Papers by Evangelos Tsotsas
Physical review fluids, Oct 21, 2020
We show that during evaporation of a pore network, liquid can refill the gas occupied pores, snap... more We show that during evaporation of a pore network, liquid can refill the gas occupied pores, snapping off a gas bubble, which then moves to a stable configuration. This phenomenon is induced by the capillary instability due to the wettability heterogeneity of the pore network and has a much smaller time scale as compared to the evaporation process. The capillary instability induced liquid refilling and bubble movement are explained in detail based on the analysis of the images obtained from the visualization experiment. The capillary valve effect, which hinders the movement of the gas-liquid interface and is induced by the sudden geometrical expansion between small and large pores, can be suppressed by the residual liquid in the large pore. For better understanding of the capillary instability induced gas-liquid two-phase transport during evaporation, a novel pore network model is developed, which considers not only the capillary and viscous forces but also the inertial forces that are seldom taken into account in the previous models. The pore network modeling results are in good agreement with the experimental data, demonstrating the effectiveness of the developed pore network model, which opens up a new route for better understanding of the role of inertial forces in two-phase transport in porous media.
In this work, an isothermal pore network model has been utilized to investigate ion transport and... more In this work, an isothermal pore network model has been utilized to investigate ion transport and crystallization in layerd porous media during drying. Said network consists of two distinct layers each with a different pore size distribution. One-dimensional approximation at the throat level describes transport phenomena for liquid, vapor, and dissolved salt. An explicit time stepping scheme has been used to obtain fluid pressure fields and ion concentration. Various simulations are carried out which indicate the effect of mean pore size disparity in the top and bottom layer, as well as the effect of drying rate on final crystal distribution.
Transport in Porous Media, Dec 25, 2014
Wet wipes are commercial products made of a fibrous substrate impregnated with lotion. The overal... more Wet wipes are commercial products made of a fibrous substrate impregnated with lotion. The overall cleaning power, wetness perception, and opacity of wet wipes depend on the lotion distribution inside the substrate. In this work, a lab-scale X-ray microtomograph is used to acquire three-dimensional digital images of dry and wet wipe samples, from which the average structural properties of the fibrous substrate as well as the lotion distribution are obtained. In addition, a pore network model is developed to simulate the lotion distribution inside a wet wipe. The void space of the substrate is approximated by three layers of pore networks. Each layer has distinct structural properties in order to reflect the structural heterogeneity of the fibrous substrates in the thickness direction. A qualitatively good agreement of the pore network simulation results with the measured lotion distributions is observed. The influence of the equilibrium contact angle and viscosity on the lotion distribution is also investigated by pore network simulations.
Aiche Journal, Oct 30, 2017
The phenomena occurring at the surface of a porous medium during drying in the capillary regime a... more The phenomena occurring at the surface of a porous medium during drying in the capillary regime are investigated by pore network simulations. The impact of the formation of wet and dry patches at the surface on the drying rate is studied. The simulations indicate an edge effect characterized by a noticeable variation of saturation in a thin layer adjacent to the porous surface. Also, the results indicate a significant nonlocal equilibrium effect at the surface. The simulation results are exploited to test Schl€under's classical model which offers a simple closure relationship between the evaporation rate and the degree of occupancy of the surface by the liquid. In addition to new insights into the surface phenomena, the results open up new prospects for improving the continuum models of the drying process.
Physics of Fluids, Feb 1, 2017
Transport in Porous Media, Jul 28, 2015
OATAO is an open access repository that collects the work of Toulouse researchers and makes it fr... more OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible.
arXiv (Cornell University), Sep 9, 2019
We show that during evaporation of a pore network, liquid can refill the gas occupied pores, snap... more We show that during evaporation of a pore network, liquid can refill the gas occupied pores, snapping off a gas bubble, which then moves to a stable configuration. This phenomenon is induced by the capillary instability due to the wettability heterogeneity of the pore network and has a much smaller time scale as compared to the evaporation process. The capillary instability induced liquid refilling and bubble movement are explained in detail based on the analysis of the images obtained from the visualization experiment. The capillary valve effect, which hinders the movement of the gas-liquid interface and is induced by the sudden geometrical expansion between small and large pores, can be suppressed by the residual liquid in the large pore. For better understanding of the capillary instability induced gas-liquid two-phase transport during evaporation, a novel pore network model is developed, which considers not only the capillary and viscous forces but also the inertial forces that are seldom taken into account in the previous models. The pore network modeling results are in good agreement with the experimental data, demonstrating the effectiveness of the developed pore network model, which opens up a new route for better understanding of the role of inertial forces in two-phase transport in porous media.
