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Wood permeability is a macroscopic property resulting from complex pathways at the cellular level. In this study, high-resolution X-ray CT scans were performed during longitudinal imbibition in Norway spruce to investigate the role of wood rays and tracheids network in liquid migration. A customized setup was designed to maintain the bottom face of a wooden cylinder in contact with water, without vibration, despite the sample rotation required during each scan. The results highlighted a contrasted role of latewood and earlywood in water migration. Liquid imbibition was fast in latewood, because the bordered pits are less prone to aspiration than in earlywood, due to their smaller diameter and larger cell wall thickness. In earlywood, the migration pattern was complex due to pit aspiration that isolate tracheids clusters. Indeed, the images revealed a significant contribution of wood rays in the migration pathways. Consequently, tracheids were not progressively filled in the longitudinal direction from one tracheid to the next, but they were filled laterally with water coming from wood rays at different heights of the cells. This explained entrapped air bubbles in the lumens and led to a 3D heterogeneous liquid distribution.

The study of moisture migration in Norway Spruce may reduce the discrepancy between measurements and simulations of transfers in buildings and thus be helpful for improving their performance and durability. For this purpose, X-ray imaging was used to study moisture migration in Norway spruce during imbibition at tissue level. Moisture content kinetics in earlywood and latewood, based on X-ray attenuation, was observed and measured through rigorous protocol. The imbibition process was studied in both longitudinal and radial directions. The results showed an easier migration of liquid in latewood than in earlywood in the longitudinal uptake. In the radial direction, the impact of compression wood as a barrier and the role of the anatomical structure in the liquid migration were highlighted by the direct visualization of filled tracheids.

International audience; Perforations are one of the recognized geometrical features that contribute to liquid redistribution in corrugated sheet packings. Our experimental study focuses on a simplified but relevant configuration: a thin liquid film flowing on either side of a vertical plate with a circular perforation. We focus on the curtain mode when the liquid fills the perforation. Confocal chromatic imaging reveals a capillary ridge upstream of the perforation, an inertial ridge downstream and a varicose capillary wave standing on the liquid curtain. We show that the wavelength is selected such that the velocity of the wave both satisfies Taylor's dispersion relation and matches the curtain local speed. We examine the effect of perforation size, supply conditions and liquid properties on the curtain transition. Lastly, we propose a simple model based on a momentum balance that describes the effect of these parameters on the Reynolds number at which curtain forms.

This study proposes a DAEM (Distributed Activation Energy Model) approach to predict the chemical alterations of lignocellulosic biomass as a function of hydrothermal treatment conditions. The model is first tuned by an original device allowing the sample shrinkage to be continuously assessed during hydrothermal treatment in saturated water vapor up to 190 °C. The shrinkage dynamic is supplied in the DAEM model as an indicator of the degree of biomass conversion. A set of chemical analyses was performed at selected residence times and treatment temperatures to correlate this degree of conversion with the resulting chemical molecules. A set of functions was then derived from this database to correlate the degree of conversion with the components concentrations. Finally, a validation database was built with different combinations of temperature levels and residence times. The model was proved to be predictive on this new dataset.

International audience; During the thermochemical valorization of biomass feedstock, the assessment of biomass powder flowability is essential for the design of feeding, storage, and handling systems. Biomass handling in valorization processes require powders to be characterized for both consolidated and static conditions, encountered in storage facilities, or under free-surface dynamic flow conditions, present in feeding systems. This paper evaluated the flow properties of two different sizes of raw, mildly and intensively torrefied poplar powders, using shear tests and the evaluation of avalanching behaviors, in a rotating drum. The size and shape descriptors showed that the particle size distributions and particle morphologies of the six samples were determined by the coupled effects of torrefaction intensity and the sieving stage. According to the shear test results, although flowability trends are stress-dependent, coarse powders have better flow properties than fine powders. A flowability ranking was established, based on the parameters extracted from the flow dynamics inside of a rotating drum, including the tendency to form particle clumps, upper angle of stability, irregularity of avalanches, and the fraction of revolution for avalanches. The relationships among the macroscopic flow parameters, obtained from both characterization techniques and the powder characteristics (particle mean size, shape, and polydispersity) were examined using adjusted correlations. Overall, intense torrefaction appears to improve the flow behaviors of powders, under both free and consolidated flow conditions.

