Influence of solid phase thermal conductivity on species separation rate in packed thermogravitational columns: A direct numerical simulation model

n this work, a direct numerical simulation model has been proposed to study the influence of porous matrix thermal properties on the separation rate in a model of packed thermogravitational column saturated by a binary mixture. The coupled flow, heat and mass dimensionless equations and boundary conditions have been derived in pore-scale and then solved over a vertical column containing fluid and solid phases. The results show that the separation rate is changed significantly by the conductivity ratio of the solid/fluid phases. The classical maximum separation at optimal Rayleigh number increases by decreasing the solid thermal conductivity. We obtained that the influence of the solid thermal conductivity for small Rayleigh number is not considerable but for intermediate Rayleigh number the separation rate initially decreases with increasing the thermal conductivity ratio and then reaches an asymptote. As the Rayleigh number increases, convection dominates and the effect of thermal conductivity ratio on separation rate becomes completely inversed

Effect of solid thermal conductivity and particle-particle contact on effective thermodiffusion coefficient in porous media

Transient mass transfer associated to a thermal gradient through a saturated porous medium is studied experimentally and theoretically to determine the effect of solid thermal conductivity and particle-particle contact on thermodiffusion processes. In this study, the theoretical volume averaging model developed in a previous study has been adopted to determine the effective transport coefficients in the case of particle-particle contact configurations. The theoretical results revealed that the effective thermodiffusion coefficient is independent of the thermal conductivity ratio for pure diffusive cases. In all cases, even if the effective thermal conductivity depends on the particle-particle contact, the effective thermodiffusion coefficient remains independent of the solid phase connectivity. We also found that the porosity can change the impact of dispersion effects on the thermodiffusion coefficients. For large values of the thermal conductivity contrast, dispersion effects are negligible and the effective thermal conductivity coefficients are the same as the ones for the pure diffusion case. Experimental results obtained for the purely diffusive case, using a special two-bulb apparatus, confirm the theoretical results. These results also show that, for non-consolidated porous media made of spheres, the thermal conductivity ratio has no significant influence on the thermodiffusion process for pure diffusion. Finally, the particle-particle contact also does not show a considerable influence on the thermodiffusion process

Discrete Salt Crystallization at the Surface of a Porous Medium

Efflorescence refers to crystallized salt structures that form at the surface of a porous medium. The challenge is to understand why these structures do not form everywhere at the surface of the porous medium but at some specific locations and why there exists an exclusion distance around an efflorescence where no new efflorescence forms. These are explained from a visualization experiment, pore-network simulations and a simple efflorescence growth model

Salt crystallisation at the surface of a heterogeneous porous medium

Evaporation of saline solutions from a porous medium often leads to the precipitation of salt at the surface of the porous medium. It is commonly observed that the crystallized salt does not form everywhere at the porous medium surface but only at some specific locations. To explain this phenomenon, we consider efflorescence formation at the surface of a saturated heterogeneous porous column (finer porous medium in coarse porous medium background) in the wicking situation. We study the impact of permeability and porosity contrasts on the efflorescence formation location from a simple visualisation experiment and Darcy's scale numerical simulations. We show that the porosity is the most sensitive parameter for our experiment and that efflorescence forms at the surface of the medium of lower porosity. A simple efflorescence growth model is then used to explain why the efflorescence continues to grow at the surface of the lower porosity medium without spreading over the adjacent surface of the greater porosity medium

Effect and propagation of water level fluctuations in a sloping sandy beach -- Unsaturated porous media II: numerical simulation test of single harmonic wave (long run)

In coastal processes, the strong water movements due to short periodic waves (such as sea swell) can induce irregular water level fluctuations in the swash zone and within the sandy beach. The measured water level fluctuations with very complex entry water level fluctuations in a wave canal with a sloping sandy beach were analyzed by using 7 capacitive sensors. Numerical simulations have also been implemented in order to complement the experimental water level signal analyses. In this paper, a numerical test of single harmonic wave (long run) with same computational domain and porous media properties as the experiment is conducted to understand the effect and propagation of the water level fluctuations in the unsaturated porous media. The key coupling parameter of the macro (sea water) porous and micro (sloping sandy beach) porous media in the Richards equation model is further identified by this numerical test, which is the basis to simulate and explain the complex experimental results

