Moisture Transport Model Research Articles

Modeling water absorption in concrete and mortar with distributed damage

The deterioration rate of concrete structures is directly influenced by the rate of moisture ingress. Modeling moisture ingress in concrete is therefore essential for quantitative estimation of the service life of concrete structures. While models for saturated moisture transport are commonly used, concrete, during its service life, is rarely saturated and some degree of damage is often present. In this work, we investigate whether classical isothermal unsaturated moisture transport can be used to simulate moisture ingress in damaged mortar and concrete and we compare the results of numerical simulations with experimental measurements of water sorption. The effect of hysteresis of moisture retention is also considered in the numerical simulations. The results indicate that the unsaturated moisture transport models well simulate early stages of moisture ingress at all damage levels, where capillary suction is the prominent mechanism. At later stages of moisture transport, where air diffusion and dissolution have a more significant contribution, simulations that consider moisture hysteresis compare most favorably with experimental results.

A Simplified Model of Moisture Transport in Hydrophilic Porous Media With Applications to Pharmaceutical Tablets

This work establishes a predictive model that explicitly recognizes microstructural parameters in the description of the overall mass uptake and local gradients of moisture into tablets. Model equations were formulated based on local tablet geometry to describe the transient uptake of moisture. An analytical solution to a simplified set of model equations was solved to predict the overall mass uptake and moisture gradients with the tablets. The analytical solution takes into account individual diffusion mechanisms in different scales of porosity and diffusion into the solid phase. The time constant of mass uptake was found to be a function of several key material properties, such as tablet relative density, pore tortuosity, and equilibrium moisture content of the material. The predictions of the model are in excellent agreement with experimental results for microcrystalline cellulose tablets without the need for parameter fitting. The model presented provides a new method to analyze the transient uptake of moisture into hydrophilic materials with the knowledge of only a few fundamental material and microstructural parameters. In addition, the model allows for quick and insightful predictions of moisture diffusion for a variety of practical applications including pharmaceutical tablets, porous polymer systems, or cementitious materials.

Time-resolved X-ray microtomographic measurement of water transport in wood-fibre reinforced composite material

Natural fibre composites are prone to absorb moisture from the environment which may lead to dimensional changes, mold growth, degradation of mechanical properties or other adverse effects. In this work we develop a method for direct non-intrusive measurement of local moisture content inside a material sample. The method is based on X-ray microtomography, digital image correlation and image analysis. As a first application of the method we study axial transport of water in a cylindrical polylactic acid/birch pulp composite material sample with one end exposed to water. Based on the results, the method seems to give plausible estimates of water content profiles inside the cylindrical sample. The results may be used, e.g., in developing and validating models of moisture transport in biocomposites.

Combined effects of sorption hysteresis and its temperature dependency on wood materials and building enclosures – Part I: Measurements for model validation

Hygroscopic materials such as wood and wood based materials have been widely used as insulation and surface moisture buffering materials due to their low thermal conductivity and high moisture capacity. And their hygrothermal performance is mainly dependent on the moisture properties, such as sorption isotherm and water vapor permeability etc. Instead of a univalued function of relative humidity, sorption isotherm of wood materials is not only affected by sorption history, but also temperature dependent. A heat and moisture transport model is formulated based on local thermodynamic equilibrium assumption, which includes thermal moisture capacity and a hysteresis model in [1]. To validate this model, sorption isotherms at 23 °C were measured using the static gravimetric method; and a moisture response test under dynamic boundary conditions was carried out in a climatic chamber. The simulation results show that the hygrothermal model with temperature dependency and sorption hysteresis can capture the dynamic moisture response to variable boundary conditions very well. Therefore, this model could be used to further analyze the individual and combined effects of sorption hysteresis and its temperature dependency through hygrothermal modeling in a companion paper.

