H42C-01
A Modified Light Transmission Visualization Method for DNAPL Saturation Measurements in 2-D Models
In this research, a light transmission visualization (LTV) method was used to quantify dense non-aqueous phase liquids (DNAPL) saturation in two-dimensional (2-D), two fluid phase systems. The method is an expansion of earlier LTV methods and takes into account both absorption and refraction light theories. Based on this method, DNAPL and water saturations can rapidly be obtained point wise across sand-packed 2-D flow chambers without the need to develop a calibration curve. The method was applied to measure, for the first time, undyed DNAPL saturation in small 2-D chambers. Known amounts of DNAPL, modeled by tetrachloroethylene (PCE), were added to the chamber and these amounts were compared to results obtained by this LTV method. Strong correlation existed between results obtained based on this method and the known PCE amounts with an R2 value of 0.993. Similar experiments conducted using dyed PCE showed a stronger correlation between results obtained by this LTV method and the known amounts of dyed PCE added to the chamber with an R2 value of 0.999. The method was also used to measure dyed PCE saturation in a large 2-D model following sparging experiments. Results obtained from image analyses following each sparging event were compared to results obtained by two independent techniques, namely gas chromatography-mass spectrometry (GC/MS) analyses and carbon column extraction. There was a good agreement between the results obtained by this LTV method and those obtained by the two independent techniques when experiments were carried out under stable light source conditions, and errors in mass balance were minor. The method presented here can be expanded to measure fluid contents in three fluid phase systems and provide a non-destructive, non-intrusive tool to investigate changes in DNAPL architecture and flow characteristics in laboratory experiments. Disclaimer This is an abstract of a proposed presentation and does not necessarily reflect EPA policy.
H42C-02
Oxygen Diffusion Measurements in Unsaturated Porous Media on the International Space Station
Oxygen supply to plant roots in unsaturated porous media is regulated by the amount of water and its distribution pattern. The design of optimal plant growth media must strike a balance between the retention of sufficient amounts of water in pore spaces by capillarity and maintenance of sufficient air-filled pore connectivity for gaseous diffusion. The challenges presented by microgravity conditions aboard spacecraft require novel management approaches to ensure optimal conditions for plant roots. We developed and tested a system for measurement of oxygen diffusion in partially saturated porous media under microgravity conditions. A sealed dual-chamber diffusion cell was constructed and controlled by an automated measurement system capable of controlling porous media water content using a metered pumping system through a porous membrane, and tensiometers to measure matric potentials concurrently. Continuous measurements of oxygen concentrations in the cells were conducted with Galvanic-based sensors providing transient response data for estimating water content-dependent diffusion coefficients. Gas diffusion was modeled as a function of air-filled porosity in mm- sized aggregated particles. Data were collected on the International Space Station between July and September 2007 as part of the ORZS-MIS experimental flight package (http://www.sdl.usu.edu/programs/orzs). Oxygen diffusion measurements in microgravity were compared with earth-based data using triplicate cell measurements in three different porous media. Preliminary results point to enhanced hysteresis in oxygen diffusion dependency on air-filled porosity in microgravity, indicating altered water distribution patterns relative to earth-based measurements. Considering air invasion during drainage, we hypothesize that a critical air-filled pathway forms at lower saturation in microgravity due to the absence of hydrostatic water distribution. A shift in the critical air-filled in microgravity would require adjustment in plant growth system management protocols and possible model development for reliable prediction of microgravity systems response.
H42C-03
Tomographic Measurements of Pore Filling at Infiltration Fronts
For certain porous media and initial conditions, constant flux infiltrations will produce a nonmonotonic saturation profile. This nonmonotonicity is purported to be a cause of gravity driven fingering, cannot be described by standard models of unsaturated flow, and is likely due to the wetting front being sharp at the pore-scale. Here we report micro-computed tomography measurements of the porous medium and measure which pores are filled by water and air at the initial wetting front as a function of flux. We find that at high fluxes all the pores are filled with water; for intermediate fluxes the pores along the edge of the column become unsaturated; and for low fluxes the pores in the bulk of the experimental column become unsaturated. This suggests that the transition between a sharp and diffuse front as a function of flux is abrupt, and that the unsaturated conditions observed at intermediate and higher fluxes are primarily an edge effect of the column.
H42C-04
Real-Time Colloid Transport Monitoring Using Synchrotron X-ray Diffraction
We investigated the mobility of freshly prepared and aged ferrihydrite colloids through sequentially encountered porous matrices with contrasting isoelectric points, quartz and calcite sands. Influent solutions varied in suspension density, arsenate concentration and pH. X-ray diffraction was performed on 1mm ID plasticrit columns mounted on an electronically controlled mobile sample stage as solutions were pumped or pushed through the columns. Two-dimensional diffraction patterns were repeatedly collected at 4-5 points along the column over the course of several hours. Colloid suspensions and the coarse matrix materials produce different characteristic diffraction patterns. The small size and numerous crystallites of colloids leads to the production of a small angle scattering peak (SASP) and/or uniform Bragg cones, recorded on an image plate detector as rings, whereas the larger matrix grains produce oriented Bragg reflections, recorded as spots or short arcs. Consequently, the appearance of the SASP or rings in the otherwise spotted diffraction pattern provides time- resolved visual evidence for accumulation of colloids at different locations in matrix. Colloid accumulation in calcite sand or in quartz sand is consistent with predictions from electrostatic considerations. This technique provides a new approach for monitoring colloid transport in real time, and identifying where colloids accumulate in heterogeneous porous media.
