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This review paper was written on the occasion of the 40th anniversary of the Drying Technology Journal. Interestingly, most of the progress made in the domain of drying modeling and simulation has occurred over the last 40 years. Indeed, 40 years ago, the comprehensive formulation of coupled heat and mass transfer in porous media was available; however, its potential use for computational simulation was only in its nascent stage. This is therefore an ideal time span in which to take a full historical context, to present the state-of-the-art and to give insights into its future trajectory. All aspects are deliberately presented in a pedagogical manner with many illustrations to make the text accessible to young scientists. However, for the benefit of experts, detailed formulations are provided in the annexes. The main points addressed in this work include: The description of the comprehensive macroscopic formulation, An historical perspective of the progress made in computational simulation, The contribution of modeling in parameter characterization, The mechanical aspects of drying: strain and stress induced by drying, including the case of large deformations, The impact of non-local equilibrium, The promising domain of the digital twin in parameter tuning and control/command. [ABSTRACT FROM AUTHOR]

The paper brings new insights into nonsymmetrical convective drying, a configuration in which stresses induce a global change of the section shape of a wooden board with five insulated faces. Two experimental devices that allow to test simultaneously “twin” wooden boards have been developed: the first board is free and its curvature is collected vs. time; the second one is constrained and the force required to keep it flat is collected vs. time. Convective drying tests have been performed on radial specimens of spruce, 5 and 15 mm thick at 45 and 65°C. The results are commented and compared with simulation runs. For this purpose, a 1-D mechanical formulation is embedded in the computational code of heat and mass transfer TransPore. The objective of the comparison between numerical and experimental results is a better understanding of the coupled phenomena and the identification of some transfer and mechanical parameters. [ABSTRACT FROM AUTHOR]

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]

The bark-water relation and the moisture transport in bark are essential for the living tree but also for the industrial utilization of bark. Thus the bark-water relation of beech (Fagus sylvatica ) and spruce (Picea abies ) have been studied in the present work. The sorption isotherms were determined at 35°C for both wood and bark and the water diffusivity was measured under steady state conditions (cup method) and under unsteady state conditions, i.e. via sorption test. The sorption isotherms were calculated based on the Hailwood-Horrobin (HH) model. It was observed that beech wood and bark have a similar hygroscopic behavior, whereas spruce bark is more hygroscopic than spruce wood in all the relative humidity (RH) ranges. The radial diffusivity was in the range 1.2 ⋅ 10-11-2.2 ⋅ 10-11 m² s-1 for spruce and beech bark, respectively, when the driving force of the diffusion is the gradient of bound water. [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]

Sorption behavior of four tropical woods coming from Cameroon is studied using a Dynamic Vapor Sorption-Intrinsic apparatus. Sorption isotherms at 20°C and 40°C of Triplochiton scleroxylon (obeche), Entandrophragma cylindricum (sapele), Sterculia rhinopetala (lotofa) and Terminalia superba (frake also called limba) are compared to those in the literature with methods using saturated salts for lotofa and frake with satisfactory results. Experimental values are fitted using two sorption isotherm models. Sorption hysteresis is greatest in sapele and least in frake. When temperature increases, differences in the hysteresis of sorption among the woods decreases. Hystereses of all studied woods ranged from 0,5 to 3%. Our studied woods have lower equilibrium moisture content probably caused by the higher values of extractives content. The water sorbed molecules on multilayer during both adsorption and desorption at 20°C and 40°C is greater in the case of frake and lower in the case of lotofa. The water sorbed molecules on multilayer of sapele is greater than those of obeche. The predominance of multilayer over monolayer of our woods is from 0,2 to 0,4 relative humidity in adsorption phase, and from 0,3 to 0,4 relative humidity in desorption phase. [ABSTRACT FROM AUTHOR]

