of equations.

Synthesis and characteristics of composite phase change humidity control materials

A new kind of phase change humidity control material (PCHCM) was prepared by using PCM microcapsules and different hygroscopic porous materials. The PCHCM composite can regulate the indoor hygrothermal environment by absorbing or releasing both heat and moisture. The PCM microcapsules were synthesized with methyl triethoxysilane by the sol–gel method. The vesuvianite, sepiolite and zeolite were used as hygroscopic materials. The scanning electron microscopy (SEM) was used to measure the morphology profiles of the microcapsules and PCHCM. The differential scanning calorimetry (DSC) and the thermal gravimetric analysis (TGA) were used to determine the thermal properties and thermal stability. Both the moisture transfer coefficient and moisture buffer value (MBV) of different PCHCMs were measured by the improved cup method. The DSC results showed that the SiO2 shell can reduce the super-cooling degree of PCM. The super-cooling degrees of microcapsules and PCHCM are lower than that of the pure PCM. The onset temperature of thermal degradation of the microcapsules and PCHCMs is higher than that of pure PCM. Both the moisture transfer coefficient and MBV of PCHCMs are higher than that of the pure hygroscopic materials. The results indicated the PCHCMs have better thermal properties and moisture buffer ability.

Coupled electromagnetics, multiphase transport and large deformation model for microwave drying

The present work involves development of a fundamentals-based coupled electromagnetics, multiphase transport and large deformation model to understand microwave drying of a hygroscopic porous material. Microwave drying is carried out in a 950W domestic microwave oven operating at 10% power level. Electric field distribution inside the oven cavity and porous material are obtained by solving Maxwell's equations for electromagnetics. Modes of fluid transport include capillarity, binary diffusion and gas pressure-driven flow. Large deformation, included by treating the solid as hyperelastic, is implemented in a novel way using the Arbitrary-Lagrangian–Eulerian framework for mesh movement. Deformation during microwave drying was found to critically alter material structure that significantly affected microwave absorption, heat and moisture transport within the material. Sensitivity analysis revealed that moisture loss and volumetric shrinkage were unaffected with changes in intrinsic permeability and elastic modulus of the material while stress state within the material was highly sensitive to elastic modulus values.

Preparation and hygrothermal properties of composite phase change humidity control materials

A novel phase change humidity control material (PCHCM) was prepared by using PCM microcapsules and different hygroscopic porous materials. The PCHCM composite can regulate the indoor hygrothermal environment by absorbing or releasing both heat and moisture. The PCM microcapsules were synthesized with methyl triethoxysilane by the sol–gel method. The vesuvianite, sepiolite and zeolite were used as hygroscopic materials. The scanning electron microscopy (SEM) was used to measure the morphology profiles of the microcapsules and PCHCM. The differential scanning calorimetry (DSC) and the thermal gravimetric analysis (TGA) were used to determine the thermal properties and thermal stability. Both the moisture transfer coefficient and moisture buffer value (MBV) of different PCHCMs were measured by the improved cup method. The DSC results showed that the SiO2 shell can reduce the super-cooling degree of PCM. The super-cooling degrees of microcapsules and PCHCM are lower than that of the pure PCM. The onset temperature of thermal degradation of the microcapsules and PCHCMs is higher than that of pure PCM. Both the moisture transfer coefficient and MBV of PCHCMs are higher than that of the pure hygroscopic materials. The results indicated the PCHCMs have better thermal properties and moisture buffer ability.

Sorption and Drying Characteristics of Xylite

The wood from which xylite is formed in a long-term process undergoes numous changes while still retaining the character of a hygroscopic porous material. To plan the optimal drying schedule of xylite, the permeability coefficient, diffusion coefficient, and surface emission coefficient were determined. The porosity of xylite is low due to the demolished capillary structure; concurrently with the blocked pits, the permeability of xylite is very low, with a permeability coefficient not higher than 2.12 × 10−10 m2/Pa s. The capillary flow of free water is insignificant, allowing the diffusion transport of water to prevail. The diffusion coefficient is considerably influenced by the xylite's density. We confirm a significant increase in the bound water diffusion coefficient as the temperature increased (from 5.5 × 10−12 m2/s at 20°C to 46.3 × 10−12 m2/s at 40°C), and we predict that the activation energy will increase as the moisture content of xylite decreases (approx. 0.63 to 1.05%/% MC changes). As a consequence of the low surface resistance, the surface emission coefficient barely influences the moisture transport below the fiber saturation point.With an appropriate drying schedule, quality dried xylite can be used for various products as an equivalent substitute for dark-colored, high-density woods like ebony.