International audience; The evaluation of the flow properties of biomass powders is essential for the design of handling systems within a thermochemical valorization context. The Discrete Element Method (DEM) is a valuable tool for simulating the bulk behavior of granular materials that has rarely been used for biomass feedstocks. This work focuses on the numerical investigation of the flow of raw and torrefied biomass particles in a loose and dynamic conditioning using a rotating drum. The relevance of DEM parameters calibrated using bulk experiments (angle-of-repose, bulk density, retainment ratio) is tested by comparison with experimental data obtained using a rotating drum system. The calibrated DEM material model considers the elongated, submillimetric and cohesive nature of the biomass powder. Several flowability descriptors (Upper Angle of Stability, size of avalanches, fraction of revolution to trigger events and irregularity of the free surface) are evaluated using both experimental data and DEM simulations. DEM results reproduced well the experimental trends and distinguished between the different cohesive extent of the samples. DEM is therefore a relevant technique for assessing flowability of biomass powders in a non-consolidated dynamic flow. This paves the way for investigating the effects of particle characteristics on bulk flow, which are briefly discussed.

International audience; Flat panels are the most spread type of photobioreactors for studying light effects on a microalgae culture. Their low thickness, usually between 1 and 3 cm, aims at ensuring light homogeneity across the culture. Yet because optical density has to remain very low, studies are still limited to low cell density cultures. To alleviate this problem, even thinner photobioreactors can be designed. Nevertheless, thin flat panel reactors are very prone to induce high shear stress. This work aimed at designing a new millimeter thin panel photobioreactor to study light effects on Chlorella and Scenedesmus genera. We proposed a numerical workflow that is capable of assessing the shear stress intensity in such a reactor. The numerical predictions were validated at three different levels: 2D preliminary simulations were able to reproduce bubble commonly known behaviors; close to the nozzle, the predictions were successfully confronted to shadowgraphy experimental reference; at the mini bioreactor scale, experimental and numerical mixing were found to be close. After these throughout validations, shear stress intensity in the photobioreactor was calculated over 1000 Lagrangian tracers. The experienced shear stress was agglomerated at the population level. From the computed shear stress, it was possible to choose the minimal reactor thickness that would not hinder cell growth.

International audience; In the wooden construction, structural elements are particularly exposed to external climatic conditions during the construction phase. To ensure their durability over their expected lifespan, the required drying time have to be known after a rain shower and before closing the timber structures. This question, involving coupled heat and mass transfer, can be investigated through numerical tools. The aim of this work is to provide experimental data through X-ray imaging to validate the prediction potential of computational models simulating transfer in porous media. Two complementary methods were developed to access the water migration in Norway spruce. The first one allowed to quantitatively measure spatial distribution of moisture content over time. It is based on a rigorous treatment of the X-ray beam attenuation. The second method, using a high-resolution 3D reconstruction, highlighted the role of wood rays on liquid migration within the porous network.

This work focuses on non-Fickian diffusion in biosourced building materials. To study these memory effects in this kind of materials, an original and automatic device was conceived and built. It allows a series of samples to be submitted, simultaneously, to a perturbation of RH at its front face and to record the evolution of sample mass and of RH at its back face. When analyzed using a classical heat and mass transfer model, the experimental data depicts a delay between RH and moisture content, the RH evolution being too fast. To address this problem, a new formulation able to model those types of behaviors using memory functions was used. To test the predictive potential of the new formulation, these memory functions were not identified but taken from a previous work, where they were determined experimentally using minute samples. A full set of experiments performed with Low Density Fiberboard (LDF) and Medium Density Fiberboard (MDF) was simulated using this new formulation feed by the experimental memory functions. The good agreement found between simulated and experimental results proves the predictive potential of the new macroscopic formulation.