Thermo–hydro-ionic transport in sea immersed tube tunnel

This paper investigates the multi-species and multi-fields transport processes in the concrete wall of sea immersed tube tunnel, exposed externally to sea water and internally to road traffic. A thermo–hydro-ionic (THI) model is established for the transport processes, taking into account the heat, mass and electrical charge conservations, the electrical neutrality and solid-solution equilibria in the representative elementary volume (REV) of material. Then, the characteristic lengths for drying and wetting are proposed for the tunnel wall through simplified model of moisture transport to define different durability patterns for tunnel wall, and the associated durability risks are identified. The THI model is applied to the immersed tube tunnel of Hong Kong–Zhuhai–Macau (HZM) sea link project, and the transport processes are simulated for a service life of 120years and different working cases. The analysis shows that: (1) the thermal gradient has more important impact on transport processes than the external sea water pressure on tunnel wall; (2) the drying and wetting extents are overlapped through the wall thickness at 120years, and the internal drying and external leaching condition the multi-species transport; and (3) the corrosion initiation range attains 65cm at 120years but the corrosion current is below the depassivation value (0.1μA/cm2).

A Fickian model for temperature-dependent sorption hysteresis in hygrothermal modeling of wood materials

The sorption isotherm is one of the most important hygric properties for modeling moisture transport in hygroscopic materials. For wood and wood based materials, the equilibrium moisture content within hygroscopic range is a function of temperature and relative humidity as well as its sorption history. The constitutive equations of the existing models in literature, which attempted to take into account temperature dependent sorption hysteresis, are either incomplete or have too many model parameters needed to be experimentally determined. As a result they are not suitable for practical applications in building physics. Therefore, a coupled heat and moisture transport model is formulated based on the classical local thermodynamic equilibrium assumption. Frandsen’s hysteresis model is adopted and implemented in this Fickian model to determine the moisture capacity and thermal moisture capacity, since it avoids the needs to store the entire moisture history. The formulated hygrothermal model is numerically resolved in Modelica by virtue of a combination of storage model and flow model. For hygroscopic material with sorption hysteresis, its water vapor resistance factor has also been recalculated as a univalued function of moisture content instead of relative humidity. With different considerations for the moisture sorption isotherms, the new model provides the possibility of numerical study under different levels depending on the way to consider moisture sorption property. A simple example case is carried out to show the performance of the new model.

Impact of Climate Change on Groundwater Resources in the Klela Basin, Southern Mali

Investigations of groundwater resources in order to understand aquifer system behavior are vital to the inhabitants of the Klela Basin, Mali, because groundwater is the only permanent water resource and is used for drinking water and irrigation. Due to climate change, this vital resource is being threatened. Therefore, MODFLOW was applied in this study to simulate groundwater dynamics. The aim of this study was to evaluate the impact of climate change on groundwater resources in the Klela basin using the RCP4.5 (Representative Concentration Scenario 4.5 W/m2) climate scenario. Climatological, geological, hydrogeological, hydraulic and demographic data were collected and used as model input data. Groundwater recharge was estimated to be approximately 165.3 mm/year using the EARTH (Extended model for Aquifer Recharge and soil moisture Transport through the unsaturated Hardrock) model. Recharge was then used as groundwater model input. The sandstone aquifer in the study area was simulated in steady and transient conditions. The results showed that hydraulic conductivity values varied from 1.1 to 13.9 m/day. The model was used for scenario quantification after model calibration and verification using three different piezometer data sets. The results of the simulated MODFLOW model showed a decrease in groundwater levels over time.

