Application of the Lattice-Boltzmann method to the modeling of population blob dynamics in 2D porous domains

AbstractIn the present paper, the Lattice-Boltzmann method is employed for the simulation of immiscible two-phase flow through a 2D porous domain when the volume fraction of the non-wetting phase is relatively low and thus it flows in the form of disconnected blobs. The flow problem is solved using an immiscible two-phase LB model where interfacial forces are expressed in terms of the chemical potential through the Gibbs–Duhem equation. We study the population dynamics of the non-wetting fluid blobs, namely the temporal evolution of the average blob size, with respect to the applied body force and the wetting phase volume fraction. Our results show that the system reaches a “steady state” where the average values of the studied parameters, such as the superficial velocities of both phases, and the number and size distribution of the blobs remain practically constant in time, although the temporal fluctuations around average values may be significant. We show that the average volume of the blobs decreases (and the population of the blobs increases) as the body force increases, namely as the viscous forces become dominant over capillary forces. The effect of the wetting volume fraction on the number of the blobs is more complex; as the wetting volume fraction decreases at constant body force, the blobs cover larger areas within the pore space producing larger pressure gradients and the dynamic breakup of blobs intensifies resulting in increasing blob numbers. However, below a critical value of the wetting volume fraction, the number of blobs begins to decrease and the non-wetting phase begins to span the entire pore network

Local mean-field study of capillary condensation in silica aerogels

We apply local mean-field (i.e. density functional) theory to a lattice model of a fluid in contact with a dilute, disordered gel network. The gel structure is described by a diffusion-limited cluster aggregation model. We focus on the influence of porosity on both the hysteretic and the equilibrium behavior of the fluid as one varies the chemical potential at low temperature. We show that the shape of the hysteresis loop changes from smooth to rectangular as the porosity increases and that this change is associated to disorder-induced out-of-equilibrium phase transitions that differ on adsorption and on desorption. Our results provide insight in the behavior of $^4$He in silica aerogels.Comment: 19 figure

Polymer-stable magnesium nanocomposites prepared by laser ablation for efficient hydrogen storage

Hydrogen is a promising alternative energy carrier that can potentially facilitate the transition from fossil fuels to sources of clean energy because of its prominent advantages such as high energy density (142 MJ per kg), great variety of potential sources (for example water, biomass, organic matter), and low environmental impact (water is the sole combustion product). However, due to its light weight, the efficient storage of hydrogen is still an issue investigated intensely. Various solid media have been considered in that respect among which magnesium hydride stands out as a candidate offering distinct advantages. Recent theoretical work indicates that MgH2 becomes less thermodynamically stable as particle diameter decreases below 2 nm. Our DFT (density functional theory) modeling studies have shown that the smallest enthalpy change, corresponding to 2 unit-cell thickness (1.6 {\AA} Mg/3.0{\AA} MgH2) of the film, is 57.7 kJ/molMg. This enthalpy change is over 10 kJ per molMg smaller than that of the bulk. It is important to note that the range of enthalpy change for systems that are suitable for mobile storage applications is 15 to 24 kJ permolH at 298 K. The important key for the development of air/stable Mg/nanocrystals is the use of PMMA (polymethylmethacrylate) as an encapsulation agent. In our work we use laser ablation, a non-electrochemical method, for producing well dispersed nanoparticles without the presence of any long range aggregation. The observed improved hydrogenation characteristics of the polymer/stable Mg-nanoparticles are associated to the preparation procedure and in any case the polymer laser ablation is a new approach for the production of air/protected and inexpensive Mg/nanoparticles.Comment: Hydrogen Storage, Mg - Nanoparticles, Polymer Matrix Composites, Laser Ablation, to appear in International Journal of Hydrogen Energy, 201

Structural, microchemistry, and hydrogenation properties of TiMn0.4Fe0.2V0.4, TiMn0.1Fe0.2V0.7 and Ti0.4Zr0.6Mn0.4Fe0.2V0.4 metal hydrides

In this work, TiFe-based alloys have been developed according to the stoichiometry Ti(1-x)A(x)Fe(1-y)B(y) (A Zr; B Mn, V). The hydrogen solubility properties have been investigated to develop dynamic hydrides of Ti-based alloys for hydrogen storage applications. The hydrogenation behavior of these alloys has been studied, and their hydrogen storage capacities and kinetics have been evaluated. Several activation modes, including activation at high temperatures under hydrogen pressure, have been attempted for the as-milled powders. In order to clarify the structural/microstructural characteristics, and chemical composition before and after hydrogenation, X-Ray Diffraction (XRD), EDAX-Mapping Analysis and Scanning Electron Microscopy (SEM), have been carried out for the samples. Modeling of the isotherms has been performed by using MATLAB programming. The maximum gravimetric density of 4.3 wt%, has been obtained on the sample with the BCC main phase. The calculated enthalpy of reaction (Delta H) is found to be about 4 kJ/mol