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A new formulation is proposed for describing the moisture sorption hysteresis in lignocellulosic products. This model introduces the concept of gripped-box. This concept allows complex experimental pathways to be fitted with only two adjustable parameters, including the closure of scanning loops without any pumping effect. The product history is embedded in a unique internal variable, which defined the actual window of interest. The formulation proposed in the present work can be easily implemented in existing computational softwares. Based on comprehensive experiments, the model parameters were defined for five lignocellulosic materials largely used in the building construction. [ABSTRACT FROM AUTHOR]

Non-Fickian diffusion in native and thermally modified wood was analyzed by the inverse method. A low quality of the identified diffusivity values was found for the diffusivity which was either constant or varying with water content. This was explained by the non-Fickian behavior. It was especially distinct for thermally modified wood for which an increased delay in obtaining the hygroscopic equilibrium was clearly shown. Such a delay was explained by time required for molecular reorganization to produce new sorption sites. This phenomenon was accounted to improve the physical model by modifying the convective boundary condition. A relaxation mechanism was used for this purpose with an adequate time constant. [ABSTRACT FROM AUTHOR]

We have developed a model on drying of two tropical woods of ayous ( Triplochiton Scleroxylon ) and frake ( Terminalia Superba ) coming from Cameroon forests. Some thermophysical parameters used in the model were experimentally obtained in this work while the remaining properties were from literature. A comparison is doing between numerical results of our model, these given by the Luikov’s model and experimental data. Numerical simulation results from the developed model give close agreement with experimental results. We note that Luikov’s model not gives a satisfaction results in the non-hygroscopic domain in the case of frake. The present model can be used to explain the drying phenomenon of these two tropical species and can be applied to others species when necessary thermophysical parameters of these species are known. In a future work, it is important to integrate the influence of anatomical direction on our numerical and experimental results. [ABSTRACT FROM AUTHOR]

Experimental measurements are often used to validate or calibrate numerical models. Many authors report good agreement between experimental measurements and numerical simulations of thermal behaviour of walls, and notice stronger discrepancy while assessing moisture transfers. One of the possible reasons for this discrepancy is the difficulty to measure precisely and robustly hygrothermal transfers under real climate variations. In order to check the robustness of hygrothermal measurements, in the present work records from two different experimental facilities are compared. Both have controlled indoor climate and similar, but not identical outdoor conditions (they are situated at about 400 km distance). The same vapour open and hygroscopic wall assembly is investigated on both cells. The results analysed show strong similarities in qualitative behaviour. Quantitative analysis using normalised values show good agreement in stable conditions. In strong dynamic conditions the analysis is more difficult; more work is needed in order to overcome the amplitude of boundary conditions. [ABSTRACT FROM AUTHOR]

To assist with the correct design of buildings, many computational models have been developed to assess the transient heat, air, and moisture (HAM) transfer within building walls. Validation of these computational models is essential to gain confidence in the codes. This paper provides datasets for validating 1-D heat and mass transfer models step by step, gradually increasing the complexity of a multilayer wall with hygroscopic components. The experiments were performed using a double climatic chamber. The climatic conditions on either side of the wall progress from the simplest boundary conditions (isothermal) to more complex ones with oscillations that mimic the mid-season period. To assess the hygrothermal response of the tested walls under such climatic conditions, the conditions of ambient air and the temperature and humidity profiles within the wall thickness are monitored during the test. This paper gives some detailed information on the chamber design, the instrumentation, the materials, and the climatic conditions so that other researchers can use the collected data for validation of their models or to build future test facilities. The results obtained with the test facility highlight the coupling that exists between heat and mass transfers across multilayer walls using hygroscopic components. The heat release by moisture adsorption is observed in the temperatures profiles. During the mid-season period, the results show the differences of the phase shift and the amplitude attenuation of the temperature and the water vapour pressure across the wall thickness. [ABSTRACT FROM AUTHOR]

