David Vidal

Effect of particle size distribution and packing compression on fluid permeability as predicted by lattice-Boltzmann simulations

Abstract Massive parallel lattice-Boltzmann method simulations of flow through highly polydispersed spherical particle packings formed using Monte-Carlo methods were performed. The computed fluid permeabilities were compared to experimental data obtained from blocks made of three natural ground calcium carbonate powders compressed at different levels. The agreement with experimental measurements is excellent considering the approximations made. A series of flow simulations was also performed for packings of spherical particles compressed at different levels with increasing polydispersity modeled with both lognormal and Weibull size distributions. The predicted permeabilities were found to follow reasonably well the Carman–Kozeny correlation although an increasing deviation towards lower predicted permeabilities with increasing polydispersity was observed. Finally, following a careful analysis of the inherent numerical errors, an expression relating the Kozeny “constant” to the size distribution and compression level was derived from the simulation results, which led to a modified correlation.

Simulation of the consolidation of paper coating structures: probabilistic versus deterministic models

Abstract Two categories of mathematical models were compared for the simulation of consolidation of paper coating structures, that is for the packing of pigments on a paper substrate under dewatering conditions. The first category uses probabilistic methods, relying on a random number generator to either determine the initial position of the pigments or their motion. The second category uses deterministic methods based on force balances. In this work, two probabilistic models and two deterministic models are described and their respective advantages and drawbacks are critically reviewed. Simulation results obtained using three of these methods are compared for the case of monodisperse and bidisperse spherical suspensions. Porosity calculations of the numerical packings obtained with the (deterministic) discrete element method (DEM) and two probabilistic methods, the Monte-Carlo (MCD) and the steepest descent (SDD) deposition methods, are compared with experimental data from the literature. These calculations reveal significant differences in the pore volume obtained with these three models. An analysis based on the bridging and relaxation phenomena that prevail in the flow of such particulate systems provide an explanation for these differences and show the strong potential of the discrete element method. The choice of the simulation method depends on the objective of the simulations. DEM will provide more accurate predictions of macroscopic quantities such as the porosity or the roughness, but requires very long computational times. MCD or SDD will only provide qualitative trends, but is computationally far less intense. A combination of strategies might be appropriate, using MCD (or SDD) to provide guidelines and DEM to enhance the results predicted by MCD.

Rheological properties of concentrated aqueous silica suspensions: Effects of pH and ions content

The rheological behavior of concentrated aqueous suspensions of nearly monodisperse submicron, spherical silica particles was studied in Couette and vane geometries. End corrections and wall depletion effects were found to be important. The apparent yield stress and shear viscosity were investigated in the light of interactions between the charged silica particles. The effects of pH as well as the addition of electrolyte were examined. The suspensions in de-ionized water without addition of acid, base, or electrolyte gave the largest apparent yield stress and shear viscosity, while the addition of base, acid, and KCl resulted in significant decreases of the apparent yield stress as well as of the viscosity. These effects have been interpreted in light of the DLVO theory and the compression of the double layer around the solid particles (Debye length).

A conservative lattice Boltzmann model for the volume-averaged Navier-Stokes equations based on a novel collision operator

The volume-averaged Navier-Stokes (VANS) equations are at the basis of numerous models used to investigate flows in porous media or systems containing multiple phases, one of which is made of solid particles. Although they are traditionally solved using the finite volume, finite difference or finite element method, the lattice Boltzmann method is an interesting alternative solver for these equations since it is explicit and highly parallelizable. In this work, we first show that the most common implementation of the VANS equations in the LBM, based on a redefined collision operator, is not valid in the case of spatially varying void fractions. This is illustrated through five test cases designed using the so-called method of manufactured solutions. We then present an LBM scheme for these equations based on a novel collision operator. Using the Chapman-Enskog expansion and the same five test cases, we show that this scheme is second-order accurate, explicit and stable for large void fraction gradients.

Numerical simulation of the hydrodynamics in a short-dwell coater

Abstract numerical model is developed to gain more insight into the conditions that ensure good short-dwell coater high-speed performance (runnability). The methodology is based on the numerical simulation of the flow field inside the pond and the prediction of coat weight and blade loading pressure by a two-dimensional finite element method. Good agreement between experimental measurements made on industrial coaters and numerical results is obtained. Investigation of short-dwell coater coating parameters is carried out in order to understand the mechanisms underlying the triggering of flow instabilities and the increase of blade loading pressure.