Difference between revisions of "Modeling of Salt and Humidity Transport"
Line 11:  Line 11:  
category = ANicolai&Salt Moisture Transport  category = ANicolai&Salt Moisture Transport  
format = ,\n* [[%PAGE%%TITLE%]],,  format = ,\n* [[%PAGE%%TITLE%]],,  
+  suppresserrors=true  
</dpl>  </dpl>  
Revision as of 06:13, 21 October 2011
Error creating thumbnail: File missing The English version of this article is being translated by Christa Gerdwilker. Error creating thumbnail: File missing

<bibimport/>
Author: Dr. Andreas Nicolai
back to Saltwiki:Community portal
Linked Article
Extension:DynamicPageList (DPL), version 3.3.2: Warning: No results.
Contents
Introduction
The transport of salts in porous materials (building materials, stone, flooring etc.)is dependent on many factors. The types of salts, their constituents and distribution, the extent of dampness, temperature, environmental conditions in general and also the microscopic structure of the material all influence the movement of salts and the appearance of decay over time.
Usually, only specific combinations of materials, salt mixtures and environmental conditions can be measured under laboratory conditions (common laboratory experiments for the determination of salt transport properties can be found in the article "Experimental calibration of salt transport parameters". Due to diverse parameters and decay scenarios it is rarely possible to directly apply such imperial measurements in the practical environment.
Alternatively, transport models can be used which realistically simulate the physical and chemical context and thus allow mathematical predictions regarding the dispersion and accumulation of salts and material decay over time. The aim of this article and linked reference material is to provide a summary of current research in the modeling of salt transport.
Due to insufficient nomenclature in relevant norms and literature, a bespoke form of annotation and symbols for salt transport models Annotation and list of symbols used for salt transport models is used for the models below.
Fundamental aspects of modeling salt transport
Due to the complexity of the involved processes, salt transport models must be able to describe a number of effects, amongst others:
 Moisture transport and moisture retention
 Thermal transfer through thermal conductivity and radiation
 Enthalpy transport, e.g. latent heat to describe cooling during evaporation
 Balanced salt phase change constituents and phase change kinetics
 Salt diffusion and dispersion
 Efflorescence (removal of salts from the calculated domain)
 Change of pore space through crystallization and resultant effect on moisture and salt transport
Because the linked thermal and moisture transport models form the basis for salt transport modeling, a summary of the current state of research in moisture transport modeling is provided. Generally, the presented model corresponds to the jointly defined transport model [Hagentoft.etal:2004]Title: Assessment Method of Numerical Prediction Models for Combined Heat, Air and Moisture Transfer in Building Components: Benchmarks for Onedimensional Cases
Author: Hagentoft, CarlEric; Kalagasidis, Angela Sasic; AdlZarrabi, Bijan; Roels, Staf; Carmeliet, Jan; Hens ,Hugo; Grunewald, John; Max Funk; Rachel Becker; Dina Shamir; Olaf Adan; Harold Brocken; Kumar Kumaran; Reda Djebbar
of the HAMSTAD Projekt.
Moisture transport modeling
Water is the transport medium for salts. In dry material salt is immobile. Only the mobilization of salts through penetrating moisture and enrichment of salts during evaporation of water in a different area of porous masonry results in decay. Subsequently a detailed moisture transport model is a primary prerequisite for every salt transport model.
Moisture transport models describe the different transport processes of moisture in a porous medium as well as the retention of moisture and subsequently the interrelation between the material conditions (moisture content and mass) and the intrinsically thermodynamic environmental conditions (capillary pressure, relative humidity, etc.).
Fundamental moisture transport mechanisms in damp masonry
Building materials, stone and generally porous materials can absorb moisture in vapor and liquid form. Accordingly the transport mechanisms for liquid water and vapor are differentiated:
 Water vapor diffusion
 Convection of vapor in air currents
 Liquid water flow induced by differential water pressure
These transport mechanisms are described in the article "Moisture transport mechanisms".