Determination of Poroelastic Parameters of Porous Building Materials

Variations of sorption moisture in the capillary porous materials result in strong fluid-skeleton interactions due to molecular and surface forces, which produce moisture-induced deformation. The effect of moisture sorption on the non-linear elastic behavior of hygroscopic porous building materials has been experimentally investigated showing strong influence of moisture especially in the lower moisture content range. In the framework poroelasticity moisture influence on elastic behavior is described by two poroelastic coefficients, which present the fluid-skeleton coupling. This paper presents an application of the procedure for the determination of the coupling coefficients for medieval brick.

Transient hygrothermal modelling of coated hemp-concrete walls

The use of hemp concrete, a sustainable material, in constructions is developing significantly. The hygrothermal properties of this hygroscopic porous material promise a good potential in terms of increasing occupants’ comfort. In this paper, numerical simulation is used to assess hygrothermal behaviour of a hemp-concrete wall submitted to transient boundary conditions. First, benchmarking the model against a well-documented experimental test case from literature proves the ability of the model to simulate heat and mass transfer in porous building material. Then, experimental results from a full-scale hemp-concrete wall exposed to transient climatic conditions are compared to numerical simulation, and a sensitivity analysis is performed. Good agreement between numerical results and experimental data is obtained for both the temperature and the relative humidity field in hemp concrete. Finally, the influence of coating layer on the hygric behaviour of the wall is investigated numerically. The ability of coating to affect moisture regulation is pointed out and the coating thickness is discussed.

A Dual-Scale Modeling Approach for Drying Hygroscopic Porous Media

A new dual-scale modeling approach is presented for simulating the drying of a wet hygroscopic porous material that couples the porous medium (macroscale) with the underlying pore structure (microscale). The proposed model is applied to the convective drying of wood at low temperatures and is valid in the so-called hygroscopic range, where hygroscopically held liquid water is present in the solid phase and water exits only as vapor in the pores. Coupling between scales is achieved by imposing the macroscopic gradients of moisture content and temperature on the microscopic field using suitably defined periodic boundary conditions, which allows the macroscopic mass and thermal fluxes to be defined as averages of the microscopic fluxes over the unit cell. This novel formulation accounts for the intricate coupling of heat and mass transfer at the microscopic scale but reduces to a classical homogenization approach if a linear relationship is assumed between the microscopic gradient and flux. Simulation result...

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.

Moisture Transfer Between Unsaturated Cement Mortar and Ambient Air

The objective of this article is to provide a description of moisture transport in building materials by application of a non-linear diffusion model, in which the effective coefficient of diffusion D eff, being a function of moisture content, is implemented. This article proposes the method of determination of this coefficient through correlation of the theoretical desorption isotherms with the experimental ones. To this aim, a number of desorption isotherms at temperature T = 20°C for several narrow ranges, being adjacent to each other, and for broad ranges of the air relative humidity \({\Delta \varphi}\) were established experimentally for cement mortar samples with water/cement ratio w/c = 0.50. The new knowledge contained in this article concerns some long-lasting measurements carried out in desiccators, for both narrow and broad ranges of the air humidity changes, and evaluation of the diffusion coefficient by applying an advanced optimization algorithm. The final conclusion is that the non-linear diffusion theory well reflects the moisture transport in hygroscopic porous materials like cement mortar.

Linear and non-linear analysis of desorption processes in cement mortar

Based on the gathered experimental data concerning adsorption/desorption processes in cement mortar, it has been stated that the rate of these processes changes in time even if they proceed in stable conditions. In this paper an attempt is made to describe such processes by applying linear and non-linear diffusion theories for comparison. The main aim of these studies is to determine the diffusion coefficient by correlating the theoretically determined desorption isotherms with the experimental ones. The validation of the diffusion coefficient was accomplished through comparison of the theoretical desorption curves with the experimental data for narrow and broad ranges of the air humidity changes. The final conclusion is that the moisture transfer in hygroscopic porous materials for broad ranges of the air humidity changes should be modeled by the non-linear diffusion theory, in which the diffusion coefficient is a function of moisture content. The new material in this paper concerns very long time measurements in desiccators, and evaluation of the diffusion coefficient by an advanced optimization algorithm.

