Gas phase advection and dispersion in unsaturated porous media

Abstract

[1] Gas phase miscible displacement experiments were conducted to quantitatively investigate the advective and dispersive contributions to gas phase transport in unsaturated porous media over a range of soil water contents. Furthermore, the independence of measured dispersivity values was evaluated through comparison of nonreactive and reactive tracer transport. Methane was used as a nonreactive tracer, while difluoromethane (DFM) and trichloroethene (TCE) were used as reactive tracers. At soil water contents below 17%, measured dispersivity values are observed to be independent of the tracer compound and of the soil water content. Conversely, the dispersivities are tracer dependent at the highest soil water contents, wherein the values for DFM and TCE are 3 and 6 times larger than that of methane's, respectively. The significantly larger dispersivity values obtained for DFM and TCE are most likely due to rate-limited mass transfer of these compounds between the gas phase and soil water, which is not observed for methane because of its minimal water partitioning. The dispersivity values obtained here range between 0.3 and 3 cm and are similar to those reported in the literature. The results indicate that within a given ''ideal transport'' range, dispersivities measured at one soil water content with a given tracer may be representative of the same porous media system at other soil water contents and for other compounds.

Turbulence Effect on Gas Transport in Three Contrasting Forest Soils

“…Thus, higher soil-water content could lead to a lower k a /e value ( Fig. 9) and probably also to higher a values (Costanza-Robinson and Brusseau, 2002). This could partly explain the relatively high D disp /D 0 values of a Vielsalm samples at low values of k a /e, which exceed the modeled values (Fig.…”

“…The phenomenon of dispersion is well-known, for example, from contaminant transport in ground water (Delgado, 2006), but is also relevant in the gas phase of porous media (Scotter and Raats, 1969;Costanza-Robinson and Brusseau, 2002).…”

“…The dispersivity a mainly depends on the geometry of the airfilled pores (Costanza-Robinson and Brusseau, 2002;Sharma and Poulsen, 2010), while the mean |v| depends on the characteristics of the pressure fluctuation at the surface and the physical properties of the soil (Poulsen and Møldrup, 2006). The |v| value at a depth is linked to the penetration of pressure fluctuations p (Pa) into the soil that can be obtained using (Fukuda, 1955):…”

Gas Dispersion and Immobile Gas Volume in Solid and Porous Particle Biofilter Materials at Low Air Flow Velocities

“…It has been observed that mechanical dispersion dominates over molecular diffusion for higher gas flow velocities through porous materials. [25][26][27][28] Gidda et al. 27 also suggest that the gas dispersivity increases for materials with a wider particle size distribution.…”

Development of a Method to Estimate the Dispersion and Knudsen Diffusion Coefficients in Soil by Using the Dusty Gas Model

“…を求めた． また，分散長に関しては，ガス流速の遅い範囲では，前 記のHibi et al 16) とCostanza-Robinson and Brusseau 6) ，ガス流速 が早い範囲についてはGidda et al 18) が一次元カラム実験を 用いて分散長を求めている．Costanza-Robinson and Brusseau 6) とGidda et al 18) は，Fickの法則に従って機械分散係数 および分散長を求めたが，Hibi et al 16) はDusty gasモデルを 加味して機械分散係数について検討した． 以上のように，屈曲度を考慮した分子拡散係数，拡散 係数に関係する屈曲度，Knudsen拡散係数，機械的分散 係数を個別または一部同時に求める試みがなされている が，一次元カラム実験を用いて，同時にこれらのパラメ ーターを求める手法は，著者が調べた範囲では開発され ていない．なお，ここで言う機械的分散係数とは，土粒 子により生じる間隙中のガスの流れの経路と分岐により 汚染物質が拡がる現象，または間隙中の土粒子とガスの 境界におけるガス流速と，間隙中のガスが流れる経路中 央におけるガス流速の違いにより生じる現象のことであ る．本研究では，2成分のDusty Gas モデルにより乾燥土 の一次元カラム実験結果からこれらのパラメーターを求 める手法を開発し，実際の実験結果からこの手法を用い て前記の各パラメーターを求めることができることを確 認した． 2. 分散係数とKnudsen拡散係数の算出方法 後述するように，今回の実験では水平方向の一次元カ ラム実験を行っているので，重力による影響を無視する ことができる．重力による影響を無視した土中ガス中の 2成分のDusrty Gas Modelは，Abriola et al 19) ，Hibi 20) ，Hibi et al 21) ，Webb and Pruess [198][199][200][201][202][203][204][205][206][207][208][209]2011.…”