Floran Pierre

Determination of the Mass Diffusion Coefficient Based on the Relative Humidity Measured at the Back Face of the Sample During Unsteady Regimes

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.

A novel device to measure gaseous permeability over a wide range of pressures: characterisation of slip flow for Norway spruce, European beech, and wood-based materials

Abstract A novel device was conceived and built to measure the apparent gaseous permeability of porous media over a large range of permeability values (10−10–10−18 m2) and mean pressures (from 1 bar down to ca. 40 mbar). An extensive series of experimental data are presented and analysed for various porous media: (1) Norway spruce (Picea abies) and European beech (Fagus silvatica) in the three anatomical directions, and, for comparison, (2) three simpler porous media, i.e. an autoclaved aerated concrete (AAC, light concrete) and two wood-based panels. For all porous media, the intrinsic permeability, the gas slippage factor, and the effective pore size were determined from the variations of the apparent permeability as a function of mean pressure. These results are in good agreement with those of previous studies for spruce and bring new insights for beech and wood-based materials, in general. For all products, the effective pore sizes identified with the new instrument are closely linked to the medium morphology. In particular, it was found that in spite of the huge anisotropic ratios between wood’s longitudinal and tangential directions, the identified pore size is similar and corresponds to anatomical features: openings in margo for spruce and scalariform perforation plates for beech. Besides, the pore size identified for beech in the radial direction suggests that radial permeability is most probably controlled by the openings in ray cells (either pits or intercellular voids).

Physical behavior of highly deformable products during convective drying assessed by a new experimental device

ABSTRACT 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.

Reduction of biomass resilience by torrefaction: apparent stiffness during failure (ASF) and specific failure energy (SFE) assessed by a custom impact device

Abstract The present work focusses on the loss of resilience of torrefied wood as an indicator of its grindability. An impact device was developed to evaluate the mechanical behaviour of wood at high compression rates with a particular emphasis on the surface area of the particles produced. It allows the energy determination needed to produce particles without a traditional grinding test. Pine (Pinus pinaster) and oak (Quercus robur) were tested in radial (R) and tangential (T) directions and for various torrefaction intensities. With increasing heat intensity, the material becomes more fragile and finally loses its fibrous character, which increases the number of peak events on the stress/strain curve and significantly reduces the deformation energy. Two indices were derived from the experimental results: the apparent stiffness during failure (ASF) and the specific failure energy (SFE). These criteria allow the quantification of the loss of mechanical strength due to torrefaction, as well as the surface area increment of particles for a given grinding energy.