Wahbi Jomaa

Application of a Coupled Thermo-Hydro-Mechanical Model to Simulate the Drying of Nonsaturated Porous Media

This study deals with drying modeling of a partially saturated deformable porous media (solid, liquid, and gas phases). In fact, during their drying, the deformable porous media undergo stresses due to volume shrinkage. The objectives of the model are to foresee the mechanical stresses in order to control the deformations of the product. Modeling of drying has been worked out for saturated media as well as for unsaturated media. The problem deals with the level of the transition between saturated and unsaturated medium where a continuous physical modeling does not exist currently. In this work, a mathematical model is proposed to describe the heat, mass, and momentum transfers applied to the drying of an unsaturated and deformable medium. The pressure gradient is the main driving force for the water migration through Darcys law. The particularity of the model is that it takes into account the strong coupling between mass transport and mechanical behavior of the material while using the notion of effective stress. The variables of coupling are the solid deformation velocity and the liquid phase pressure. The model is validated for clay material in various convective drying conditions. The simulations show the feasibility of the model to describe the drying and the deformation of a partially saturated media.

Two-Phase Shrinking Porous Media Drying: A Modeling Approach Including Liquid Pressure Gradients Effects

This article deals with the drying modeling of a saturated deformable porous media (solid + fluid). This modeling takes into account the mechanical behavior of the product as well as its initial conditions such as its moisture content and geometry. The effect of fluid pressure gradient on the fluid transfer is described by using Darcys law, and the fluid compressibility is characterized by its bulk modulus of elasticity. The solid phase is supposed to be incompressible and the porous media permanently diphasic (ideal shrinkage hypothesis). The description of the strong thermo-hydro-mechanical coupling that takes place within the material during drying is taken into account in the light of Biots consolidation theory by adopting Terzaghis effective stress principle. The numerical resolution is performed by FEM and ALE methods. A comparison between experimental results (convective drying of an alumina gel) and numerical results improves the ability of this modeling to describe thermo-hydro-mechanical coupling. It further highlights a current lack of precision in the determination of the thermophysical parameters values that control the model.

Crack Appearance during Drying of an Alumina Gel: Thermo-Hydro-Mechanical Properties

Most heterogeneous catalyst supports used in refineries are composed of porous alumina ceramics. Drying has been identified as a critical process for final product mechanical strength. In the literature, numerous papers deal with drying-induced stresses, which can lead to crack initiation. However, there are few papers devoted to experimental study of drying conditions that promote cracking. The objective of this work is to enhance knowledge of cracking behavior, specifically by studying alumina gel drying. First, the relation between drying conditions and first crack initiation is studied experimentally. Then a complete thermo-hydro-mechanical characterization of the alumina gel is made, including moisture content as a parameter.

Convective Drying of Gels: Comparison Between Simulated and Experimental Moisture Profiles Obtained by X-ray Microtomography

In this article, internal moisture profiles obtained either by simulation or experimentally during the convective drying of resorcinol-formaldehyde gels are compared. Such a comparison constitutes an attractive way to validate drying simulation models. X-ray microtomography, a powerful 3D imaging technique, coupled to image analysis is used to determine the internal moisture profiles in a nondestructive way. A thermo-hygro-mechanical coupled model is used to simulate the moisture profiles developing during drying. Resorcinol-formaldehyde gels are used because their degree of shrinkage can be easily controlled. Results show a fairly good agreement between experimental and simulated profiles, especially at high moisture contents.

Crack formation and self-healing behavior during the drying of alumina gels: Experimental studies

ABSTRACT The high shrinkage of alumina gels during the drying process leads to cracks. In this work, the behavior of four alumina gel formulations was experimentally studied following two drying procedures: (i) at ambient temperature and ambient humidity, outside a convective dryer and (ii) inside the dryer, allowing the application of different conditions of temperature and humidity. During the experiments, the observations in real time showed shrinkage of gels and the formation of cracks on them due to drying. Two of the gels displayed a capacity for self-healing, requiring numerical treatment of pictures to distinguish the closure of cracks due to shrinkage from those due to self-healing. The results show the precise determination of shrinkage steps, the estimation of the “shrinkage diffusivity” and proved that the manifestation of the chemical reaction is responsible for self-healing, activated by both applied temperature conditions and residual water of gels. Additional in situ investigations at microscopic scale using an environmental scanning electronic microscope corroborate the self-healing phenomenon noticed at macroscopic scale.