Absorption of impinging water droplet in porous stones

This paper presents an experimental investigation and numerical analysis of the absorption of water droplets impacting porous stones. The absorption process of an impinging droplet is here fully characterized from spreading to evaporation in terms of absorbed mass during droplet depletion and moisture content distribution in a time-resolved manner for three different natural stones. High-speed imaging and neutron radiography are used to quantify moisture absorption in porous stones of varying moisture properties from deposition until depletion. During impact and spreading, the droplet exhibits a dynamic non-wetting behavior. At maximum spreading, the droplet undergoes pinning, resulting into the contact radius remaining constant until droplet depletion. Absorption undergoes two phases: initially, absorption is hindered due a contact resistance attributed to entrapped air; afterwards, a more perfect capillary contact occurs and absorption goes on until depletion, concurrently with evaporation and further redistribution. A finite-element numerical model for isothermal unsaturated moisture transport in porous media captures the phases of mass absorption in good agreement with the experimental data. Droplet spreading and absorption are highly determined by the impact velocity of the droplet, while moisture content redistribution after depletion is much less dependent on impact conditions.

Moisture-induced stresses in glulam frames

Wood is a hygroscopic and moisture-sensitive material that seeks to achieve equilibrium moisture content (EMC) with its surrounding environment. For softwood timber structures exposed to variations in climate throughout their service life, this behaviour results in variable moisture-content gradients that cause moisture-induced stresses in the direction of and perpendicular to the fibres. Although Eurocode 5 (EC5) states that moisture-induced stresses should be considered, they are often not adequately dealt with in building design due to the difficulties in predicting the stresses involved by hand. Accordingly, there is a need for advanced computer tools to study how the long-term stress behaviour of timber structures is affected by creep and cyclic variations in climate. A beam model to simulate the overall hygro-mechanical and visco-elastic behaviour of (inhomogeneous) glulam structures is presented. A two-dimensional transient, non-linear moisture transport model for wood is also developed and linked with this beam model. The combined models are used to study the long-term deformations and stresses in a curved frame structure exposed to both mechanical loading and cyclic climate conditions. It is shown that the moisture-induced deformations and stresses are of such magnitude that the design codes employed should take them into account. Thus it is argued that climate-related loads should be treated as separate load contributions that can be included in different load combinations.

Triphasic Model of Heat and Moisture Transport with Internal Mass Exchange in Paperboard

Mixture theory is used to derive a triphasic model to describe processes in paperboard consisting of solid fiber, bound water and gas. The gas is viewed as a miscible mix of the two constituents dry air and water vapor. The governing equations are mass conservation laws for bound water, dry air, water vapor, and mixture energy balance. Constitutive relations are found by exploiting the macroscale dissipation inequality. Resulting constitutive equations include Fickian diffusion of water vapor and dry air, Darcian flow for gas and Fourier heat conduction for the mixture. Mass exchange between bound water, and water vapor due to adsorption/desorption is driven by the difference in chemical potential. The interaction function is based on equilibrium considerations for the bound water–water vapor system. From the description of the sorption isotherm, expressions for net isosteric heat and free energy related to water–fiber interaction are derived. The resulting thermodynamically consistent model is used to simulate moisture and heat dynamics for paperboard rolls. Simulation results are presented for a paperboard roll with anisotropic material properties subjected to a change in ambient relative humidity from 50 to 80 %.

Mathematical modeling of shrinkage deformation in eggplant undergoing simultaneous heat and mass transfer during convection-oven roasting

This study aimed to analyze experimentally and by modeling the shrinkage deformation of Japanese eggplant (Solanum melongena) during roasting by simultaneous heat and moisture transport model coupled with a structural mechanics model applicable to a body undergoing volumetric changes, as a consequence of moisture removal. Cylindrical slices (45 mm × 10 mm) were roasted for up to 15 min. Roasting experiments were performed using a convection-oven at 250 °C. Several coefficients and parameters required for calculations were experimentally obtained. An implicit finite element method was applied using a FORTRAN program. In this study, an approximately equal reduction rate in moisture content losses and volume shrinkage was observed (R2 ≥ 0.99). The volumetric (Sv) and directional shrinkage were successfully estimated by developed empirical equations (e.g. R2 > 0.98 for Sv). The simulation of temperature and moisture distribution undergoing shrinkage deformation confirmed the importance of internal evaporation in the transport model.

