Water, effective diffusivity

In a porous medium saturated with water effective diffusion coefficients have values between n-lO and n-lO" cm -s For the black earth and podsol loam they are determined to be equal 3-10 to 4-10 cm -s for a quartz sand, 3-10" to 7-10 cm -s", for kaolin and montmorillonite clays, 5-10 to 8-10 cm -s" and for morainic loams and loesses, 0.5-10 to M0 erne s. The methane diffusion coefficient in water-saturated clays is equal to 2.12-10 cm -s" and for propane under the same conditions it is 0.56-10" cm -s. ... 

Intermittency is considered as an intensification scenario depending on both product and drying process. Mounir et al. (2012) and Allaf and Allaf (2014) analyzed the different intensification modes of conventional air drying. The first intensification depends on the airflow characteristics, which are the flow rate, temperature, and humidity. The drying rate can effectively increase with airflow rate till a maximum value. The latter depends on water effective diffusivity ... 

Silicon nitride (Si3N4) is an excellent electrical insulator, which is increasingly replacing Si02 because it is a more effective diffusion barrier, especially for sodium and water which are maj or sources of corrosion and instability in microelectronic devices. As a result, it can perform... 

Lin, TH Phillies, GDI, Prohe Diffusion in Polyacrylic Acid Water—Effect of Polymer Molecular Weight, Journal of Colloid and Interface Science 100, 82, 1984. 

Strictly speaking, in this formulation the effective diffusion coefficient, is replaced by an empirical dispersion coefficient, D, to account for the effect of water flow on diffusion. However, in practice, the rate of transpirational water flow is sufficiently slow that dispersion effects are minimal and Eq. (8) can be used without error. This is because the Peclet number (see Sect. F.2) is small. For the same reason, in almost all cases diffusion is the most important process in moving nutrients to the root and the convection term can be omitted entirely. 

Membranes of plastics and rubbers that are used to control liquid water penetration and water vapor diffusion are effective in controlling air movement as well. If they can be adequately sealed at the joints and penetrations and installed intact, then they could also provide a mechanical barrier to radon entry. 

Similarly, for diffusion in water a molecular diffusivity of 2 x 10-6 m2/h is reduced by a factor of 20 to an effective diffusivity of 10 7 m2/h, which is combined with a path length of 0.05 m to give an effective soil-to-water mass transfer coefficient of ksw 2 x 10 6 m/h. 

J. H. Nam and M. Kaviany. Effective diffusivity and water-saturation distribution in single- and two-layer PEMEC diffusion medium. International Journal of Heat Mass Transfer 46 (2003) 4595-4611. 

A poorly balanced water distribution in the fuel cell can severely impair its performance and cause long-term effects due to structural degradation. If PEMs or CLs are too dry, proton conductivity will be poor, potentially leading to excessive joule heating, which could affect the structural integrity of the cell. Too much water in diffusion media (CLs and GDLs) blocks the gaseous supply of reactants. As these examples show, all processes in PEECs are linked to water distribution and the balance of water fluxes. 

The reduction of the long-range diffusivity, Di by a factor of four with respect to bulk water can be attributed to the random morphology of the nanoporous network (i.e., effects of connectivity and tortuosity of nanopores). For comparison, the water self-diffusion coefficient in Nafion measured by PFG-NMR is = 0.58 x 10 cm s at T = 15. Notice that PFG-NMR probes mobilities over length scales > 0.1 /rm. Comparison of QENS and PFG-NMR studies thus reveals that the local mobility of water in Nafion is almost bulk-like within the confined domains at the nanometer scale and that the effective water diffusivity decreases due to the channeling of water molecules through the network of randomly interconnected and tortuous water-filled domains. ... 

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