Moisture Transfer In Porous Materials Research Articles

Improved engineering model for water absorption in softwoods

Modeling of moisture transfer in porous materials is typically partitioned into liquid transport and vapor diffusion. Coefficients for liquid transport are often derived from laboratory measurements of water absorption by partial immersion. Although the assumption that capillary liquid transport is dominant while vapor diffusion is negligible is a reasonable assumption for mineral-based materials, recent work indicates that it is not representative of water absorption in wood across the grain. This article builds on earlier work and develops an improved engineering model for liquid water transport in softwoods. The approach uses a constant liquid diffusivity value, relies primarily on ordinary hygrothermal property measurements, and partitions vapor and liquid transport in a practical way that is informed by current understanding of wood–water interactions. The model is optimized using one-dimensional simulations calibrated against measured water uptake across a range of softwood species. Laboratory measurements were conducted to investigate key properties of eastern white cedar, which had the highest relative error in a previous version of the model, and to examine the practicality and relevance of vacuum saturation and free saturation for southern yellow pine. The model accuracy is improved by paying closer attention to the moisture storage function and capillary saturation value.

Influence of variability in hygrothermal properties on analytical results of simultaneous heat and moisture transfer in porous materials

Depending on the data source used, the material hygrothermal properties that are used in the numerical analysis of simultaneous heat and moisture transfer will not be consistent. Differences in measurement methods and the individuality of specimens account for this. It is necessary to choose values from these different physical property sets to conduct a numerical calculation, which can cause the calculated results to differ. The subsequent range of variation in the calculated results should be quantitatively evaluated. In this study, the physical properties of several types of porous building materials were first gathered from four databases. The data were then categorized based on the kind of material and compared in terms of each physical property (density, porosity, specific heat, moisture capacity, thermal conductivity, and vapor permeability). The density, porosity, and specific heat varied by 10% on average, and the moisture capacity, thermal conductivity, and vapor permeability varied by 20% or more for all types of materials. In particular, the vapor permeability of plywood and moisture capacity of gypsum board differed by 50%. The influence that these physical property value variations had on hygrothermal calculation results was then quantitatively demonstrated for moisture and heat flow rate under a step change in the relative humidity or temperature of indoor air for a single layer wall. The moisture and heat flow rate into a single layer wall fluctuated by approximately 10%–40% due to differences in the vapor permeability and moisture capacity of the materials. For all types of materials, moisture was transferred more slowly than heat. Therefore, differences in moisture property values, such as vapor permeability and moisture capacity, influenced the results more significantly. Moreover, the moisture flow was accompanied by a phase change. The differences in moisture property values thus affected the heat flow.

Critical assessment of efficient numerical methods for a long-term simulation of heat and moisture transfer in porous materials

The issue to predict the behavior of building materials during wide horizons of time is still challenging. Experimental set-ups, since they require to perform tests for several years, are costly, never at the full scale and inconvenient. Building Performance Simulation (BPS) programs are designed to perform predictions on computational machines and cut experimental costs significantly. Nonetheless, in the recent review of state–of–the–art, it was indicated that despite the wide range of programs, there are still some drawbacks in terms of the accuracy and the high computational cost. This paper investigates the application of an innovative numerical method, called Super–Time–Stepping (STS) method. It allows performing accurate simulations with time-steps much larger than with standard explicit approaches. These “super” time-steps also enable us to reduce the computational cost. In addition to that, the design of the method allows easier application for models in higher dimensions and with nonlinear parameters. The efficiency of the method is tested on linear and nonlinear academic cases. Further study for the reliability of the model is performed on an experimental case study. The experiment has been carried out on a rammed earth wall during almost 14 months. Obtained data is presented in this article and implemented into proposed model. As a result of the case studies, it is shown that in comparison to the euler explicit method, the STS methods can cut costs by more than five times while maintaining high accuracy and efficiency. A very fine analysis of the physical phenomena is also performed.