This study provides the sorption isotherm, its hysteresis and their mass transfer properties of four Central Africa Tropical woods widely used for building construction: frake (Terminalia Superba), lotofa (Sterculia Rhinopetala), sapele (Entandrophragma Cylindricum) and ayous (Triplochiton Scleroxylon). Characterization of these four species in particular and Central Africa tropical woods in general were necessary to develop conservation and treatment of wood after first transformation using the drying. Also, moisture transport on wooden material used such as wall buildings can be facilitating to found the thermal comfort. Measurements of isotherms were performed using a dynamic vapor sorption apparatus (Surface Measurement Systems) at 20 and 40°C with air relative humidity ranged from 0% to 90%. Mass diffusivity was determined in steady state using a specific vaporimeter. Air permeability was determined using a specialized device developed to measure over a wide range of permeability values. Permeability and mass transfer properties were determined in the tangential direction with a "false" quartersawn board (sapele and lotofa) and in the radial direction with a flatsawn board (ayous and frake). Samples of sapele, ayous and frake are heartwood when lotofa contains as well as heartwood than sapwood. Results obtained showed that the temperature effect on sorption behavior was quite low. We observed also a low difference between the sorption behavior of these different species and hysteresis of sorption decreases when temperature increases. Hailwood-Horrobin model's explains plausibly the experimental sorption isotherms data. Results on characterization of mass transfer properties showed that, in the steady state, mass diffusivity decreases exponentially when basal density increases. Mass diffusivity was higher in desorption than in adsorption phase. The gaseous permeability of these species was between than those of Australian hardwoods and temperate woods. It was difficult to define a relationship between permeability and mass diffusivity. [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]

In this work, the effects of moisture content variations produced by oscillating drying conditions have been studied by means of non-symmetrical drying (flying wood) and loaded drying (3-point bending test). The oscillating conditions are intended to produce a mechano-sorptive creep that is able to reduce drying stresses. However, our experimental data seem to prove that the activation of the mechano-sorptive creep by oscillations of the moisture content is very limited. Besides, the simulations showed that noisy conditions due to the kiln regulation already activate the mechano-sorptive creep, even for non-oscillating conditions, and could confuse the results. Contrary to the experimental observations, the implementation of classical mechano-sorptive models in the drying simulation model is far too optimistic, which means that these models are not suitable when the material is submitted to numerous oscillations. [ABSTRACT FROM AUTHOR]

Energy consumption during timber drying has become an increasingly important issue, alongside conventional concerns such as quality, cost, and drying time. This paper proposes a detailed analysis of the energy consumption evolution in conventional kiln drying using a sophisticated modelling approach. The formulation of energy consumption proposed by Perré et al.[ 1 ] was embedded in a multiscale computational model to consider kiln drying of a stack of boards. The drying of each board in the stack is simulated using a full version of a heat and mass transfer code (the 1-D version of TransPore), which allows all boards to be different and facilitates consideration of wood variability, in addition to the effect of position within the stack. For simple configurations, the energy consumption predicted by this complex computational tool is in agreement with the global approach.[ 1 ] Moreover, this sophisticated model is able to account for the standard deviation of the final moisture content (MC), which allows a compromise to be struck between quality, drying time, and energy consumption. In addition, this model can also be applied to test innovative, energy-saving drying strategies with an equivalent drying quality, i.e. sorting boards of similar properties before drying, or sorting boards at the end of drying to re-dry those boards that have a final MC that is still too high. [ABSTRACT FROM AUTHOR]

This article proposes a dual-scale computational model of wood drying in a batch lumber kiln. The stack may consist of a large number of boards (typically 100) arranged in layers. Each single board of the stack is simulated using one module of TransPore , a comprehensive computational model for heat and mass transfer in porous media. Timber variability is taken into account by a Monte-Carlo method. Such a dual-scale model allows the drying simulation of a 100-board stack to be completed in less than 30 s on a PC with a 2.8-GHz Xeon processor. Sample simulations are presented to depict the great potential and prospects of this new tool. [ABSTRACT FROM AUTHOR]

The purpose of this study is to investigate the effect of the position of the solar collector absorber and stagnant air relative to the airflow path on the thermodynamic properties of traversing air and the overall thermal performance of the collector. A numerical model was developed and validated using five different absorber design positions. Numerical results are validated using experimental data obtained from Epinal (France). The model was subsequently used to parametrically study the influence of these absorber positions on the density, temperature, velocity variations of the airflow through the collector and the thermal efficiency of each case scenario. The study showed the importance of stagnant air in solar collector thermal performance. Also, it showed that when one absorber is used, it is better to locate the absorber on the top of the airflow path than below the airflow path. Sandwiching the airflow path between double absorbers with the upper absorbers having stagnant air between it and the plexiglass cover and the down absorber having stagnant air between it and the bottom of the collector can produce a thermal efficiency of 31% which is 2% higher than the closest case and 13% higher than the worst-case design. [ABSTRACT FROM AUTHOR]