Evaporation with sodium chloride crystallization in a capillary tube

Sodium chloride crystallization induced by evaporation of aqueous solution is studied from visualization experiments in a circular capillary tube. In agreement with recent studies, the onset of crystallization is observed with a significant supersaturation. Detailed information on the precipitation kinetics and transport of ions is obtained from numerical computations of the ion mass fraction field during the evaporation process. It is shown that the precipitation kinetics is fast compared to transport so that the crystal growth is mostly controlled by the transport. This offers possibles implifications for analyzing more involved situations such as crystallization in porous media. The fact that the significant super- saturation does not lead to tube damage is explained

Efflorescence fairy ring and salt centripetal colonization at the surface of a drying porous medium containing a salt solution. Impact on drying curve

We study experimentally the drying of sintered glass porous pellets initially saturated with a sodium chloride solution. The objective is to develop a better understanding of the interplay between evaporation, ions transport and crystallization. The focus is on the characterization of the drying kinetic and its dependence on the development of salt crystals at the porous media surface. Together with analysis of the drying curves in relation with direct visualizations of the sample surface, quantitative pieces of information on the efflorescence growth and structure are obtained from X-ray computed tomography imaging. The study shows that markedly different results are obtained depending on the mean pore size of pellet. This is explained by the completely different efflorescence structure resulting from the drying process. A very thin salt crust is obtained for sufficiently small pores whereas cauliflower like efflorescence structures are obtained for sufficiently large pores. The salt crust tends to severely reduce the evaporation rate whereas the cauliflower like efflorescence does not affect the evaporation rate compared to pure water drying as long as the efflorescence is wet. In the case of the cauliflower type, the X-ray computed tomography images indicate a greater efflorescence growth at the periphery of the porous medium surface. The analysis of the data also suggests that the mean pore size in the efflorescence is significantly smaller than in the underlying porous medium

Wave front migration of endothelial cells in a bone-implant interface

The neo-vascularization of the host site is crucial for the primary fixation and the long-term stability of the bone-implant interface. Our aim was to investigate the progression of endothelial cell population in the first weeks of healing. We proposed a theoretical reactive model to study the role of initial conditions, random motility, haptotaxis and chemotaxis in interactions with fibronectin factors and transforming angiogenic factors. The application of governing equations concerned a canine experimental implant and numerical experiments based upon statistical designs of experiments supported the discussion. We found that chemotaxis due to transforming angiogenic factors was attracting endothelial cells present into the host bone. Haptotaxis conditioned by fibronectin factors favored cells adhesion to the host bone. The combination of diffusive and reactive effects nourished the wave front migration of endothelial cells from the host bone towards the implant. Angiogenesis goes together with new-formed bone formation in clinics, so the similarity of distribution patterns of mineralized tissue observed in-vivo and the spatio-temporal concentration of endothelial cells predicted by the model, tended to support the reliability of our theoretical approach

Packing fixed bed reactors with cylinders: influence of particle length distribution

In this work, we are interested in a better understanding of the local packing structure of fixed bed reactors made of cylindrical pellets packed in cylindrical tubes. Packing of cylindrical particles is simulated using Grains3D DEM code and results are analyzed in terms of void fraction using 3D analysis tools. With tube diameter as a secondary parameter, we investigate the porous structure depending on the pellet length distribution: average porosity, radial porosity, orientation of the pellets and variability of those indicators when repeating the numerical experiments

Etude de la thermogravitation dans une couche fluide horizontale

Nous présentons une étude analytique et numérique de l’action conjuguée de la thermodiffusion et de la convection, dite diffusion thermogravitationnelle, dans les mélanges binaires. Il s’agit de quantifier l’importance de la séparation dans le cas d’une cavité parallélépipédique horizontale remplie d’un mélange binaire et soumise à des gradients horizontaux de température constants sur les deux parois horizontales, les autres parois sont imperméables à la matière et thermiquement isolées. Cette nouvelle procédure diffère de celle habituellement utilisée et qui consiste à obtenir la séparation thermogravitationnelle dans une cellule verticale dont deux parois verticales sont maintenues à des températures constantes et différentes