This work is devoted to diffusion mechanisms in low-density fiberboard (255 kg m−3). Experiments were performed under unsteady state conditions (relative humidity step from 37% to 71%) with different thicknesses (half thickness ranging from 1 mm to 20 mm). The mass diffusivity was determined by inverse analysis from the experimental moisture content evolution, using a comprehensive macroscopic model of coupled heat and mass transfer. A clear failure of Fickian's law becomes evident regarding the effect of thickness. A dual-scale model, based on the concept of distributed microstructure models with coupled heat and mass transfer at both scales, was used to simulate the experiments. The large number of dual-scale simulations proposed in this work were also analyzed by the inverse method. These dual-scale simulation results were successfully confronted to the experiment. The good fit with the experimental data is obtained for a diffusivity of the microscopic phase (the storage phase) equal to 1.10−13 m2 s−1 for a fiber radius of 20 μ m. As the main recommendation, we advise that the dual scale effect can be neglected for this kind of fiberboard for a total thickness larger than some centimeters, depending on the panel density. This also means that this effect must be considered in material characterization or when capturing the buffering effect of the surface layers of the envelope. [Display omitted] • Transient diffusion performed on fiberboard samples for a wide range of thicknesses. • Inverse analysis using a macroscopic model highlighted the non-Fickian behavior. • A thorough dual-scale model explains and predicts the non-Fickian behavior. • A full dataset is proposed at both the macroscopic and microscopic levels. • A chart (effects of density and thickness) provided for practical recommendations. [ABSTRACT FROM AUTHOR]

This paper investigates the role of moisture content on the measurement of the thermal conductivity of wood-based low-density fibreboards (LDFs) used for building insulation. To that purpose, a computational code of coupled transfers was used to simulate the widely used Heat Flow Meter (HFM) method. To widen the range of study, the thermal conductivity is obtained with a mixing law model to account for the properties and fractions of the constitutive phases. This mix law model was fitted from thermal conductivity values reported for commercial LDFs. The prediction of the heat flow density during the HFM test was compared with the measurements for dry and wet LDF. Through a series of case studies, the simulations showed that, in the case of moist products, the thermal conductivity determined by the HFM method was overestimated by up to 20%. This overestimate depends on the density distribution, thickness, moisture content, and measurement parameters such as time and the equilibrium criterion. Recommendations are made to limit this error. [ABSTRACT FROM AUTHOR]

Abstract: A comprehensive heat and mass transfer computational model is used to analyse the intricate two-way coupling arising from the activated chemical reactions involved in the heat treatment of wood. The 2D version of a drying code known as TransPore is used to simulate the coupled heat and mass transfer phenomena. This code accounts for the internal pressure in the porous medium. The pyrolysis model describing the chemical reactions occurring in the main constituents within the cell walls of wood (cellulose, hemicelluloses and lignins) is derived using data taken from the literature. Refined computational strategies were required to address the two-way coupling between the heat and mass transfer and chemical mechanisms, including the thermal activation of the chemical reactions, together with the treatment of heat sources (or sinks) and the production of volatiles. The experimental set-up allows the overall weight loss, and the internal temperature and pressure at specific locations within the board to be determined during processing. The reported simulations highlight that the model is able to capture two particular phenomena observed during the heat treatment of wood: the double pressure peak due to water evaporation and volatiles production; and the temperature overshoot during the heat treatment phase. [Copyright &y& Elsevier]

