HR: 1340h
AN: H23D-1626    [Abstracts]
TI: Effects of flow focusing on enhancement of transverse-mixing limited reactions at the pore and continuum scale
AU: Willingham, T
EM: thomas_willingham@yahoo.com
AF: Univesity of Illinois at Urbana-Champaign, Department of Civil and Environmental Engineering 205 North Mathews Ave., Urbana, IL 61801, United States
AU: * Werth, C J
EM: werth@uiuc.edu
AF: Univesity of Illinois at Urbana-Champaign, Department of Civil and Environmental Engineering 205 North Mathews Ave., Urbana, IL 61801, United States
AU: Valocchi, A J
EM: valocchi@uiuc.edu
AF: Univesity of Illinois at Urbana-Champaign, Department of Civil and Environmental Engineering 205 North Mathews Ave., Urbana, IL 61801, United States
AB: For groundwater contaminants that originate from a persistent source, recent studies indicate that degradation occurs primarily along plume fringes, where limiting substrates and trace nutrients mix with the contaminant plume transverse to the direction of flow. In many cases, groundwater environments are heterogeneous, and plume fringes are located in sediments with large variations in permeability. Permeability variations can give rise to flow focusing, and a recent study [Werth et al., 2006] indicates that this can give rise to enhanced transverse mixing and reaction along plume fringes. While a number of studies have examined streamline focusing and enhanced mixing in individual pores, none have evaluated flow focusing at the pore scale, and subsequent enhancement of transverse-mixing limited reactions. Herein we present results from micromodel experiments and a pore-scale model that examine enhanced transverse mixing and chemical reaction in preferential flow structures. Micromodels are pore-scale representations of porous media etched into silicon wafers, and pore- scale modeling is performed using the lattice-Boltzmann method for flow, and a finite volume model for reactive transport. In both experiments and the model, two substrates are introduced into a network of pores via two separate and parallel fluid streams, and they mix due to transverse dispersion and react. In both the experiments and the model we control the exact porous-media structure, and therefore are able to directly evaluate how changes in preferential flow structure, mixing zone location, width of flow focused zone, length of focused zone, permeability contrast between high and low permeability zones, and intrinsic reaction rate affect mixing and product formation. Excellent agreement was obtained between the micromodel and LB-FVM results, indicating that the latter adequately captures the pore-scale behavior observed in the former. Results indicate that the extent of reaction for flow-focused scenarios increased by over 40% compared to identical structures not containing the flow focused region. Simulations evaluating the effect of flow focusing location on extent of product formation demonstrated that the longitudinal location does not have an effect on the overall extent of reaction for fast reactions, but does play a role when reaction rates are limiting. The length of the flow focusing region has a measurable but small effect on the overall extent of product formation. Comparison of pore-scale to continuum- scale results indicates that total product formation is similar, but that product formation at the pore-scale lags spatially behind product formation at the continuum-scale. Possible reasons for this will be discussed.
DE: 1829 Groundwater hydrology
DE: 1831 Groundwater quality
SC: Hydrology [H]
MN: 2007 Fall Meeting