arXiv (Cornell University), Aug 28, 2019
A discrete thermo-mechanical drying model is developed to investigate the interaction between the... more A discrete thermo-mechanical drying model is developed to investigate the interaction between the porous structure and the drying characteristics of dense particle aggregates. The solid phase consists of polydisperse spherical particles in the micrometer range and the void space is constructed by a complementary network of tetrahedral pores. A modified version of the classical invasion percolation algorithm is set up to describe the preferential evaporation of the confined liquid in the pores. Thus, the evolution of the liquid distribution throughout the complex disordered medium can be simulated. In a one-way coupling scheme, capillary forces caused by both fluid pressure and surface tension are computed over time from the filling state of pores and they are applied as loads on each primary particle in the discrete element method. Based on this robust approach the drying kinetics and the mechanical behavior of several different aggregates with various fractions of small and large particles are simulated and quantified.
Transport in Porous Media, Jul 4, 2016
In this work, a two-dimensional pore network model is developed to study the heat and mass transf... more In this work, a two-dimensional pore network model is developed to study the heat and mass transfer inside a capillary porous wick with opposite replenishment in the dry-out regime. The mass flow rate in each throat of the pore network is computed according to the Hagen-Poiseuille law, and the heat flux is calculated based on Fourier's law with an effective local thermal conductivity. By coupling the heat and the mass transfer, a numerical method is devised to determine the evolution of the liquid-vapor interface. The model is verified by comparing the effective heat transfer coefficient versus heat load with experimental observations. For increasing heat load, an inflation/deflation of the vapor pocket is observed. The influences of microstructural properties on the vapor pocket pattern and on the effective heat transfer coefficient are discussed: A porous wick with a non-uniform or bimodal pore size distribution results in a larger heat transfer coefficient compared to a porous wick with a uniform pore size distribution. The heat and mass transfer efficiency of a porous wick comprised of two connected regions of small and large pores is also examined. The simulation results indicate that the introduction of a coarse layer with a suitable thickness strongly enhances the heat transfer coefficient.
Ullmann's Encyclopedia of Industrial Chemistry
International Journal of Heat and Mass Transfer, 2021
Abstract Experimental and numerical pore network simulation studies on convective drying of capil... more Abstract Experimental and numerical pore network simulation studies on convective drying of capillary porous media under slow isothermal conditions are presented in this work. As a physical model of a real capillary porous medium, two dense packings of particles filled with monodisperse spherical glass beads (mean diameter 0.8 mm) and initially saturated either with distilled water or with a salt solution are prepared. Two controlled drying experiments with these packings are carried out using a custom-made setup installed in a lab-scale X-ray microtomograph. Based on in-situ tomograms (voxel size 16.4 μm) the time evolution of three-dimensional (3D) structures of liquid and salt deposit in the packings during drying are characterized. The respective results clearly demonstrate the formation of capillary liquid rings at the wedge-shaped pores located near the particle-particle contacts. The rings remain connected over a long distance to the packing surface during a significant period of drying. To highlight the crucial impact of liquid rings on drying, a 3D discrete pore network model that explicitly accounts for the ring effect is developed. Pore network simulations in the presence and absence of this effect are compared with measurements in terms of drying kinetics and saturation profiles. It is found that liquid rings act as additional hydraulic pathways for moisture transport from the interior of the pore/particle network to the surface and thus notably accelerate the drying process, and they lead to a spatially homogeneous distribution of the liquid down to low saturation. This study can be considered as a step forward in discrete modeling of drying of capillary porous media with 3D secondary capillary structures and should be of interest for various applications in the field of complex multiphase flow phenomena in porous media.