Математические предпосылки к вопросу о передвижении влаги в почве при капельном орошении

The article discusses a mathematical models to describe the process of movement of moisture with drip irrigation. In the Federal target program «Development of reclamation of agricultural lands of Russia for 2014−2020» the task is saving water. One such water-saving technologies is drip irrigation. This method of irrigation is widespread in different countries of the world. It is noted that the accuracy and effectiveness of the regulation of the water regime is defined as frequent and less watering. Indicates that the application in practice of traditional methods of calculation of drip irrigation causes farmers a lot of questions. In the absence of engineers, reclamation specialists on the production of farmers can not apply developed in a large number of recent recommendations and are forced to set their own water supply mode, because of this reduced efficiency in the use of drip irrigation systems. It is shown that observation of the movement of moisture using the balance method with drip irrigation is difficult. It is noted that numerous studies have shown the high accuracy and efficiency the calculation of irrigation regimes using models of moisture transport, among which the most widespread are based on the basis of an expression L. Richards.

Numerical modelling of transient heat and moisture transport in protective clothing

The paper presents a complex model of heat and mass transfer in a multi-layer protective clothing exposed to a flash fire and interacting with the human skin. The clothing was made of porous fabric layers separated by air gaps. The fabrics contained bound water in the fibres and moist air in the pores. The moist air was also present in the gaps between fabric layers or internal fabric layer and the skin. Three skin sublayers were considered. The model accounted for coupled heat transfer by conduction, thermal radiation and associated with diffusion of water vapour in the clothing layers and air gaps. Heat exchange due to phase transition of the bound water were also included in the model. Complex thermal and mass transfer conditions at internal or external boundaries between fabric layers and air gaps as well as air gap and skin were assumed. Special attention was paid to modelling of thermal radiation which was coming from the fire, penetrated through protective clothing and absorbed by the skin. For the first time non-grey properties as well as optical phenomena at internal or external boundaries between fabric layers and air gaps as well as air gap and skin were accounted for. A series of numerical simulations were carried out and the risk of heat injures was estimated.

Influence of sorption hysteresis on moisture transport in wood

The independent domain theory is used to analyze the influence of sorption hysteresis on the behavior of wood exposed to environmental conditions. Due to hysteresis, sorption history of wood has an impact on its performance under varying moisture conditions. An integrated Preisach–Mayergoyz (IPM) approach that is phenomenologically sound and insightful while mathematically and computationally straightforward to implement is applied, where the parameters of the IPM space are determined from experiments. The implementation of the IPM approach in a heat and moisture transport model is validated based on independent measurements of dynamic vapor sorption in spruce wood. The transport model, including sorption hysteresis, is further used to simulate the response of a wood beam to weekly humidity variations and hourly climatic humidity and temperature variations. The analysis shows that hysteresis of wood is strongly dependent on the magnitude of moisture changes. As moisture penetrates the material faster in longitudinal than in radial direction, due to a lower vapor resistance factor, the effect of hysteresis is more pronounced deeper into the wood in longitudinal than transversal directions. Furthermore, the simulations imply that errors in moisture content of more than 20 % (or 30 %) can be made when using the main adsorption curve (or main desorption curve) instead of the hysteresis model for wood sorption behavior. Sorption hysteresis must thus be accounted for in heat and mass transport models to assess moisture-related damage risks such as mold growth, rot or moisture-induced cracking, and the IPM approach offers such an appropriate avenue.