International audience; The process of aging wine in oak barrels causes various mechanisms that modify wine quality. This study is devoted to understanding the gas and wine transfers that occur through the barrels depending on ambient conditions. Four barrels were instrumented in a cellar to monitor the liquid's height and evaporation rates, the ullage space pressure, the content of dissolved oxygen in the wine and the environmental conditions. The liquid wine progressed slowly through the wood and reached a stabilized position after approximately 100 days. Evaporation was initially very limited and then achieved an average rate of 20 mL per day after linearization of the moisture content profile. The evaporation rate was directly impacted by the cellar conditions. The negative pressure inside the barrels, created following the loss of wine, was also affected by the cellar conditions. Above a certain negative pressure, an air percolation threshold was reached, implying the rapid entry of air into the barrel.

In this study, we use a novel method to determine the mass diffusion coefficients of hygroscopic materials. The experiment involves subjecting one face of the sample to relative humidity (RH) variations over time and measuring the RH at the back face. The imposed RH and temperature are measured during the experiment, and they are used as boundary conditions in a comprehensive computational code to address heat and mass transfer in porous media. This new method is utilized to characterize the following building materials: autoclave aerated concrete, plaster, poplar wood, pine wood, medium-density fiberboard and low-density fiberboard. This paper provides the details regarding the determination of the sorption isotherms of the selected materials that are required for the determination of mass diffusivity. The obtained electrical tortuosity values are compared with geometrical tortuosity values determined via microtomography and three-dimensional image processing. The agreement between the two tortuosity values is applicable only for porous media with narrow pore size distribution.

Vibration is a promising way to intensify the drying process through heating-up due to viscous dissipation, activation of internal liquid transfer and increase of external transfer. To better assess the possible contribution of these effects, we choose a multiscale approach. This paper is focused on the pore scale, simulated by a capillary tube partially filled with water subjected to sinusoidal vibrations. We studied the displacement of water inside this tube through image analysis. This configuration mimics the moisture transfer inside the pores of a porous media during drying. The experimental device developed in this study is applicable to a wide range of configurations, such as symmetrical or asymmetrical vibrations. Keywords: drying intensification, acoustic vibration, pore scale, symmetrical and asymmetrical signals, image processing.

International audience; This work proposes an original innovative in-house device able to characterize the evolution of viscoelastic properties of lignocellulosic products during hydrothermal pretreatment (water-saturated conditions up to 190 °C). The main objective is to assess the cooking phase before steam explosion. We designed and built a novel experimental device for analyzing the dynamic mechanical analysis of the biomass soaked in water or acidic water. The device allows the sample to be loaded/unloaded during the test with an accurate measurement of deformation and force despite the severe conditions. This system can perform the mechanical test on immersed samples under controlled pressure (up to 1.5 MPa) and temperature (up to 190 °C). Experiments were performed with poplar via harmonic and periodic tests. The experimental data collected on poplar allows the characteristic time constants of the thermo-chemical reactions to be obtained as a function of the operating conditions.

International audience; We experimentally study drops formed from a nozzle into an immiscible, cross-flowing phase. Depending on the operating conditions, drops are generated either in dripping or jetting mode. We investigate the impact of the continuous and dispersed phase velocities, dispersed phase viscosity, and interfacial tension on the drop generation mode and size. We find that a dripping to jetting transition (DJT) takes place at a critical inner Weber number, a function of the outer capillary and Ohnesorge numbers. Two jetting regimes occur depending on the phase velocity ratio. When the continuous phase velocity is significantly greater (respectively lower) than the dispersed phase velocity, jet narrowing (respectively widening) occurs. In jet widening, the critical inner Weber number depends little on the outer capillary number whereas in jet narrowing, it sharply decreases as the outer capillary number increases. We propose a comprehensive model to describe the DJT based on the attached drop equation of motion. The model satisfactorily predicts the DJT and the effect of the outer capillary number on the critical inner Weber number. It also well accounts for the drop diameter in jet narrowing.

