Graduate Thesis Or Dissertation
 

Use of pore-scale network to model three-phase flow in a bedded unsaturated zone

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  • Contamination of ground water by non-aqueous phase liquids (NAPLs) has received increasing attention. The most common approach to numerical modeling of NAPL movement through the unsaturated zone is the use of the finite difference or finite element methods to solve a set of partial differential equations derived from Darcy's law and the continuity equations (Abriola and Pinder, 1985; Kaluarachchi and Parker, 1989). These methods work well in many settings, but have given little insights as to why certain non-ideal flow phenomena will occur. The network modeling method, which considers flow at the pore-scale, was used in this study to better understand macroscopic flow phenomena in porous media. Pore-scale network models approximate porous medium as a connected network of pores and channels. Two and three-dimensional pore-scale network models were constructed in this study. A uniform statistical distribution was assumed to represent the random arrangement of pore and tube sizes. Both hysteristic scanning curves and intermediate fluid distribution are studied. The simulation results suggested that network models may be used to predict the characteristic curves of three-phase systems. The results also suggested that three-dimensional models are necessary to study the three-phase problems. Two-dimensional models do not provide realistic results as evidenced by their inability to provide scale-invariant representation of flow processes. The network sizes used in this study ranged from 10 x 5 (50) to 156 x 78 (12168) pores for two-dimensional and from 10 x 5 x 5 (250) to 100 x 50 x 5 (25000) pores for three-dimensional domains. The domain size of 100 x 50 x 5 pores was large enough to provide descriptions independent of the domain scale. The one important limitation of network models is the considerable computational requirements. The use of very high speed computers is essential. Except for this limitation, the network model provides an invaluable technique to study fluid transport mechanisms in the vadose zone.
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