moisture transport in porous materials

Transport phenomena in porous media can be separated in to: diffusion, saturated viscous flow and capillary (liquid) transport.

DIFFUSION at iso thermal conditions is driven by a difference in vapor pressure, and The 3-D diffusion process can be describe by Fick's law:

flow (g) = - d*div(pv): d=moisture permeability; g density of moisture flow rate.

VISCOUS SATURATED FLOW is driven by a difference in pressure. The flow depends both on the geometry of the porous material and on properties of the fluid itself. The saturated flow is often described by Darcy's law

Straube, J. F. and Burnett, E. F. P., 1998

Drainage is a liquid flow mechanism driven by gravity. Capillary transport, driven by gradients in suction stress, is another possible mechanism for transporting liquid moisture, although capillarity transport can only redistribute moisture, not remove it, from the enclosure. Vapor diffusion, driving by gradients in the vapor content of the air, and convection, driven by air pressure differences, are the primary mechanisms transporting vapor.

Model porous media by individual grains (such as sand): Garboczi, E. J., D. P. Bentz, and K. A. Snyder, Ed., An electronic monograph: modelling and measuring the structure and properties of cement-based materials

"Moisture Transport Mechanisms (http://www.durable-wood.com/pdfs/moisturebulletin-e.pdf)

The migration of moisture into and through building assemblies generally takes place by any of four moisture transport mechanisms: liquid flow, capillarity, convection or diffusion. Liquid flow and capillarity into the building envelope occur primarily with exterior source moisture such as rainwater and groundwater, whereas movement of moisture into the building envelope by diffusion or air movement can occur with interior or exterior source moisture.

Liquid flow is the movement of water under the influence of a driving force (such as gravity, or suction caused by air pressure differences).

Capillarity is the movement of liquid water in porous materials resulting from surface tension forces. Capillarity, or capillary suction, can also occur in the small space created between two materials.

Air movement refers to the movement of water vapour resulting from air flow through spaces and materials.

Diffusion is the movement of water vapour resulting from a vapour pressure difference.

Of the four transport mechanisms, liquid flow and capillarity are the most significant. Thus, it is not

surprising that rain penetration and groundwater control has been the primary focus of builders and

designers for generations. Air movement and vapour diffusion are important, though less significant and

obvious contributors to moisture problems."

Tidbits

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Characterising porous media, by Webber, J.B.W., 2000

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Infrared thermography as a tool for studying the movement of water through some building materials. Part 1. Capillary moisture, by Gayo, E., Palomo, A. and Macias, A., 1993

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Kinetics of hygrothermal treatment of capillary porous materials, by Kharin, V. M., G. V. Agafonov, et al., 2001

genral descriptioin and equations

Methods of measuring the moisture diffusivity at high moisture levels, by Janz, M., 1997

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Model for the calculation of combined chemical reactions and transport processes and its application to the corrosion of mineral-building materials: Part I. Simulation model, by Schmidt-doehl, F. and Rostasy, F. S., 1999

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Moisture transfer in porous materials exposed to combined humidity and temperature gradients, by ASHRAE, 1997

survey of status before 1985

The theory of heat and moisture transfer in porous media revisited, by De Vries, D. A., 1987