Drying of a porous medium with multiple open sides using a pore-network model simulation (original) (raw)
Related papers
Pore-Network Modeling of Isothermal Drying in Porous Media
Transport in Porous Media, 2005
In this paper we present numerical results obtained with a pore-network model for the drying of porous media that accounts for various processes at the pore scale. These include mass transfer by advection and diffusion in the gas phase, viscous flow in the liquid and gas phases and capillary effects at the liquid-gas interface. We extend our work by studying the effect of capillarity-induced flow in macroscopic liquid films that form at the pore walls as the liquid-gas interface recedes. A mathematical model that accounts for the effect of films on the drying rates and phase distribution patterns is presented. It is shown that film flow is a major transport mechanism in the drying of porous materials, its effect being dominant when capillarity controls the process, which is the case in typical applications.
Pore-network study of the characteristic periods in the drying of porous materials
Journal of Colloid and Interface Science, 2006
We study the periods that develop in the drying of capillary porous media, particularly the constant rate (CRP) and the falling rate (FRP) periods. Drying is simulated with a 3-D pore-network model that accounts for the effect of capillarity and buoyancy at the liquid-gas interface and for diffusion through the porous material and through a boundary layer over the external surface of the material. We focus on the stabilizing or destabilizing effects of gravity on the shape of the drying curve and the relative extent of the various drying periods. The extents of CRP and FRP are directly associated with various transition points of the percolation theory, such as the breakthrough point and the main liquid cluster disconnection point. Our study demonstrates that when an external diffusive layer is present, the constant rate period is longer.
Influence of heating mode on drying behavior of capillary porous media: Pore scale modeling
Chemical Engineering Science, 2008
Invasion percolation (IP) rules under non-isothermal conditions are applied to model the pore-scale events occurring during drying of capillary porous media, namely displacement of immiscible phases and cluster formation. A saturated two-dimensional network with a bimodal pore size distribution is dried by applying two different heat transfer boundary conditions; one corresponds to convective drying and the other to less resistive contact drying. Simulated macroscopic drying behavior is presented in conjunction with freely evolved microscopic temperature fields and phase distributions for both heating modes. Convective heating exhibits similar invasion patterns as those in isothermal simulations; both are dominated by the spatial distribution of pore radii. However, in contact heating, temperature dependency of surface tension produces significantly different invasion patterns.
Isothermal and non-isothermal drying of pore structures has been experimentally investigated using 2D square network models of interconnected etched channels with different (Gaussian) distributions of the channel width. In experiments with imposed temperature gradients, the temperatures either increase from the open side of the network with increasing network depth (referred to as the positive temperature gradient) or the temperatures decrease with increasing distance from the network opening (i.e. a negative temperature gradient). Experiments reveal that the observed phase patterns, or the distributions of liquid and gas, during drying are significantly depending on the direction of the temperature gradient; but also the presence of macro channels can have a strong effect on the phase patterns as well as on drying time.
Drying of porous media by concurrent drainage and evaporation: A pore network modeling study
International Journal of Heat and Mass Transfer
Drainage and evaporation can occur simultaneously during the drying of porous media, but the interactions between these processes and their effects on drying are rarely studied. In this work, we develop a pore network model that considers drainage, evaporation, and rarefied multi-component gas transport in nanopores. Using this model, we investigate the drying of a liquid solvent-saturated nanoporous medium enabled by the flow of purge gas through it. Simulations show that drying progresses in three stages, and the solvent removal by drainage effects (evaporation effects) becomes increasingly weak (strong) as drying progresses through these stages. Interestingly, drainage can contribute considerably to solvent removal even after evaporation effects become very strong, especially when the applied pressure difference across the porous medium is low. We show that these phenomena are the results of the coupling between the drainage and evaporation effects and this coupling depends on the operating conditions and the stage of drying.
A pore network study of evaporation from the surface of a drying non-hygroscopic porous medium
Aiche Journal, 2017
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.
Consideration of heat transfer in pore network modelling of convective drying
International Journal of Heat and Mass Transfer, 2008
The influence of heat transfer on the drying behaviour of capillary porous media saturated with water is studied. To overcome the limitations of continuum approaches, a pore network model based on statistical physics and invasion percolation is used. The presented non-isothermal model is the first of its kind to describe free evolution of temperatures in convective drying. Gas-side mass transfer
Drying processes in the presence of temperature gradients --Pore-scale modelling
The European Physical Journal E - Soft Matter, 2002
The influence of temperature gradients on the drying of water-saturated porous networks has been studied. We have focussed on the influence of the temperature on the drying process via the equilibrium vapor density ρe, because this is the most sensitive parameter with respect to variations of the temperature T . We have used a 2D model which accounts for both capillary and buoyancy forces. Invasion events by air or water are handled by standard rules of invasion percolation in a gradient (IPG). Vapor fluxes are calculated by solving a discretized version of the Laplace equation. In the model the temperature T varies linearly from the open side T 0 to the closed side TL. The temperature gradients strongly influence the cluster evolution during the process, because they facilitate vapor transport through wet regions. When T0 < TL, the movement of the front is inhibited and dry patches develop after a certain time at the closed side. When T0 > TL, the front movement is enhanced and the air ingress in the wet region behind the front is inhibited. The behavior of 3D systems differs from that of 2D systems, because the point where air percolates the system and the point where the water network breaks up in isolated clusters do not coincide. Before the latter fragmentation point the temperature will mainly influence the drying rates. After this point also the water distribution becomes sensitive to the temperature profile.
Scaling theory of drying in porous media
Physical Review E, 1999
Concepts of immiscible displacements in porous media driven by mass transfer are utilized to model drying of porous media. Visualization experiments of drying in two-dimensional glass micromodels are conducted to identify pore-scale mechanisms. Then, a pore network approach is used to analyze the advancing drying front. It is shown that in a porous medium, capillarity induces a flow that effectively limits the extent of the front, which would otherwise be of the percolation type, to a finite width. In conjuction with the predictions of a macroscale stable front, obtained from a linear stability analysis, the process is shown to be equivalent to invasion percolation in a stabilizing gradient. A power-law scaling relation of the front width with a diffusionbased capillary number is also obtained. This capillary number reflects the fact that drying is controlled by diffusion in contrast to external drainage. The scaling exponent predicted is compatible with the experimental results of Shaw ͓Phys Rev. Lett. 59, 1671 ͑1987͔͒. A framework for a continuum description of the upstream drying regimes is also developed. ͓S1063-651X͑99͒04604-8͔