Numerical simulations of high viscosity DNAPL recovery in highly permeable porous media under isothermal and non-isothermal conditions.

We developed a decimetric size model based on coupling generalized Darcy's law and heat-transfer equations to model viscous dense non-aqueous phase liquid (DNAPL) pumping through highly permeable porous media under non-isothermal conditions. The presence of fingering and non-wetting phase ganglia was modeled through an unsteady capillary diffusion coefficient and an arbitrary heterogeneous permeability field. The model was validated using existing experimental data of a simple case, an oil injection in a 2D tank packed with glass beads. Next, we compared the results of this model against a DNAPL extracting situation in the 2D tank to better understand the two-phase flow behavior in highly permeable porous media. We found that natural convection during heating plays an essential role in heat transfer, especially in the wetting phase zone. By adding the dynamic effect (unsteady conditions) we were better able to describe the presence of the ganglia in porous media. We observed good agreement between modeled and experimental oil saturation curves until the breakthrough point, with a mean relative error of about 10% for low and high flow rates, and 8% and 16% after breakthrough for low and high flow rates, respectively. Extracting viscous oil at low flow rates and high temperature generates less fingering and is well described by the generalized Darcy's law. The remobilization of residual non-wetting ganglia after the breakthrough point at the outlet is, however, difficult to simulate using the generalized Darcy's law. In the end, we treated this issue by using a perturbed permeability field to simulate the observed fingering in the 2D tank.

Experimental study of thermally enhanced recovery of high-viscosity DNAPL in saturated porous media under non-isothermal conditions.

Thermal enhancement is known to be an efficient way to decrease the residual saturation of some common dense non-aqueous phase liquids (DNAPLs) after pumping. However, the effect of transient heat transfer during the recovery of a high-viscosity contaminant, such as coal tar, in highly permeable porous media is still unknown. A 2D tank experimental setup allowing monitoring of temperature and saturation fields during DNAPL pumping has been developed. Experiments were run under isothermal and non-isothermal conditions, at low and high flow rates. We investigated the presence of viscous fingering and how that influences the shape of the cone of depression, as well as the residual saturation. The saturation fields show that less viscous fingering occurs in pre-heated cases and that heating increases the recovery efficiency. Increasing the temperature increases the critical velocity and the viscosity ratio and helps to stabilize the interface between the non-wetting and wetting phase. Observations were first made on an oil and ethanol fluid pair because its properties were known, before extending the experiments to a coal tar and water fluid pair. Residual oil saturation after pumping was decreased by 6-16% in all pre-heated conditions. Pumping at low flow rate in these conditions leaves the smallest oil residual saturation (20%) after pumping. A low flow rate increases the recovery efficiency by reducing viscous fingering and by spreading the generated heat to a larger part of the tank. Finally, results on coal tar pumping show that the high thermal conductivity of water helps in keeping the temperature high during pumping. The residual coal tar saturation was reduced from 40% at 20 °C to 28% when pre-heating the tank. Operating at a low flow rate and with a uniform temperature is the key to recovering the highest amount of a viscous DNAPL such as coal tar from the soil and satisfying cleanup goals when using thermally enhanced pumping.

Determination of Chlorinated Solvent Sorption by Porous Material-Application to Trichloroethene Vapor on Cement Mortar.

Experiments have been performed to investigate the sorption of trichloroethene (TCE) vapor by concrete material or, more specifically, the cement mortar component. Gas-flow experiments were conducted using columns packed with small pieces of cement mortar obtained from the grinding of typical concrete material. Transport and retardation of TCE at high vapor concentrations (500 mg L-1) was compared to that of a non-reactive gas tracer (Sulfur Hexafluoride, SF6). The results show a large magnitude of retardation (retardation factor = 23) and sorption (sorption coefficient = 10.6 cm3 g-1) for TCE, compared to negligible sorption for SF6. This magnitude of sorption obtained with pollutant vapor is much bigger than the one obtained for aqueous-flow experiments conducted for water-saturated systems. The considerable sorption exhibited for TCE under vapor-flow conditions is attributed to some combination of accumulation at the air-water interface and vapor-phase adsorption, both of which are anticipated to be significant for this system given the large surface area associated with the cement mortar. Transport of both SF6 and TCE was simulated successfully with a two-region physical non-equilibrium model, consistent with the dual-medium structure of the crushed cement mortar. This work emphasizes the importance of taking into account sorption phenomena when modeling transport of volatile organic compounds through concrete material, especially in regard to assessing vapor intrusion.

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.