SSRN Electronic Journal, 2021
International Journal of Heat and Mass Transfer, 2021
From micro-scale to macro-scale modeling of solute transport in drying capillary porous media. (2... more From micro-scale to macro-scale modeling of solute transport in drying capillary porous media. (2021) International Journal of Heat and Mass Transfer, 165 (Part B). 120722.
Chemical Engineering Science, 2020
A two equation continuum model is developed to simulate the mass transfer in drying porous media.... more A two equation continuum model is developed to simulate the mass transfer in drying porous media. The main goal is to capture the so called non equilibrium effect To this end , we opera te in a regime where the liquid phase is immobile so that non equilibrium mass exchange between liquid and vapor phase dom inates. The formulation of the model relies on an upscaling technique. This notably permits to formula te the non local equilibrium phase change term on a firmer basis. The upscaling also indicates that there is no reason to consider an enhancement factor in the vapor diffusion model. The macroscopic model parameters are determined from pore network drying simulations. The same simulations are also used as a reference to compare with the predictions of the non local equilibrium continuum model. The solu tion of the two equation continuum model proves that this model simulates the non local equilibrium effect with reasonable accuracy. Also, the simulations indicate that the non local equilibrium effect is especially significant at the porous medium surface.
Physical Review Fluids, 2020
Drying Technology, 2019
In this work, the drying behavior of single rice grains at moderate and elevated air temperatures... more In this work, the drying behavior of single rice grains at moderate and elevated air temperatures (i.e. 60 C, 120 C, and 140 C) is experimentally investigated by using a magnetic suspension balance system. To describe the experimental drying behavior, a diffusion model is developed. The reference moisture diffusivity of the rice grains is determined by using an inverse method, whereas other thermo-physical properties are measured. Next, the diffusion model is reduced to a semi-empirical model within the frame of reaction engineering approach (REA). Both models are successfully benchmarked against the experimental observations. The results indicate that the moisture diffusivity of rice grains can be represented as a linear function of the moisture content and the drying process at high temperatures is controlled by both intra-and extra-particle mass transfer resistances. In addition to these results, this study favors a practical application of the REA in drying calculations to derive a simple, robust, and extrapolative semi-empirical model since the parameter of this model seems to be insensitive to variations of drying conditions.
Powder Technology, 2018
The morphology of agglomerates has been mainly investigated by approximating constituent primary ... more The morphology of agglomerates has been mainly investigated by approximating constituent primary particles with equivalent spheres. However, this simplified spherical primary particle model (SPPM) is questionable, because most agglomerates in food, pharmaceutical and chemical industry are made of irregularly shaped primary particles. Therefore, in the present work the SPPM is compared with a model that is based on complete data of the real structure (real structure model, RSM). Individual agglomerates of different sizes, produced from maltodextrin powder in a spray fluidized bed, have been scanned in an X-ray microtomographic device. After a series of image processing steps, the data has been used to derive various 3D morphological descriptors (such as coordination number, coordination angle, radial distribution of primary particles and open porosity) by both the SPPM and the RSM. The results of the two models delineate noticeable differences, indicating that the SPPM may not provide a precise characterization of maltodextrin agglomerates. Therefore, the RSM is the more appropriate method to study the morphology of agglomerates that consist of soft and deformable primary particles of varying size and irregular shape.
International Journal of Heat and Mass Transfer, 2018
In this work, a continuous model is developed to describe the dynamics of heat and mass transfer ... more In this work, a continuous model is developed to describe the dynamics of heat and mass transfer in cellular plant porous media during the superheated steam drying process at atmospheric pressure. This model accounts for the advective liquid and vapor flows in the intercellular void space as well as for the diffusive liquid flow across the solid cell membranes of the porous medium. The numerical results are verified against drying experiments for potato samples, which were carried out by a magnetic suspension balance at three different superheated steam temperatures (160°C, 180°C, 200°C). A comparison between the simulation results and the measured data shows that the drying characteristics of a plant porous medium can fairly be predicted by using the continuous model developed herein. The influence of the cell membrane water conductivity on the spatio-temporal distribution of the moisture content and of the temperature within the porous medium is studied by numerical simulations. It is observed that the water diffusion across the cell membranes controls the dynamics of the heat and mass transfer in the porous medium, and thus the drying kinetics.
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Papers by Evangelos Tsotsas