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A thermal conductivity measurement method designed for wet porous materials applied to a wood fiber based thermal insulator


Y. Jannot1, V. Félix1, A. Degiovanni1 and C. Moyne1

1Laboratoire d'Energétique et de Mécanique Théorique et Appliquée, INPL, France

Keywords: heat transfer, mass transfer
property: thermal conductivity
material: wet porous material

Wood fiber based materials are increasingly used for buildings thermal insulation mainly because they are presented as ecological. Since wood is a hygroscopic material [1], their water content and thermal conductivity varies with the ambient air humidity. For an actual estimation of its insulating performance in real use conditions, it is thus important to measure precisely the thermal conductivity as a function of the ambient air humidity.

This paper presents experimental results of the thermal conductivity of a material containing more than 95% of wood fibers.

The thermal characterization was achieved using the centered hot plate method [2]: a plane heating element is set between two samples of the material and submitted to a heat flux step. The unheated face of the sample is maintained at a constant temperature. The temperature at the center of the element is recorded. The analysis of the residues between the experimental and the modelled temperature both for dry and wet samples emphasizes the influence of water transfer on heat transfer during the experiment. A new model has been established enabling temperature simulation, it takes into account coupled heat and mass transfers. The analysis of the residues obtained using the coupled model shows a significant improvement for the wet materials. It was found that the thermal conductivity values estimated with a stationary method such as the guarded hot plate or by the classical heat transfer model applied to the transient hot plate are closed to the values estimated using the coupled model with a maximum deviation of 5% for the tested material.

The sorption isotherm giving the product water content as a function of the ambient air humidity has been determined by the saturated salt method. This sorption isotherm finally enables to obtain the thermal conductivity as a function of the ambient air humidity.

References
  1. S. Raji, Y. Jannot, P. Lagière,J.R. Puiggali. Construction and Building Materials, 23 (2009)

  2. Y. Jannot, V. Félix, A. Degiovanni. Meas. Sci. and Tech., 21 (2010)

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