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Title: Pore-Scale Simulation of Intragranular Diffusion: Effects of Incomplete Mixing on Macroscopic Manifestations

Abstract

Diffusive mass transfer into and out of intragranular micropores ("intragranular diffusion") plays an important role in the large-scale transport of some groundwater contaminants. We are interested in understanding the combined effect of pore-scale advection and intragranular diffusion on solute transport at the effective porous medium scale. To study this problem, we have developed a three-dimensional pore-scale model of fluid flow and solute transport that incorporates diffusion into and out of intragranular pore spaces. Our model is based on the Smoothed Particle Hydrodynamics (SPH) simulation method, which represents fluid and solid phases by a mesh-free particle discretization. In the pore spaces, fluid flow is simulated by discretizing the Navier-Stokes equations using the SPH approach. Solute transport is represented by advection, diffusion within the fluid phase, and diffusion between the fluid and solid phases. Our model is implemented on large-scale parallel computing hardware, allowing us to simulate millions of computational particles and represent fully three-dimensional systems of pores and grains with arbitrarily complex physical geometry. We have used this model system to perform numerical experiments using various model porous media systems, which allows us to draw comparisons between macroscopic measures computed from the pore-scale simulations (such as breakthrough curves) and those predictedmore » by macroscopic formulations that assume complete mixing over the representative volume. In this paper we present results of 3D simulations of pore-scale flow and transport, including cases with and without intragranular diffusion, in two model porous media, one with randomly-packed uniform spherical grains and a second with randomly-packed spheres drawn from a binary grain size distribution. Breakthrough curves were computed from the 3D simulations at various transport distances. Comparable breakthrough curves were computed using 1D macroscopic models with parameters determined independently to the degree possible. Based on comparisons of the pore-scale and macroscopic model results, we draw two primary conclusions. First, non-Fickian behavior is persistent and ubiquitous at the scales considered, and most cases are better represented by a multi-rate mass transfer model even when there is no distinct secondary porosity (i.e., no intragranular diffusion). This suggests that diffusive mass transfer processes between preferential flow paths and relatively immobile zones within the primary porosity may have significant impact on transport, particular in low-concentration tails. Second, the application of mass transfer rate parameters based on an assumption of well-mixed concentrations at the pore-scale tend to overestimate the amount of mass transfer that occurs in heterogeneous pore geometries in which preferential flow leads to incomplete pore-scale lateral mixing.« less

Authors:
; ; ;
Publication Date:
Research Org.:
Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
Sponsoring Org.:
USDOE
OSTI Identifier:
1093512
Report Number(s):
PNNL-SA-91962
42319; KP1702030
DOE Contract Number:  
AC05-76RL01830
Resource Type:
Journal Article
Journal Name:
Water Resources Research, 49(7):4277-4294
Additional Journal Information:
Journal Name: Water Resources Research, 49(7):4277-4294
Country of Publication:
United States
Language:
English
Subject:
mass transfer; multirate; pore-scale; simulation; intragranular diffusion; smoothed particle hydrodynamics; Environmental Molecular Sciences Laboratory

Citation Formats

Scheibe, Timothy D., Hou, Zhangshuan, Palmer, Bruce J., and Tartakovsky, Alexandre M. Pore-Scale Simulation of Intragranular Diffusion: Effects of Incomplete Mixing on Macroscopic Manifestations. United States: N. p., 2013. Web. doi:10.1002/wrcr.20333.
Scheibe, Timothy D., Hou, Zhangshuan, Palmer, Bruce J., & Tartakovsky, Alexandre M. Pore-Scale Simulation of Intragranular Diffusion: Effects of Incomplete Mixing on Macroscopic Manifestations. United States. doi:10.1002/wrcr.20333.
Scheibe, Timothy D., Hou, Zhangshuan, Palmer, Bruce J., and Tartakovsky, Alexandre M. Mon . "Pore-Scale Simulation of Intragranular Diffusion: Effects of Incomplete Mixing on Macroscopic Manifestations". United States. doi:10.1002/wrcr.20333.
@article{osti_1093512,
title = {Pore-Scale Simulation of Intragranular Diffusion: Effects of Incomplete Mixing on Macroscopic Manifestations},
author = {Scheibe, Timothy D. and Hou, Zhangshuan and Palmer, Bruce J. and Tartakovsky, Alexandre M.},
abstractNote = {Diffusive mass transfer into and out of intragranular micropores ("intragranular diffusion") plays an important role in the large-scale transport of some groundwater contaminants. We are interested in understanding the combined effect of pore-scale advection and intragranular diffusion on solute transport at the effective porous medium scale. To study this problem, we have developed a three-dimensional pore-scale model of fluid flow and solute transport that incorporates diffusion into and out of intragranular pore spaces. Our model is based on the Smoothed Particle Hydrodynamics (SPH) simulation method, which represents fluid and solid phases by a mesh-free particle discretization. In the pore spaces, fluid flow is simulated by discretizing the Navier-Stokes equations using the SPH approach. Solute transport is represented by advection, diffusion within the fluid phase, and diffusion between the fluid and solid phases. Our model is implemented on large-scale parallel computing hardware, allowing us to simulate millions of computational particles and represent fully three-dimensional systems of pores and grains with arbitrarily complex physical geometry. We have used this model system to perform numerical experiments using various model porous media systems, which allows us to draw comparisons between macroscopic measures computed from the pore-scale simulations (such as breakthrough curves) and those predicted by macroscopic formulations that assume complete mixing over the representative volume. In this paper we present results of 3D simulations of pore-scale flow and transport, including cases with and without intragranular diffusion, in two model porous media, one with randomly-packed uniform spherical grains and a second with randomly-packed spheres drawn from a binary grain size distribution. Breakthrough curves were computed from the 3D simulations at various transport distances. Comparable breakthrough curves were computed using 1D macroscopic models with parameters determined independently to the degree possible. Based on comparisons of the pore-scale and macroscopic model results, we draw two primary conclusions. First, non-Fickian behavior is persistent and ubiquitous at the scales considered, and most cases are better represented by a multi-rate mass transfer model even when there is no distinct secondary porosity (i.e., no intragranular diffusion). This suggests that diffusive mass transfer processes between preferential flow paths and relatively immobile zones within the primary porosity may have significant impact on transport, particular in low-concentration tails. Second, the application of mass transfer rate parameters based on an assumption of well-mixed concentrations at the pore-scale tend to overestimate the amount of mass transfer that occurs in heterogeneous pore geometries in which preferential flow leads to incomplete pore-scale lateral mixing.},
doi = {10.1002/wrcr.20333},
journal = {Water Resources Research, 49(7):4277-4294},
number = ,
volume = ,
place = {United States},
year = {2013},
month = {7}
}