Choix d'un modèle de pyrolyse ménagée du bois à l'échelle de la microparticule en vue de la modélisation macroscopique

Par définition la pyrolyse ménagée du bois est la décomposition physique et chimique de matières organiques sous l'action de la chaleur et en absence d'oxygène. Comprendre ce phénomène passe d'abord par l'identification des mécanismes réactionnels et la détermination des paramètres cinétiques mis en jeu lors de la dégradation thermique du bois et de ses constituants majeurs, c'est-à-dire cellulose, hémicelluloses et lignines. La richesse et la diversité des résultats issus de la littérature spécialisée rendent compte de la difficulté à expliquer ces cinétiques complexes. Cette étude se propose de contribuer de façon innovante au bois traité à haute température en proposant une approche théorique à l'échelle de la microparticule en vue d'une modélisation macroscopique des phénomènes couplés « thermiques, chimiques et physiques ». En restituant les résultats d'une étude bibliographique poussée, nous avons fait le choix de retenir une approche analytique cherchant à séparer les trois composés principaux du bois et à caractériser chacun d'entre eux séparément

WAVET*, a custom device able to measure viscoelastic properties of wood under water saturated conditions (*WAVET : Environmental Vibration Analyser for Wood)

International audienceThis work presents an original experimental device conceived to characterise the viscoelastic properties of wood. Classically, the dynamic mechanical analysis of wood is performed using a commercial apparatus like a DMA (Dynamic Mechanical Analyser). However, when analysing wood with this type of apparatus, many problems related to the hygroscopic behaviour and the orthotropic structure of wood may be encountered. This is why an original apparatus perfectly adapted to the wood features has been developed. The WAVET is able to measure the viscoelastic properties of wood samples under water-saturated conditions, in the temperature range of 5°C to 95°C at frequencies varying between 0.005 Hz and 10 Hz. Samples are tested in a cantilever configuration. The whole experiment has been designed to withstand the severe conditions of temperature and humidity. At the same time, an analytical model based on Kelvin's elements has been developed. This model is able to correct experimental measurements performed close to the resonance frequency. Results obtained for beech samples in radial and tangential directions using the WAVET and a commercial apparatus (DMA 2980 TA Instruments) are compared and discussed. This comparison underlines the relevance of the WAVET device

Effects of Composite Processing Methods on Wood Particle Development and Length Distribution: Consequences on Mechanical Properties of Wood-Thermoplastic Composites

The relationship between structure and properties of high-density polyethylene (HDPE) filled with wood particles and processing techniques—injection molding, compression molding, and extrusion—was investigated. Wood particles were hammer-milled, sieved, and compounded into pellets at 35% by weight with HDPE using a twin-screw extruder. Coupling agent (ethylene-maleic anhydride copolymer) was added at 2% by wood filler weight. The pellets were used to produce test samples using the three processing techniques. The sensitivity of jack pine and several other wood particles (eastern white cedar, black spruce, and jack pine bark) to composite processing was analyzed. Bark particles showed higher propensity to generate fines than wood particles, possibly because of a higher thermal sensitivity. The major reduction in mean particle length was found to occur in the compounding process. Extrusion and injection molding contributed to particle length reduction to a lesser extent. Conversely, compression molding did not cause significant damage to wood particles. Stiffness and strength increased linearly with weight-averaged length

A physical interpretation of the use of fractional operators for modelling the drying process

[EN] Fractional order derivatives provide useful alternatives to their integer order counterparts due to their ability to model memory and other properties of the porous medium, such as nonlocal behaviour. These phenomena are driven by the constrained interactions within the complex and non-homogeneous microstructures evident at the pore scale. In this work, we investigate the suitability of time and space fractional operators for modelling drying processes and provide a physical interpretation of these operators. At first, the concept and the general formulation in the case of a 1-D finite domain is summarised. Then a selection of simulations allowed us to analyse the physical effects of these operators on the solution. In particular, we elucidate:(I )the ability of these operators to break the fundamental relationship between mean square displacement and time in the simple example of diffusion in an open space, (ii) the caution to be taken with the formulation of boundary conditions and source terms to obtain consistent balance equations, (iii) the effect of fractional in space diffusion as a way to alter the MC profiles compared to standard diffusion, therefore potentially avoiding the dependence of the diffusivity on the variableThis work was supported financially by a visiting Professorial Fellowship that enabled Turner to work at CentraleSupélec, Université Paris-Saclay, France for a period of three months. Both authors acknowledge the financial support for this research received through the Australian research Council (ARC) grant DP150103675.Perre, P.; Turner, I. (2018). A physical interpretation of the use of fractional operators for modelling the drying process. En IDS 2018. 21st International Drying Symposium Proceedings. Editorial Universitat Politècnica de València. 545-552. https://doi.org/10.4995/IDS2018.2018.7885OCS54555