H42C-05
Verification of Chaotic Advection Enhanced Mixing in Porous Media Using Real Time Imaging
Chaotic advection refers to the highly complicated particle trajectories observed in the Lagrangian frame of reference under simple well-behaved velocity fields. Recent theoretical work indicates that time-periodic oscillations in low Reynolds (laminar) regimes can cause laminar flows to exhibit very complicated particle trajectories and cause substantial mixing in porous media. If proved successful, the chaotic advection methodology would allow the processes of bioremediation to occur much faster by enhancing the mixing of nutrients and microorganisms. This work intends to verify the theoretical work using a newly developed light reflection imaging system that is capable of real time quantitative monitoring of low concentrations of fluorescent dye and colloids. A fluorescent dye is injected into a decimeter-scale flow chamber filled with clean quartz sand via a well-triplet located at the center of the chamber. Dye distributions within the flow chamber are imaged over time and converted to absolute concentrations based on calibration curves. Two flow schemes are used, a time- dependent oscillatory flow scheme and a constant flow scheme (control). To establish the oscillatory flow, one of the three wells is randomly assigned a pumping magnitude with realistic constrains and a direction (injection or withdrawal), and the magnitude is then randomly partitioned to the other wells, which are assigned the opposite flow direction of the first well. For the control experiment, one well is constantly injecting while the other two are both withdrawing at half the rate of the first well. Our results show that the dye plume produced by the oscillating flow is more contained than the dye plume from the control experiment, and the dye concentration is higher in the more contained plume. Dye concentrations at certain points around the wells show higher fluctuations over time for the chaotic advection experiment than those for the control experiment. Explanations on how the chaotic flow can better contain a plume while simultaneously increasing mixing within the plume will be discussed.
H42C-06
Evaluation of segmentation algorithms for x-ray-based microtomographic imaging of multi- phase flow in porous media
Significant strides have been made in recent years in imaging fluid flow in porous media using x-ray computerized microtomography (CMT) with 1-20 micron resolution; however, one of the remaining hurdles is precise quantification of the results. Tomographic imaging was for many years focused on volume rendering and the more qualitative analyses necessary for rapid assessment of the state of a patient's health. In recent years, many highly quantitative CMT-based studies of fluid flow processes in porous media have been reported; however, many of these analyses are made difficult by the complexities in processing the resulting grey-scale data into reliable applicable information such as pore network structures, phase saturations, interfacial areas, and curvatures. Yet, relatively few rigorous tests of these analysis tools have been reported so far. The work presented here was designed to evaluate segmentation and surface generation algorithms as they were applied to CMT images of gas-fluid configurations in a number of glass capillary tubes. Interfacial areas calculated with these algorithms were compared to actual interfacial geometries and we found very good agreement between actual and measured surface and interfacial areas. (The test images used are available for download at the website listed below) http://cbee.oregonstate.edu/research/multiphase_data/index.html
H42C-07
Validation of Lattice Boltzmann Modeling of Multiphase Fluids in Porous Media with Micro-X- ray Tomography Data
A parallel processing version of the 3D Shan-and-Chen (SC) multi-component, multiphase lattice Boltzmann method (LBM) was developed and used to simulate the distributions of two immiscible fluids in porous media. The model was tested for conformance to the laws of Laplace (fluid-fluid interfacial tension) and Young (contact angles). Then, the SC LBM was successfully validated against cone beam X-ray microtomographic data for the distribution of oil (decane), water, and air phases in a 5-mm cube of porous medium composed of packed quartz sand grains. The results confirm that LBM models allow for the straightforward incorporation of complex pore space geometry as defined from X-ray microtomography measurements and that simulated wetting and non-wetting phase distributions are consistent with X-ray observations on both macroscopic and microscopic scales.
H42C-08
Numerical and Experimental Investigation on the Mobilization of Residual NAPL by Seismic Waves
Applying seismic waves might improve the efficiency of conventional pump-and-treat technologies for cleaning up aquifers contaminated by nonaqueous phase liquids (NAPL). We investigate the hypothesis that the mobilization of trapped NAPL blobs can be enhanced by exploiting capillary-induced resonance. We have developed a theory that allows us to estimate the response of a trapped NAPL blob to variable frequency excitation including its resonant frequency. Physical experiments to image the response of blobs to seismic waves in single pore channels and sphere packings at a high spatial and temporal resolution are under way. To obtain an unobstructed view of the blobs we match the optical refraction indices of the liquids and the solid phase and illuminate the porous medium with a laser light sheet. Finally, 3D Lattice-Boltzmann simulations will be used to demonstrate that a trapped NAPL blob in a porous medium exhibits resonance.