Liquid water diffusivity of wood from the capillary pressure-moisture relation

This study focuses on liquid water transport in wood above the fiber saturation point in the nonhygroscopic region. The liquid water transport of hygroscopic porous materials including wood is usually described by Darcy’s law. It requires knowledge of capillarity and intrinsic and relative permeabilities. In this study, the capillary pressure-water relation and relative permeability were investigated using experimental data for wood available in the literature. The performance of three models (Brooks-Corey model, van Genuchten model, and Durner’s bimodal pore-size distribution model) was investigated for the capillary pressure-water relation. These models have advantages in that each shape parameter has qualitative physical meaning for the pore-size distribution. Most species had unimodal pore distributions except for aspen, which had a bimodal pore distribution. The van Genuchten model represented the capillary pressure-water relation better than the Brooks-Corey model. Durner’s bimodal model was found to be the most appropriate for the capillary pressure-moisture relation of aspen. The relative permeability was calculated by using Mualem’s model, which was compared with the value from the Couture model. From the results, the liquid water diffusivity divided by intrinsic permeability of wood was estimated. This approach may be promising for adopting the liquid water diffusivity for the numerical simulation of drying and sorption, although there might be considerable variation within and between trees.

Moisture Loss Kinetics of Apple During Convective Hot Air and Isothermal Drying

Effective moisture diffusivity of cylindrical apple samples (0.7 and 1.4 cm diameter) was determined under convective hot air drying (40, 50, 60, and 70° C and 1.5 and 3 m/s air velocity). The effective moisture diffusivities obtained were in the range of reported values and were found to be temperature dependent according to the Arrhenius relationship. However, the prediction of moisture loss obtained from Fick's diffusion model failed to follow experimental drying curves. Temperature profiles during convective hot air drying showed temperature gradients. This lack of isothermal conditions may lead to inaccurate predictions of moisture loss. Therefore, a combined microwave-convective hot air apparatus, capable of providing isothermal drying conditions, was used to quantify the drying kinetics. Differences in effective diffusivities determined from convective (9.6–18.5 3 10210 m2/s) and isothermal (20.3–37.6 3 10210 m2/s) drying experiments were shown. Using effective diffusivities obtained under isothermal conditions, the Fickian model still did not predict moisture loss during isothermal drying. Due to the developing porous structure of apple during drying, it was hypothesized that drying of a hygroscopic porous material is limited by evaporation of water to water vapor. Therefore, an irreversible first-order kinetic model was proposed to predict isothermal drying of apple. Using the rate constant calculated from the slope of the normalized drying curves (0.08–0.17 min21), the model predicted accurate moisture loss at each temperature throughout the entire moisture range of 0.06–8.5 g/g dry solid.

NUMERICAL SIMULATION OF COUPLED HEAT AND MASS TRANSFER IN HYGROSCOPIC POROUS MATERIALS CONSIDERING THE INFLUENCE OF ATMOSPHERIC PRESSURE

A model of simultaneous transport in hygroscopic porous materials was developed. Water in fabrics is considered to be present in three forms: liquid water in the void space between fibers, bound water in the fibers, and vapor. It is assumed that the heat and mass transport mechanisms include movement of liquid water due to the capillarity and atmospheric pressure gradient, diffusion of vapor within interfibers due to the partial pressure gradient of vapor and total gas pressure gradient, diffusion of vapor into fiber, evaporation, and condensation of water. The moisture diffusion process into hygroscopic porous materials such as wool fabrics was simulated. At normal atmospheric pressure, the results were compared with experimental data on the temperature and water content changes reported in the literature. The distribution of temperature, moisture concentration, liquid water saturation, and atmospheric pressure in the void space between fibers at different boundary conditions are numerically computed and compared. The conclusion is that atmospheric pressure gradient has significant impact on heat and mass transport processes in hygroscopic porous materials.

Effect of the environmental atmosphere on heat, water and gas transfer within hygroscopic fabrics

A dynamic model of simultaneous heat and mass transfer in hygroscopic porous materials was developed. In the model, evaporation/condensation and movement of water, sorption/desorption of fiber and effect of atmosphere on mass transfer in porous media were considered. The theoretically calculated results agree well with the experimental data reported in the literature. The distributions of temperature, moisture concentration, liquid water saturation and atmosphere at different boundary conditions are numerically computed and compared. It could be concluded that the atmospheric pressure has significant impact on heat and mass transport processes in hygroscopic porous materials.