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Determination of the Liquid Water Diffusivity from Transient Moisture Transfer Experiments

Department of Civil Engineering, K.U. Leuven, Kasteelpark Arenberg 51, B-3001 Leuven, Belgiumjan.carmelie{at}bwk.kuleuven.ac.be

H. Hens

S. Roels

Department of Civil Engineering, K.U. Leuven, Kasteelpark Arenberg 51, B-3001 Leuven, Belgium

O. Adan

H. Brocken

TNO Building and Construction Research, P.O. Box 49, 2600 AA Delft, The Netherlands

R. Cerny

Z. Pavlik

Faculty of Civil Engineering, Department of Structural Mechanics, Czech Technical University, Thakurova 7, CZ-16629 Prague 6, Czech Republic

C. Hall

Centre for Materials Science and Engineering, University of Edinburgh, The King’s Buildings, Edinburgh EH9 3JL, UK

K. Kumaran

Institute for Research in Construction, National Research Council Canada, 1200 Montreal Road, Ottawa ON K1A 0R6, Canada

L. Pel

Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands

The Boltzmann transformation method is used to determine the liquid water diffusivity from moisture content profiles as measured in a capillary water absorption experiment. An inter-laboratory comparison for analyzing the reliability of the determination method showed that the inaccuracy in the liquid water diffusivity is caused by scatter in the transformed data and by uncertainty in the boundary conditions at the intake surface and ahead of the steep moisture front. A methodology is proposed based on (1) the evaluation of the validity of the diffusion approach, (2) a simplified handling of the boundary conditions, (3) smoothing of the scattered data and (4) the evaluation of the quality of the determined liquid water diffusivity. For HAM (Heat-Air-Moisture transport) calculations values of the liquid water diffusivity for moisture contents higher than the capillary moisture content are disregarded. The liquid water diffusivity can be described by an exponential function limited at a lower moisture content bound.

To describe the moisture diffusivity including liquid water and water vapour transports, a new parametric description of the moisture diffusivity is presented, which shows sufficient flexibility both in the hygroscopic and overhygroscopic ranges. When permeability is calculated from diffusivity, the permeability should monotonically increase with decreasing capillary pressure. In the hygroscopic region it should coincide with the measured water vapour permeabilities.

Key Words: liquid water diffusivity • Boltzmann transformation • determination method • moisture permeability

Journal of Thermal Envelope and Building Science, Vol. 27, No. 4, 277-305 (2004)
DOI: 10.1177/1097196304042324


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