On the importance of heat and mass transfer coupling during characterization of hygroscopic insulation materials

[EN] The present work is focused on mass transfer characterization of hygroscopic materials used for insulation, such as Low Density Fibreboards. Due to their particular morphology, these panels present a very high mass diffusivity in the connected gaseous phase and a very low thermal conductivity. This combination of properties exacerbates the coupling between heat and mass transfer in transient state. Based on experimental data obtained with an original set-up and relevant simulations performed using a comprehensive physical formulation, a throughout vision of this question is proposed in the present study. In particular, we emphasize on: - The impressive change in core temperature in terms of magnitude and duration, - The great impact of the internal temperature gradient, which slows down mass diffusion, - The dramatic error on mass diffusivity value if the coupling is ignored.Perre, P.; Challansonnex, A. (2018). On the importance of heat and mass transfer coupling during characterization of hygroscopic insulation materials. En IDS 2018. 21st International Drying Symposium Proceedings. Editorial Universitat Politècnica de València. 537-544. https://doi.org/10.4995/IDS2018.2018.7884OCS53754

Non-Fickian diffusion in biomaterials

[EN] The knowledge of water vapour diffusion in biomaterials is very important in several fields of application (drying, building materials, food packaging…). The process of vapour water transfer in biomaterials is usually described by Fick’s law of diffusion. Nevertheless, sometimes the experimental data of transient experiments in biosourced materials do not respect the standard Fickian model. The precise determination and formulation of this non-Fickian behaviour is therefore an important topic. This work presents a new methodology to quantify the non-Fickian diffusion. Based on memory functions and imbedded in a comprehensive heat and mass transfer computational model, the method is robust and prone to be applied to different biomaterials. Examples of application are proposed in the case of biofilms and lignocellulosic materials.Almeida, G.; Perre, P. (2018). Non-Fickian diffusion in biomaterials. En IDS 2018. 21st International Drying Symposium Proceedings. Editorial Universitat Politècnica de València. 2043-2052. https://doi.org/10.4995/IDS2018.2018.7943OCS2043205

Reconciling biodiversity and carbon stock conservation in an Afrotropical forest landscape

Protecting aboveground carbon stocks in tropical forests is essential for mitigating global climate change and is assumed to simultaneously conserve biodiversity. Although the relationship between tree diversity and carbon stocks is generally positive, the relationship remains unclear for consumers or decomposers. We assessed this relationship for multiple trophic levels across the tree of life (10 organismal groups, 3 kingdoms) in lowland rainforests of the Congo Basin. Comparisons across regrowth and old-growth forests evinced the expected positive relationship for trees, but not for other organismal groups. Moreover, differences in species composition between forests increased with difference in carbon stock. These variable associations across the tree of life contradict the implicit assumption that maximum co-benefits to biodiversity are associated with conservation of forests with the highest carbon storage. Initiatives targeting climate change mitigation and biodiversity conservation should include both old-growth and regenerating forests to optimally benefit biodiversity and carbon storage

Architectures for Cognitive Radio Testbeds and Demonstrators – An Overview

Wireless communication standards are developed at an ever-increasing rate of pace, and significant amounts of effort is put into research for new communication methods and concepts. On the physical layer, such topics include MIMO, cooperative communication, and error control coding, whereas research on the medium access layer includes link control, network topology, and cognitive radio. At the same time, implementations are moving from traditional fixed hardware architectures towards software, allowing more efficient development. Today, field-programmable gate arrays (FPGAs) and regular desktop computers are fast enough to handle complete baseband processing chains, and there are several platforms, both open-source and commercial, providing such solutions. The aims of this paper is to give an overview of five of the available platforms and their characteristics, and compare the features and performance measures of the different systems

Vers une méthode de conception HYGRO-thermique des BATiments performants : démarche du projet HYGRO-BAT

Cet article présente la démarche mise en oeuvre dans le projet collaboratif ANR Hygrobat, qui vise à donner des outils pour quantifier de manière fiable l'impact des transferts de masse sur les transferts de chaleur dans les parois de bâtiments comprenant des matériaux fortement hygroscopiques. Les matériaux sélectionnés dans ce projets (fibre de bois, bois massif, OSB) ont été soigneusement caractérisés. Leurs propriétés hygrothermiques ont été mesurées en laboratoire, puis ils ont été mis en oeuvre dans des parois, soumises à des conditions aux limites de variées. L'originalité de l'étude réside dans : (i) la démarche "pas à pas" de complexité croissante, partant des caractérisations d'un seul matériau en conditions stationnaires, jusqu'aux mesures sur une paroi multicouche en climat réel ; (ii) les vérifications croisées de mesures expérimentales, qui sont effectuées dans deux des laboratoires partenaires du projet, parfois sur des dispositifs différents ; (iii) association des benchmarks expérimentaux et numérique