A discussion of “Analysis of the water absorption test to assess the intrinsic permeability of earthen materials”

Abstract In February 2019, this journal published the article “Analysis of the water absorption test to assess the intrinsic permeability of earthen materials”. The author of this discussion considers the article’s simplification of moisture transfer in porous materials to a “competition between the kinetics of water ingress within the wall, which is driven by intrinsic permeability, and water evaporation, which is rather driven by resistance to vapour diffusion at the wall surfaces” in contradiction with the state-of-the-art on this topic. It is therefore reiterated that decades of research demonstrate the intrinsically unsaturated nature of moisture transfer in porous materials. The discussion continues by reviewing the article’s application of Kiessl’s/Kunzel’s exponential diffusivity. Whereas the article observes that this exponential diffusivity overestimates the capillary absorption, the theory on this formulation states that it should correctly reproduce the capillary absorption. It is furthermore noted that a similar application of the exponential diffusivity by one of the article’s authors in a related work contrarily results in an underestimation of the capillary absorption. These contradictions can only be explained by the faulty application of the exponential diffusivity by the article’s authors, rather than the exponential diffusivity being unsuited for earthen materials, as currently concluded in the article.

(Semi-)analytical solution of Luikov equations for time-periodic boundary conditions

The paper addresses the problem of coupled heat and moisture transfer in porous materials with the time-periodic boundary conditions applied. The solution of Luikov equations [1], which describe coupled heat and moisture transfer, is presented. Laplace transform is used, where some terms of the inverse Laplace transform ought to be solved by Gaussian quadrature, meaning that the solution is semi-analytical. The time-periodic boundary conditions are applied to simulate the humidity and temperature oscillations of natural environment. Therefore, the proposed solution is appropriate to evaluate the distribution of moisture and temperature within the porous material exposed to everyday natural cycles. The paper presents convergence tests, validation of semi-analytical solution and application to different building materials are presented in the paper.

Stable explicit schemes for simulation of nonlinear moisture transfer in porous materials

Implicit schemes have been extensively used in building physics to compute the solution of moisture diffusion problems in porous materials for improving stability conditions. Nevertheless, these schemes require important sub-iterations when treating nonlinear problems. To overcome this disadvantage, this paper explores the use of improved explicit schemes, such as Dufort–Frankel, Crank–Nicolson and hyperbolization approaches. A first case study has been considered with the hypothesis of linear transfer. The Dufort–Frankel, Crank–Nicolson and hyperbolization schemes were compared to the classical Euler explicit scheme and to a reference solution. Results have shown that the hyperbolization scheme has a stability condition higher than the standard Courant–Friedrichs–Lewy condition. The error of this schemes depends on the parameter τ representing the hyperbolicity magnitude added into the equation. The Dufort–Frankel scheme has the advantages of being unconditionally stable and is preferable for nonlinear transfer, which is the three others cases studies. Results have shown the error is proportional to . A modified Crank–Nicolson scheme has been also studied in order to avoid sub-iterations to treat the nonlinearities at each time step. The main advantages of the Dufort–Frankel scheme are (i) to be twice faster than the Crank–Nicolson approach; (ii) to compute explicitly the solution at each time step; (iii) to be unconditionally stable and (iv) easier to parallelize on high-performance computer systems. Although the approach is unconditionally stable, the choice of the time discretization remains an important issue to accurately represent the physical phenomena.

Network numerical simulation of coupled heat and moisture transfer in capillary porous media

In this paper we present the numerical solution of the problem of coupled heat and moisture transfer in porous materials. A network model introducing an analogy based in the equivalence mass transfer-electrical current is proposed to obtain the numerical solutions. An algorithm based on a finite-difference scheme for the spatial variable (as in the lines method because the time remains as a continuous variable) is formally equivalent to those derived from the mathematical model. An implementation of the Network Simulation Method is used to derive a solution to the heat and mass conservation coupled equations in porous media for the cases of distributed and lumped models. The obtained values by the theoretical model are compared with other author data to demonstrate the efficiency of the method used, moreover all the cases the convergence is always quickly reached.