• Extended hygrothermal characterization of unstabilized rammed earth. • Low influence of density and grain size distribution on sorption isotherm. • High influence of density on thermal conductivity and vapor resistance factor. • Same linear dependency of thermal conductivity on moisture content for different soil compositions. • Vapor transfer resistance factor for a large span of relative humidity. Although hygrothermal properties of rammed earth represent great potential for improving thermal comfort during hot summers, still little knowledge is available about this topic. The literature lacks complete sets of hygrothermal characteristics of rammed earth, although they are fundamental for heat and moisture transfer modeling necessary to predict earthen building performances. The present work reports the results of a large collaboration between four academic laboratories in order to obtain an extended hygrothermal characterization of rammed earth, focusing on the material used in a modern construction in Lyon, France. The aim of the article is to provide for the same kind of soil an extend characterization on a large span of relative humidity to provide complete set of material properties for heat and moisture transfer simulation. The hygrothermal measurements cover adsorption and desorption isotherm, intermediate isotherm loop, specific heat capacity, dry thermal conductivity, moisture-dependent thermal conductivity and water vapor resistance factor for a large span of relative humidity. In addition, the capillary rise test was performed to estimate the liquid transport coefficient for suction and redistribution. The results show that variations in the density of the samples have a low impact on sorption isotherm, while they have a non-negligible impact on thermal conductivity and water vapor permeability. Surprisingly, variations in the grain size distribution do not produce a significant modification in the sorption isotherm, while hysteresis slightly reduces when bigger grains are added to the soil composition. Approximation of empirical model coefficients for the determination of moisture-dependent thermal conductivity is proposed for rammed earth materials. [ABSTRACT FROM AUTHOR]

• Thermo-migration of water in wood investigated by in-situ micro-CT imaging. • An original setup was conceived to maintain the temperature gap during CT scanning. • The analysed scans show the dynamics of moisture migration from hot to cold sides. • Above the FSP point, liquid water appears close to the cold side as localized spots. • Simulations prove that the vapor density gradient is the relevant driving force. Wood used as a building material is exposed to non-isothermal and unsteady state conditions involving coupled heat and mass transfer within the porous media. These exchanges contribute to the regulation of relative humidity in a passive way and influence the energy performance of buildings. In order to characterize the importance of the hygrothermal behaviour of softwoods in this context, an experimental setup was developed to access to moisture fluxes and/or moisture content profiles in the presence of a known temperature difference imposed on both sides of the wood sample. This original setup was conceived to maintain the temperature gap during micro-tomography acquisition. Image processing allows the moisture content profiles to be obtained over time after changes of the thermal gradient. The simulation using a computational model of coupled heat and mass transfer confirmed that the gradient of vapor density is a relevant unique, "synthetic", driving force to account for moisture migration in non-isothermal conditions. [ABSTRACT FROM AUTHOR]

• Beech wood torrefaction at different conditions using comprehensive instrumentation. • Heat treatment heterogeneity can be assessed by water vapour sorption measurements. • The data-set allowed a complete computational code to be validated. • Torrefaction schedule should be adapted to the particle size and its initial MC. • Treatment conditions meeting users' expectations found by probabilistic optimization. This paper uses a comprehensive computational model to propose optimal wood torrefaction conditions by probabilistic optimization. Its main outcome is to propose tailor-made heat treatment conditions (temperature levels-duration of mild pyrolysis at temperature levels ranging from 200 to 300 °C) to meet users' expectations in terms of overall mass loss, duration and homogeneity of treatment. To this purpose, beech wood boards were torrefied with a usual 3-steps treatment schedule (drying, heating and cooling) under contrasting configurations in a well-instrumented device. The heterogeneity of the treatment within the wood sample was assessed through X-ray attenuation profile and water vapour sorption isotherm. These results allowed the model to be validated. In particular, it predicts the evolution of the mass loss and internal temperatures with good accuracy, including the temperature overshot. The results highlight the need to adjust the heat treatment schedule to each input parameter such as the wood piece dimensions and its initial moisture content or density, in order to limit the effect of exothermic reactions. The torrefaction model was then embedded in a probabilistic optimization process. A case study demonstrates the ability of the model to propose an alternative 3-steps treatment schedule able to reach the target mass loss while controlling the temperature overshot within the wood piece. [ABSTRACT FROM AUTHOR]