Correlation between driving forces and moisture content
The correlation between a volume or mass based amount of e.g. moisture content and the intrinsic (and therefore volume independent) amount of relative humidity or capillary pressure are created by the moisture retention capacity. This is differentiated into
 The sorption isotherm and
 The moisture retention capacity (MRC)
The sorption isotherm is commonly defined for standard conditions and with that, constant temperature (hence the term isotherm) and establishes the correlation between moisture content and relative humidity Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \phi} . The moisture retention capacity establishes the correlation between the moisture content and capillary pressure Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle p_c} .
The moisture content can be measured as moisture mass over volume of material i.e. as moisture mass density Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho^{m_{w+v}}} or as water volume over material volume i.e. the moisture content Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \theta_\ell} .
Relative humidity and capillary pressure are interrelated (in saltfree! material) which is expressed in the Kelvin equation
Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \ln \phi = \frac{p_c}{\rho_w R_v T} }
The capillary pressure is defined as negative pressure in contrast to tension caused by suction which is defined as negative capillary pressure.
The article "Moisture retention in porous materials" further elaborates on this interrelation.
Phasechanging processes (without salt)
Even without the presence of salt in porous materials the following phase changes within the pore system need to be considered:
 Evaporation
 Condensation,
 Freeze
 Thaw
The related phase change enthalpies are always of critical issue here.
The article "Modeling of the phase changes between ice, water and vapor" discusses the common approaches to linked hygrothermal transport models.
Balance equations
Following the discussion and illustration of the individual processes involved in moisture transport and retention, these can be summarized in the equations below.
The general equation for moisture and ice mass (for the kinetic description of ice formation) is
Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \frac{\partial \rho^{m_{w+v}}}{\partial t} &=  \nabla \left( j^{m_{w}} + j^{m_{v}}_{dif\!f} + j^{m_{v}}_{conv} \right)  \sigma_{w \rightarrow \text{ice}}}
Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \frac{\partial \rho^{m_{\text{ice}}}}{\partial t} &= \sigma_{w \rightarrow \text{ice}}}
The second part of the balance equation and phase change term Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \sigma_{w \rightarrow \text{ice}}} can be ignored if the crystallization of ice is not of interest. The use of the term ‘isothermal process’ to describe moisture transport is rarely sufficient. It is not applicable where cooling during evaporation and warming during condensation form substantial processes in practice. Therefore, the moisture mass balance equation is complemented by the energy balance equation.
Thermal conductivity, thermal transfer and energy equation
Heat retention
Heat retention is differentiated into:
 Sensible heat i.e. heat which is stored in the kinetic energy caused by atom vibration or the movement energy of molecules
Every material has a specific thermal capacity, which together with density and temperature changes, result in changes to the stored sensitive heat: Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \Delta U = c_T \rho \Delta T}  latent Wärme, i.e. phase change enthalpies
Latent heat is commonly defined in relation to the state of aggregate e.g. the liquid aggregate state of water. During heating above boiling point steam absorbs latent heat which is released again during condensation. Analogously, latent heat is released during freezing which has to be reintroduced during the melting of ice. Latent heat is several magnitudes bigger than sensible heat.  Chemically bound heat
This energy is balanced in reaction equations e.g. during the crystallization or solution of salts.
The stored energies can be summarized in relation to the reference temperature Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle T_{Ref}} and produce, in the case of a salt free material, the energy density difference Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho^U  \rho^{U_{Ref}}} :
Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho^U  \rho^{U_{Ref}} = \rho_b c_T \left(T  T_{Ref} \right) \\ + \quad \quad \rho^{m_v} \left[ c_{T,v} \left(T  T_{Ref} \right) + h_v\right] \\ + \quad \quad \rho^{m_w} c_{T,w} \left(T  T_{Ref} \right) \\ + \quad \quad \rho^{m_\text{ice}} \left[ c_{T,\text{ice}} \left(T  T_{Ref} \right)  h_{\text{ice}} \right] }
Simplified, the energy density Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho^U} can be seen as a differential value and, therefore, an explicit specification of the reference temperature can be omitted.
Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rho^U = \rho_b c_T T + \rho^{m_v} \left( c_{T,v} T + h_v \right) + \rho^{m_w} c_{T,w} T + \rho^{m_\text{ice}} \left( c_{T,\text{ice}} T  h_{\text{ice}} \right) }
The reference phase here is the liquid phase, so that phase change enthalpy is added to the steam and the relevant enthalpy is deducted from ice. The first term of the added energy density is the energy stored within the material matrix.
Illustrative example
The following mental experiment can be conducted during consideration of the equation for energy density:
 The control volume is adiabatic so that the energy density is constant
 Water is to evaporate so that Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \Delta \rho^{m_v} =  \Delta \rho^{m_w}} , which results in a smaller liquid density and a larger vapor mass density.
 If the energy density equation is now adjusted for temperature, a lower temperature than before is obtained; this cooling through evaporation corresponds with our expectations.
Thermal transfer mechanisms
Different mechanisms of thermal transfer are considered:
 Thermal conductivity
 Thermal transfer through short/ longwave radiation (shortwave radiation is only of significance during the description of frame conditions whereas long wave radiation is important within constructions e.g. inside buildings).
 Convection of latent and sensible heat
The article "Mechanisms of thermal transfer" discusses models for the different thermal transfer mechanisms, in particular thermal conductivity Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle j^Q} and enthalpy transport Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle h j^m} of the different components.
Energy balance equation
The equation for the energy stored in a saltfree porous materials is Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \frac{\partial \rho^U }{\partial t} &=  \nabla \left( j^Q + h_w j^{m_{w}} + h_v j^{m_{v}}_{dif\!f} + h_v j^{m_{v}}_{conv} \right)}
No expanding or reducing terms for the phase change enthalpy are given in the energy equation. The change enthalpies are considered in the energy density (see paragraph above).
The enthalpy of dry air Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle h_a \rho^{m_a}} (dry air = all gas phase components except water vapor) is much smaller than the enthalpy of water vapor so that it is commonly neglected. The energy equation can be expanded accordingly in specific applications (e.g. compressed air drying of porous materials). In typical application scenarios the speed of air currents in porous substances can be neglected so that Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle h_v j^{m_{v}}_{conv} } can also be disregarded.
Modeling of salt transport
The previously described modeling approaches and equations initially apply to saltfree materials with the added restriction of a nonvariable material structure. If salts are also to be considered, several influential factors will need to be integrated into the model.
Transport mechanisms for salts/ions and modeling approaches
Generally, salts can only be transported within building materials in the presence of liquid phase i.e. capillary water. This transport happens through diffusion or convection.
 Diffusion describes the (masscentric) exchange of ions
 Convection describes the transport of dissolved salts together with the liquid phase
During the convection of a salt solution through porous material, different current paths through the pore network can result in higher salt concentrations fronts "forging ahead" or "lagging behind" in some areas. The observed spread of a concentration front, similar to a diffusion process, is termed dispersion. The effect of dispersion and diffusion is similar and difficult to differentiate in the presence of convection. In still liquids only diffusion will take place.
Phase changes
 Crystallization Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \sigma_{s\rightarrow p}} and solution Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \sigma_{p\rightarrow s}}
 Hydration Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \sigma_{p \rightarrow p,\text{hyd}}} and dehydration Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \sigma_{p,\text{hyd} \rightarrow p}}
 Deliquescence Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \sigma_{p\rightarrow s,\text{del}}}
The detailed description of the phase changing models is shown in the article "Modeling of the phase change reaction of salts ".
Influence of salts on moisture retention
To describe:
 Increased hygroscopic moisture absorption in the presence of salt loads
 Reduction in vapor pressure, water activity
 Surface tension + Kelvinequation
Influence of salts on moisture transport
 Viscosity
 Influence on density/gravitational forces
Crystallization within and outwith the pore network
 Decrease in pore space due to crystallization, resultant effect on reservoir capacity and transport mechanisms
 EfflorescenceModels
Literature
Note: The listed literature is still to be shown and reviewed
[Koniorczyk.etal:2008]Title: Heat and Moisture Transport in Porous Building Materials Containing Salt
Author: Koniorczyk, Marcin; Gawin, Dariusz
[Filter missing]