The improvement of durability and dimensional stability of wood properties via modification of the microstructure and wood–water interaction has been widely utilised. This study investigated polyester treatments, a possible alternative, using environmentally friendly chemicals such as malic acid to improve the beech wood (Fagus sylvatica) properties. The modified properties have been studied with four treatments using malic acid, glycerol, butanediol and succinic anhydride, mixing polycarboxylic acids and polyols. Results showed that the anti-swelling-efficiency (ASE) improved up to 70%, and the bulking coefficient improved around 23%, exhibiting an efficient penetration within the cell walls. The leaching rates (LR) of treatments and the extractables remained low, between 0.05 and 2.4%. The equilibrium moisture content (EMC) decreased by 50% for the four treatments, compared to untreated beech wood. [ABSTRACT FROM AUTHOR]

Drying is the most energy consuming process in the industrial transformation of wood. The current energy and climate crisis makes it imperative to adapt the process to energy availability, in terms of quantity, cost and temperature level. Wood drying schedules are historically based on practice. In the present work, a mechanistic drying model is used as a predictive tool to adapt conditions to specific situations. A multiscale computational model, Multi_Wood_DryS, has been combined with a probabilistic optimization code to propose tailor-made drying schedules that meet operators' expectations in terms of energy consumption, quality, drying time and cost. Optimizations of drying schedules of a stack of boards are proposed. The cost-optimized schedule is advantageous for all criteria with a 48% reduction in drying time. The resulting metamodel is a first step toward an intelligent controller for wood drying. [ABSTRACT FROM AUTHOR]

An X-ray density analyzer was used to determine the evolution of the moisture content (MC) profiles of a wood board during drying. Tests were performed on quartersawn boards of five wood species (oak, beech, Scotch pine, birch, alder) of two thicknesses (20 and 40 mm) dried at low temperature. The water vapor diffusion coefficient and liquid permeability of each wood species were determined by inverse analysis of the profiles using a computational model of coupled heat and mass transfer. These parameters were also measured by classical methods for validation purposes. The inverse method gives values close to the measured ones when using the evolution of the moisture profile, but also quite remarkably when using only the evolution of moisture at the core of the board. On the contrary, the method is less accurate when using only the average kinetics as experimental information. [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]

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. [ABSTRACT FROM AUTHOR]

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. [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]

We investigated the possibilities to use polychromatic X-rays sources to study wood properties that usually require of monochromatic radiation. Three applications using X-ray tubes without monochromator together with high-resolution energy-sensitive X-rays detectors are presented. In a first experiment, a standard X-ray source and a spectrometric detector were placed on both sides of a wood dryer which operates at 70°C and 50°C for dry-bulb and wet-bulb respectively. Digital cartography obtained can offer the possibility to identify some phenomenological parameters of fluid migration in wood. In a second part, an innovative device designed and made by the authors to record X-ray scatter intensities is presented. The feasibility to use the ratio of diffraction to attenuation for moisture content determination is discussed. This device has been also used for microfibril angle determination by recording and analyzing the X-ray scattered intensities of cellulose (002) lattice. The microfibril angle determination was successful; however, a background of X-rays scattering from amorphous component of wood prevented the determination of moisture content. [ABSTRACT FROM AUTHOR]

• 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]

Intermittent drying may be of interest in the future to address energy issues. Such drying conditions are also likely to help stress relaxation through mechanosorptive creep. The influence of applying oscillations of temperature and relative humidity during the drying of beech timber on time and drying stresses is discussed in the paper by means of nonsymmetrical and loaded drying. Experimental data were used to validate a numerical model in the case of intermittent drying. The model was then used to perform a numerical investigation of the possibility of using an intermittent energy source to dry wood. [ABSTRACT FROM PUBLISHER]

In this study, the drying of thin layers of the “Violet de Galmi” onion (a variety mainly grown in West Africa) is presented in this article, along with the reaction engineering approach (REA) modeling for a comprehensive understanding of the drying kinetics. The experiments were conducted on a lab-scale dryer to form thin layer of cylindrical onion slice. By performing this experiment, the standard activation energy is evaluated and modeled. The model is validated by simulating the drying rates under various drying conditions. The comparison of simulation and experimental data is found to be satisfactory. This approach allows the determination of the internal characteristics of the onion for the further studies such as design of solar dryer for onion. [ABSTRACT FROM PUBLISHER]