Moisture content prediction of rain-exposed wood: Test and evaluation of a simple numerical model for durability applications

Decay prediction models are frequently used to estimate the service life of wooden components. These models require knowledge of how the material climate, i.e. moisture content and material temperature, varies over time. In the present study, the performance of a simple numerical moisture transport model was evaluated for use in decay prediction models. First, a model based on Fick's second law of diffusion was calibrated against laboratory measurements where wooden boards were exposed to artificial rain. Second, the model was tested against field-test measurements on wooden boards exposed outdoors above-ground. The influence of rain was investigated by studying the difference between sheltered and exposed specimens over time. Finally, the model was applied to a number of Swedish climates and two different decay-prediction models were used to evaluate the decay rate. The influence of rain on the moisture content in wooden specimens was reproduced with sufficient accuracy for decay prediction. The error between the numerical result and the measurements tended to increase at high moisture contents and with decreasing temperature. However, the total error was reduced when the moisture content history was post-processed in a decay-prediction model as the rate of decay tends to decrease with decreasing temperature. The estimated service life varied with depth and the different decay models.

A non-equilibrium model for soil heating and moisture transport during extreme surface heating: the soil (heat–moisture–vapor) HMV-Model Version 1

Abstract. Increased use of prescribed fire by land managers and the increasing likelihood of wildfires due to climate change require an improved modeling capability of extreme heating of soils during fires. This issue is addressed here by developing and testing the soil (heat–moisture–vapor) HMV-model, a 1-D (one-dimensional) non-equilibrium (liquid–vapor phase change) model of soil evaporation that simulates the coupled simultaneous transport of heat, soil moisture, and water vapor. This model is intended for use with surface forcing ranging from daily solar cycles to extreme conditions encountered during fires. It employs a linearized Crank–Nicolson scheme for the conservation equations of energy and mass and its performance is evaluated against dynamic soil temperature and moisture observations, which were obtained during laboratory experiments on soil samples exposed to surface heat fluxes ranging between 10 000 and 50 000 W m−2. The Hertz–Knudsen equation is the basis for constructing the model's non-equilibrium evaporative source term. Some unusual aspects of the model that were found to be extremely important to the model's performance include (1) a dynamic (temperature and moisture potential dependent) condensation coefficient associated with the evaporative source term, (2) an infrared radiation component to the soil's thermal conductivity, and (3) a dynamic residual soil moisture. This last term, which is parameterized as a function of temperature and soil water potential, is incorporated into the water retention curve and hydraulic conductivity functions in order to improve the model's ability to capture the evaporative dynamics of the strongly bound soil moisture, which requires temperatures well beyond 150 °C to fully evaporate. The model also includes film flow, although this phenomenon did not contribute much to the model's overall performance. In general, the model simulates the laboratory-observed temperature dynamics quite well, but is less precise (but still good) at capturing the moisture dynamics. The model emulates the observed increase in soil moisture ahead of the drying front and the hiatus in the soil temperature rise during the strongly evaporative stage of drying. It also captures the observed rapid evaporation of soil moisture that occurs at relatively low temperatures (50–90 °C), and can provide quite accurate predictions of the total amount of soil moisture evaporated during the laboratory experiments. The model's solution for water vapor density (and vapor pressure), which can exceed 1 standard atmosphere, cannot be experimentally verified, but they are supported by results from (earlier and very different) models developed for somewhat different purposes and for different porous media. Overall, this non-equilibrium model provides a much more physically realistic simulation over a previous equilibrium model developed for the same purpose. Current model performance strongly suggests that it is now ready for testing under field conditions.

Study of Two Hemp Concrete Walls in Real Weather Conditions

The use of bio-based materials in construction is significantly improving. One of the bio-based materials is hemp concrete which is a composite obtained by mixing a binder and the non-fibrous part of hemp, called shiv. This specific hemp concrete developed by Vicat is made with Prompt Natural Cement (PNC) as binder. In situ hygro-thermal performance assessment of this material was performed in this paper. For this both simulation and experimental measurements are conducted in parallel. In the experimental study two PASSYS test facilities are used. They are parallelepiped test cells around 30 m3 volume each. One test wall is built with the interesting façade while the five other walls are highly insulated and are non-tested walls. Test walls are subject to weather solicitations outside and hydrothermal regulation inside. The two test cells are next to each other, and the hempcrete wall is south oriented. The only difference between cells is the external coating in order to analyze the coating effect on moisture transfer: one cell is supplied with traditional hemp lime render and the other one with cement based render. In parallel, 2D numerical simulation of hygrothermal behavior of the wall was carried out using a coupled heat and moisture transport model with boundary condition from the monitored data (outside weather conditions and inside regulation). Results show a good agreement between measured and simulated data.

