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

Modeling of hygrothermal transfers through a bio-based multilayered wall tested in a bi-climatic room

International audienceA bio-based multilayered wall has been developed in the framework of the European ISOBIO project. A key point was to be able to perform proper simulations of the hygrothermal transfers occurring through the wall local predictions are of first importance to characterize the behavior of the wall and thereafter its ability to ensure comfortable hygrothermal conditions inside buildings. A previous study proved that the conventional assumption of an 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 slow sorption kinetics occur. In the present study, an improved expression of the local kinetics is proposed and validated by sorption experiments. Then, Moisture Buffer Value tests are performed (Nordtest project's protocol). The simulations are adjusted to these measurements by using the inverse method in order to refine the knowledge of some critical parameters. Depending on the studied materials, the local kinetics constants are between 0.15 and 14 day-1/(kg.m-3). Finally, the ISOBIO wall is studied in a bi-climate room under a wide range of operating conditions. Simulations carried out with the conventional approach (TMC) and the local kinetics approach (TMCKIN) are compared to measurements this clearly shows that the latter is able to predict well the relative humidity dynamics while the former underestimates it by a factor up to 66%. These results highlight the relevance of the new expression of the local kinetics and its ability to describe well the local hygric dynamics is certainly an achievement

Effects of Initial Conditions on Simulations of Hygrothermal Transfers Through a Bio-Based Multi-layered Wall Subjected to a Real Climate

International audienceThis modeling study investigates the effects of the initial conditions on a bio-based multi-layered wall by hygrothermal simulations. Such a study cannot be found in the relevant literature. In a first step, the work consists in numerically generating different climate histories for the studied wall, using two different real climates: a winter and a spring climate. Then, the wall preloaded with these two different climate histories and therefore different hygric and thermal initial conditions is subjected to several cycle of 18 days of the spring climate. This study highlights the significant effects of the initial conditions on the subsequent simulations: strong differences are predicted about relative humidity and water content. Many cycles of 18 days of the spring climate have to be applied to obtain a hygric convergence of the two simulations, i.e., to erase the wall hygric history. The analytic analysis of the results leads to define characteristic times of hygric dependency of about 35–40 days. It appears that the hygric predictions of the simulations become independent of the initial conditions after about three times these characteristic times, i.e., about 3.5 months. Comparatively, the thermal history takes only a few days to be erased. © 2022, The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd

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

Preparation and characterization of isotactic polypropylene​/zinc oxide microcomposites with antibacterial activity

In this study, we investigated the influence of ZnO particles obtained by spray pyrolysis with submicron dimensions on the structure, morphology, thermal stability, photodegradation stability, mechanical and antibacterial properties of isotactic polypropylene (iPP)/ZnO composites prepared by melt mixing. The results of the morphological analyses indicate that, despite the surface polarity mismatch between iPP and ZnO, the extrusion process and the unique characteristics of the utilized particles allow a composite with a fair distribution of particles to be obtained, although some agglomeration phenomena can occur, which primarily depends on the composition of the composite. The addition of ZnO particles imparts significant improvements on the photodegradation resistance of iPP to ultraviolet irradiation, which confirms that ZnO particles act as screens for this type of radiation. The thermal stability of the iPP/ZnO composites is improved with respect to that of neat iPP and increases with the content of ZnO. The iPP/ZnO composites exhibit significant antibacterial activity against Escherichia coli. This activity is dependent on exposure time and composition