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Dense Gas Flow in Volcanic Ash Soil: Effect of Pore Structure on Density-Driven Flow

^a Dep. of Biological and Environmental Eng., Graduate School of Agric.and Life Sci., Univ. of Tokyo, 1-1-1 Yayoi Bunkyo-ku, Tokyo, 113-8657, Japan
^b Dep. of Global Agricultural Sciences, Graduate School of Agric. and Life Sci., Univ. of Tokyo, 1-1-1 Yayoi Bunkyo-ku, Tokyo,113-8657, Japan

View larger version (15K): [in this window] [in a new window]   Fig. 1. Water retention curves of Tachikawa loam and Toyoura sand fitted by the Durner's bimodal pore size distribution model (Durner, 1994) and the van Genuchten model (van Genuchten, 1980), respectively. The pF equals log(–) in centimeters of H2O, where is the soil-water matric potential.

View larger version (12K): [in this window] [in a new window]   Fig. 2. Schematic diagram of the experimental setup.

View larger version (11K): [in this window] [in a new window]   Fig. 3. (a) Gas diffusivity and (b) air permeability of Tachikawa loam and Toyoura sand as a function of air content. The solid and dotted lines in gas diffusivity represent the predicted curves of the Porosity Enhanced (POE) model (Moldrup et al., 2005) against measured data of both soils. The solid and dotted lines in air permeability indicate the approximations of measured data by the soil-water-characteristic-based model (Moldrup et al., 2003a) for both soils. Bars show the standard error of measured data (n = 5).

View larger version (13K): [in this window] [in a new window]   Fig. 4. Gas density profiles for gas samples taken at 5, 15, and 25 cm from the column inlet of (a) Tachikawa loam and (b) Toyoura sand at 40% air content in horizontal experiments. The plots show the average of three replicates. The solid and dashed lines show the Fickian model predictions (Eq. [1]) with retardation factor R = 1.0 (no gas adsorption) and R = 2.0, respectively. Coefficient of variation of three replicates at each plot was <5%.

View larger version (13K): [in this window] [in a new window]   Fig. 5. Gas density profiles for gas samples taken at 5, 15, and 25 cm from the column inlet of (a) Tachikawa loam and (b) Toyoura sand at 40% air content in vertical downward experiments. The plots show the average of three replicates. The solid and dashed lines show the Fickian model predictions (Eq. [1]) with retardation factor R = 1.0 (no gas adsorption) and R = 2.0, respectively. Coefficient of variation of three replicates at each plot was <5%.

View larger version (10K): [in this window] [in a new window]   Fig. 6. (a) Tortuosity and (b) equivalent pore diameter of Tachikawa loam and Toyoura sand calculated from Eq. [5] and [6], respectively. The data sets of the averaged gas diffusivity and air permeability at each air content (Fig. 2 and 3) were used for the calculation.