Partial saturation, porous

This method is based on the principle of capillary equilibrium, providing a nondestructive technique in which the total porosity and capillary pressure-saturation data are measured [198,199]. The idea is that when two partially saturated porous materials are in contact, the system moves toward an equilibrium state in which the capillary pressure of a liquid in one of fhe samples is the same for fhe ofher material. 

The traditional, continuum-based approach uses Darcy s law, modified for partially saturated porous media, to quantify the flux of water ... 

Richards equation (Richards 1931), based on a mass conservation balance, together with Eq. 9.1, can be used to describe the transient flow of water through a partially saturated porous medium. In one dimension (vertically), Richards equation is given as... 

In this chapter, we examine the various mechanisms that influence chemical redistribution in the subsurface and the means to quantify these mechanisms. The same basic principles can be applied to both saturated and partially saturated porous media in the latter case, the volumetric water content (and, if relevant, volatilization of NAPL constiments into the air phase) must be taken into account. Also, such treatments must assume that the partially saturated zone is subject to an equilibrium (steady-state) flow pattern otherwise, for example, under periods of heavy infiltration, the volumetric water content is both highly space and time dependent. When dealing with contaminant transport associated with unstable water infiltration processes, other quantification methods (e.g., using network... 

Gray, W.G. and Schrefler, B.A. (2001) Thermodynamic approach to effective stress in partially saturated porous media, Eur. J. Mech. A/Solids 20, 521-538... 

Knowledge of detailed liquid-vapor configurations enables separation of capillary and adsorptive contributions to the interfacial area as shown in Fig. 1-1 la (note the log-log scale). We denote liquid-vapor interfacial areas associated with menisci (curved interfaces at pore comers) as capillary contributions, and those associated with films as adsorptive contributions. The results in Fig. 1-1 la illustrate the dominant contribution of liquid films to the total liquid-vapor interfacial area of a partially saturated porous medium (Millville silt loam). Note that the flat region in Fig. 1-1 la (changes in SA with no change in p) reflects pore snap-off processes. 

The pore scale model and the associated unitary approach were upscaled to represent a sample of partially saturated porous medium. 

Kim, H., P.S.C. Rao, and M.D. Annable. 1997. Determination of effective air-water interfacial area in partially saturated porous media using surfactant adsorption. Water Resour. Res. 32 2705-2711. 

The model describing the partially saturated porous materials is composed of three balance equations. They express water mass conservation, air mass conservation and energy conservation. They can be found in Chavant (2001). The capillary pressure, the gas pressure and the temperature have been chosen as the main variables of the system. The three conservation equations are non-linear and coupled. As an example, these notions are explained on liquid conductivity expression, written as equation 3 ... 

We present now the extension of the constitutive equation (7) to partially saturated porous media. The material is assumed to be saturated by a liquid phase (noted by index w) and a gas mixture (noted by index g ). The gas mixture is a perfect mixture of dry air (noted by index da) and vapour (noted by index va). Based on most experimental data of unsaturated rocks and soils (Fredlund and Rahardjo 1993), and on the theoretical background of micromechanical analysis (Chateau and Dormieux 1998), the poroelastic behaviour of unsaturated material should be non-linear and depends on the water saturation degree. We consider here the particular case of spherical pores which are dried or wetted under a capillary pressure equal to the superficial tension on the air-solid interface. By adapting the macroscopic non-linear poroelastic model proposed by Coussy al. (1998) to unsaturated damaged porous media, the incremental constitutive equations in isothermal conditions are expressed as follows ... 

Vlahini, I., Jennings, H. M., Andrade, J. E. and Thomas, J. J., 2011. A novel and general form of effective stress in a partially saturated porous material The influence of microstructure . Mechanics of Materials, 43, 25-35. 

Physically, is a heterogeneity factor for vaporization exchange in the partially saturated porous electrode, while AHvapPw/Mw is the energy consumed for the evaporation of a unit volume of liquid water (P - P ) is the driving force for evaporation or condensation (depending on the sign), which is proportional to the difference of saturated and actual water vapor pressures. The product describes... 

The main processes are electrochemical reactions at electrified metal-electrolyte interfaces reactant diffusion through porous networks proton transport in water and at aggregates of ionomer molecules electron transport in electronic support materials water transport by gasous diffusion, hydraulic permeation, and electro-osmotic drag in partially saturated porous media and vaporization/condensation of water at interfaces between liquid water and gas phase in pores. 

A highly water-permeable PEM would facilitate water removal via liquid transport toward the anode, alleviating the problem of cathode flooding and anode dehydration. Erom a system perspective, it is deemed beneficial to make use of internal humidification of CLs and PEM by water that is produced at the cathode. This mode of internal water management obviates the need for external humidifiers. It demands, however, precise control of water permeation rates through the PEM and of vaporization rates in partially saturated porous electrodes. Therefore, it is cmcial to know how relevant parameters of water transport (diffusion, hydraulic permeation, electro-osmotic drag, vaporization, and condensation) depend on PEM morphology and thermodynamic conditions. 

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