We experimentally investigate single-drop formation from a vibrating capillary tube immersed in a second immiscible liquid. At set forcing frequencies, significantly smaller drops are generated when a threshold amplitude is reached. We show that a drop grows and resonates in first mode once the drop resonance frequency and the forcing frequency coincide. The drop then detaches by a stretching mode if its elongation ratio exceeds a critical value, function of the drop to pore diameter ratio. The detached drop diameter is well described by the Bisch et al. expression for bound drop resonance frequency. Otherwise, the drop grows, leaves the first-mode resonance range and detaches at a larger size by dripping. The threshold and drop diameters are well predicted by a simple LFHO-based model.

International audience; Drop generation from an axially vibrating nozzle exhibits a transition in drop diameter when varying the vibration amplitude. Below a threshold amplitude, forcing has essentially no effect on drop size and drops form in dripping mode. Above the threshold, drop size is controlled by forcing: drops detach at resonance, i.e., when the first eigenfrequency of the growing drop coincides with the forcing frequency. We experimentally study the impact of the nozzle inner diameter, dispersed phase flow rate, interfacial tension, and dispersed phase viscosity on this transition. Drop diameter is well correlated to the mode 1 eigenfrequency of Strani and Sabetta for a drop in partial contact with a spherical bowl. We propose a transient model to describe drop dynamics until detachment. The drop is modelled as a linearly forced harmonic oscillator, with the eigenfrequency of Strani and Sabetta. Since the dispersed phase does not wet the nozzle tip, an additional damping coefficient is introduced to account for the viscous dissipation in the film of continuous phase between the drop and nozzle surface. The model adequately reproduces the effect of the different parameters on the threshold amplitude.

International audience; This article presents an experimental device specially designed for the continuous measurement of mass and dimensions of samples submitted to convective drying. This experimental device consists of a magnetic suspension balance and an image acquisition system. The sample deformation is determined by image correlation using a custom software. The entire system is able to achieve accurate mass and dimension measurements over a wide range of temperature and relative humidity. To present the potential of this equipment, experimental data of highly deformable food products (potato and apple) submitted to several drying conditions are presented. The obtained results confirm that it is required to consider the actual sample surface to determine the mass flux. This allowed a first drying stage to be observed for potato. Clear differences in physical behavior of these two vegetable products were also observed, for example, an isotropic shrinkage behavior for potato and an anisotropic behavior for apple. These results were explained by the anatomical structure of the products.

International audience; This work proposes a new method for the determination of the mass diffusion coefficient in hygroscopic materials. The experiment consisted of submitting one face of the sample to a variation in time of the relative humidity (RH) and measuring the RH on its back face. The imposed RH and temperature were measured during the test and served as boundary conditions in a comprehensive computational code to solve heat and mass transfer in porous media. This model uses a physical engine embedded in the inverse procedure to determine the mass diffusion coefficient. Compared with classical methods, this new method has several advantages:It allows several samples to be measured simultaneously, simply by multiplexing the RH sensors.Accurate values can be obtained even when starting and ending out of equilibrium, which allows the characterization time to be drastically reduced.The external mass transfer coefficient has a negligible effect on the identified value.Nonstandard Fickian behaviors can be detected by the disagreement between the measured and simulated curves.The results show that the diffusivity obtained for spruce wood is in good agreement with those found with classical methods. In contrast, the fiber board results differed between the experiment and model, or yielded unrealistic values, which confirms the dual-scale nature of mass transfer that occurs in this kind of material.