Hygrothermal effects and moisture kinetics in a bio-based multi-layered wall:Experimental and numerical studies

International audienceA bio-based multi-layered reference wall has been developed within the framework of the European ISOBIO project. One of the key points of this project was to be able to perform proper simulations of the hygrothermal transfers occurring inside such walls. Previous published investigations, also performed in the framework of this project, have demonstrated that the classic assumption of instantaneous equilibrium between local relative humidity and water content according to the sorption isotherm is not relevant for bio-based porous materials, where, in practice, a rather slow kinetics of sorption occurs. The theoretical background developed in this previous study is used here to determine the kinetic constants of the bio-based construction materials and to perform 1D hygrothermal simulations. The kinetics constants are determined thanks to a 1D cylindrical tool based on the local kinetics approach, validated against several experiments of sorption. Then, heat and hygric transfers recorded on a demonstrator building (The HIVE, Wroughton, UK) are analyzed and are simulated using two modeling tools: TMC based on the Künzel approach and TMCKIN based on the local kinetic approach. From the simulations, the local kinetics improves the small timescale RH dynamics. The comparison with measurements performed in the demonstrator confirms the relevance of the local kinetics approach

Determination of the elongational properties of polymers using a mixed numerical-experimental method

International audienceThe biaxial properties of materials such as rubbers or polymers are often difficult to identify, and generally a simple isotropic behaviour derived from classical 1D tensile test is considered. However, biaxial properties are useful for the simulation of plastic-processing operations such as blow moulding [1] or thermoforming. As previously reported, the rheological behaviour and the mechanical properties of rubbers and polymers can be obtained by using a bubble inflation rheometer [2-4], or multi-axial tensile test [5]. In this work, experimental data provided by optical measurements on tensile tests and bubble inflation tests are coupled with Finite Element Method simulations for identifying the rheological behaviour. This work is actually based on natural rubber and will be extend to thermoplastic (PP and PET) materials. A bubble inflation rheometer has been developed in the laboratory. It allows to blow under r r controlled pressure rubber or thermoplastic membranes [2]: a circular membrane, clamped at the rim, is inflated by applying air pressure to its bottom face (see Fig. 1, left). In the case of thermoplastic, a heating step is necessary before applying the pressure. The heating can be performed by air convection, by conduction (heating cartridge at the rim) and by IR radiation. FIGURE 1. Bubble inflation rheometer (detail, left) and shape contour extraction (right). Two experimental optical techniques based on non-contact measurement by CCD cameras have been developed for in situ measurements: (i) images acquisition of the 2D projection of the bubble is done during the inflation process, giving shape contours versus inflated pressure (see Fig. 1, right); (ii) Digital Image Stereo-Correlation (DISC) [6] is applied using a calibrated stereo rig in order to obtain the three-dimensional description of the strain fields on the surface of the bubble. f To perform DISC directly on the rubber bubble, several difficulties are to be solved (large level of deformation, semi-transparent aspect of the materials, t lighting, etc.). In addition, tensile tests have been performed using DISC due to the high level of strain. Tensile test performed on standard specimen give, on the one hand, the stress/strain curve. On th

Water Transport in Bio-based Porous Materials A Model of Local Kinetics of Sorption—Application to Three Hemp Concretes

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Rarefied Pure Gas Transport in Non-isothermal Porous Media: Validation and Tests of the Model

Viscous flow, effusion, and thermal transpiration are the main gas transport modalities for a rarefied gas in a macro-porous medium. They have been well quantified only in the case of simple geometries. This paper presents a numerical method based on the homogenization of kinetic equations producing effective transport properties (permeability, Knudsen diffusivity, thermal transpiration ratio) in any porous medium sample, as described by a digitized 3D image. The homogenization procedure -- neglecting the effect of gas density gradients on heat transfer through the solid -- leads to closure problems in R^6 for the obtention of effective properties ; they are then simplified using a Galerkin method based on a 21-element basis set. The kinetic equations are then discretized in R^3 space with a finite-volume scheme. The method is validated against experimental data in the case of a closed test tube. It shows to be coherent with past approaches of thermal transpiration. Then, it is applied to several 3D images of increasing complexity. Another validation is brought by comparison with other distinct numerical approaches for the evaluation of the Darcian permeability tensor and of the Knudsen diffusion tensor. Results show that thermal transpiration has to be described by an effective transport tensor which is distinct from the other tensors