Modeling of moisture over-saturation and vapor pressure in die-attach film for stacked-die chip scale packages

Die-attach film failure during soldering reflow is of particular concern for reliability of 3D ultra-thin stacked-die chip scale packages (CSPs), as extremely high vapor pressure can be generated from vaporized moisture to cause severe damages. Under rapid heating, pressure-driven moisture vapor flow could become as significant as concentration-driven diffusion, and should be included in moisture transport models. In this study, a convection–diffusion (CD) model is presented for analyzing vapor pressure and moisture behavior of soft die-attach films for 3D CSPs, with both vapor flow and the effect of temperature on saturated moisture content considered. By using a 1D model according to experimental observations, the over-saturation phenomenon in the thin film and vapor pressure evolution are investigated. It is found that the reflow profiles and substrate thickness could have great impact on the magnitude of vapor pressure, which agrees well with the experimental results. Sensitivity analyses show that a high vapor permeability and a large porosity could lead to low vapor pressure. Being a vapor pressure-based model, the CD model is suitable to study multiple-material systems without discontinuity issue at the material boundary. It is concluded that the CD model could serve as an effective method for predicting vapor pressure and moisture behaviors during soldering reflow for 3D stacked-die CSPs.

Concrete desorption isotherms and permeability determination: effects of the sample geometry

The isotherm sorption curve is a first-order parameter used in the Finite Element modelling of concrete moisture transport, shrinkage and creep behaviour. An original experimental campaign was developed by EDF R&D in order to characterise the first desorption isotherm at room temperature of a laboratory concrete. Long-term drying tests were carried out on three sample geometries: radial and axial one-dimensional drying on thin discs and multi-dimensional drying on representative elementary volumes (REV), in order to evaluate the possibility of accelerating the tests. Porosity, densities and mass loss curves are measured and the first-desorption isotherms obtained for the three different configurations are compared. Several analyses of these results are proposed including the assessment of a criterion for the determination of the moisture content final balance (estimation of the asymptotic mass loss) and the back analysis of equivalent permeability. The tests results show the significant time gain using thin (2.5 mm) concrete half samples drying in radial direction compared to the REV samples.

Determining soil moisture by assimilating soil temperature measurements using the Ensemble Kalman Filter

This study investigates the potential to estimate the vertical profile of soil moisture by assimilating temperature observations at a limited number of depths into a coupled heat and moisture transport model (Hydrus-1D). The method is developed with a view to assimilating temperature data from distributed temperature sensing (DTS) to estimate soil moisture at high resolution over large areas. The correlation between temperature and soil moisture in the shallow soil (top ∼ 50 cm) ensures that soil moisture can be estimated using just soil temperature observations. Synthetic tests across a range of soil textures show that with data assimilation both modeled temperature and the moisture profile are improved considerably compared to the ensemble open loop model simulations. In addition, employing data assimilation provides a means to quantitatively account for different sources of uncertainty. This is particularly relevant in the context of DTS given the influence of spatial variability in soil texture and its impact on estimation error. The data assimilation approach could also be used to determine, the number of temperature observations required and the depths at which they should be made. Results suggest that temperature observed at two depths is typically sufficient to estimate soil moisture using this approach. The root mean square error (RMSE) in soil moisture was reduced by up to 75% in the top 20 cm. Furthermore, this approach solves many of the challenges identified in the application of an inversion approach to estimate soil moisture from DTS.