Thermal diffusion of water vapour in porous materials: Fact or fiction?

The reliable evaluation of moisture transfer in porous materials is essential in many engineering applications, among which building science. One key aspect is a correct description of moisture flow phenomena and their transport potentials. While different issues can be debated in that respect, thermal diffusion of water vapour in porous materials – diffusion driven by temperature gradients – currently stands out, due to the contradictory findings on the topic. Thermal diffusion proponents uphold that, complementary to vapour pressure gradients, temperature gradients equally yield substantial diffusion. Thermal diffusion opponents, on the other hand, assert that these thermal transports are negligibly small.This paper resolves that contradiction. A critical analysis of the investigations supporting the occurrence of thermal diffusion reveals that all are flawed. A correct reinterpretation of all measurements allows concluding that no consistent nor significant thermal diffusion can be observed. This brings these investigations in line with their earlier opponents. This conclusion also agrees with thermodynamics, which confirms the actual existence of thermal diffusion, but also indicates its negligible magnitude. It can in conclusion be stated that thermal diffusion is of no importance for building science applications, leaving vapour pressure as the sole significant transport potential for the diffusion of water vapour in porous materials.

Non-isothermal Moisture Transport calculations with DIM 3.1 / Nicht-isotherme Berechnungen des Feuchtigkeitstransportes mit Hilfe des Porgrammes DIM 3.1

Abstract The application of a general thermodynamical mass and energy transport model to the coupled heat and moisture transfer in porous materials results in a balance equation system and the related constitutive equations of the considered quantities. The constitutive equations describe moisture transport in a phase-separated manner leading into phase-divided hygric transport coefficients (liquid water permeability, water vapour diffusivity). A conceptual model is presented in the paper in order to circumvent the difficulties resulting from non-isothermal overlaying moisture transport processes. Since phase-divided hygric transport coefficients are not directly measurable, but moisture transport coefficients in distinct hygric ranges, moisture conductivities and a phase dividing function are introduced. The moisture conductivities include liquid water and water vapour transport. For a known phase dividing function, the phase-divided hygric transport coefficients of the balance equation system can be calculated from the measurable moisture conductivities. The influence of a variation of the introduced phase-dividing function on non-isothermal moisture transport processes is investigated by means of computer simulations.

An analytical solution to coupled heat and moisture diffusion transfer in porous materials

An analytical method is proposed for solving the problem of coupled heat and moisture transfer in porous materials. The coupled partial differential equations and boundary conditions are first subjected to Laplace transformation, the equations are reduced to ordinary differential equations, then the equations are converted into a single fourth-order ordinary differential equation by introducing a transformation function. The solution of the equation can be easily obtained, and thus, the temperature and moisture distributions in the transform domain can be determined. Finally, the transformed values are analytically or numerically inverted to obtain the time domain results. Therefore, the transient solution at any given time can be evaluated. The results are identical with published analytical solutions for a special case using decoupling technique, and they agree with a published analytical solution for wood slab. The method is compact enough to be generally applied to problems of heat and mass transfer in porous media.

Coupled heat air and moisture transfer in building structures

Abstract Based on the energy, mass and linear momentum conservation laws as well as the entropy law, a coupled, partial, non-linear, differential equation system for the heat, air and moisture transfer in porous materials follows, and for the more-dimensional solution of it, a suitable algorithm including the simulation software has been developed. Its efficiency is demonstrated by a recent building structure—chuted concrete between fibre board with polystyrol-foam outside insulation-under the boundary conditions of the TRY Essen: hourly values of outdoor climate. As a result of the high built-in moisture, moisture damages (mould growing) in the base level (exterior wall/concrete plate over the basement garage) and inadmissible high heat losses caused by the moisture depending thermal conductivity and the additional enthalpy flows coupled to the moisture movement and phase changing are observed in the first years.

Temperature distribution and moisture transfer in porous materials

Temperature distribution and moisture transfer in porous materials