David R. Rector

Three-dimensional thermo-fluid electrochemical modeling of planar SOFC stacks

A simulation tool for modeling planar solid oxide fuel cells is demonstrated. The tool combines the versatility of a commercial computational fluid dynamics simulation code with a validated electrochemistry calculation method. Its function is to predict the flow and distribution of anode and cathode gases, temperature and current distributions, and fuel utilization. A three-dimensional model geometry, including internal manifolds, was created to simulate a generic, cross-flow stack design. Similar three-dimensional geometries were created for simulation of co-flow, and counterflow stack designs. Cyclic boundary conditions were imposed at the top and bottom of the model domains, while the lateral walls were assumed adiabatic. The three cases show that, for a given average cell temperature, similar fuel utilizations can result irrespective of the flow configuration. Temperature distributions however, which largely determine thermal stresses during operation, are dependent on the chosen design geometry/flow configuration. The co-flow case had the most uniform temperature distribution and the smallest thermal gradients, thus offers thermo-structural advantages over the other flow cases.

Enhanced Heat Transfer Through the Use of Nanofluids in Forced Convection

Much attention has been paid in recent years to the use of nanoparticle suspensions for enhanced heat transfer. The majority of this work has focused on the thermal conductivity of these nanofluids, which can be as much as 2.5 times higher than that of the plain base fluid. The present work moves beyond measurements of non-flowing liquids, to explore the role that nanofluids can play in enhancing convective heat transfer within microscale channels. A unique pseudo-turbulent flow regime is postulated, which simulates turbulent behavior at very low Reynolds numbers, in what are nominally laminar flows. The resulting fluid mixing has the potential to raise the average convective heat transfer coefficient within the channel. Numerical modeling, using the lattice Boltzmann method, confirms the existence of the pseudo-turbulent flow regime. Finally, experimental results are presented which demonstrate a significant heat transfer enhancement when using nanofluids in forced convection. The current results are especially relevant to microchannel heatsinks, where the low Reynolds numbers impose limitations on the maximum Nusselt number achievable.Copyright

Thermal Measurements in Rectangular Microchannels

This paper reports on an experimental study of heat transfer in high aspect ratio (width/depth), rectangular micro-channels. A single channel with width of 10 μm was cut into polycarbonate spacers of various thicknesses, resulting in channel depths of 128 μm, 263 μm and 521 μm. Heat transfer experiments were performed with a constant heat flux boundary condition applied at the surface of the channel. Experiments conducted for refrigerant R-124 working fluid in the range Re = 300 − 900 and Pr = 3.6 − 3.8 showed small or no departure from macro-scale predictions for channels with hydraulic diameters larger than 500 μm. Results for the 80:1 aspect ratio channel showed a significant departure from theoretical predictions. Experimental values of local Nusselt numbers were approximately 25 percent lower than predicted using macro-scale theory.Copyright

A semi-implicit lattice method for simulating flow

We propose a new semi-implicit lattice numerical method for modeling fluid flow that depends only on local primitive variable information (density, pressure, velocity) and not on relaxed upstream distribution function values. This method has the potential for reducing parallel processor communication and permitting larger time steps than the lattice-Boltzmann method. Several benchmark problems are solved to demonstrate the accuracy of the method.

Bench-Scale Enhanced Sludge Washing and Gravity Settling of Hanford Tank S-107 Sludge

This report summarizes the results of a bench-scale sludge pretreatment demonstration of the Hanford baseline flowsheet using liter-quantities of sludge from Hanford Site single-shell tank 241-C-106 (tank C-106). The leached and washed sludge from these tests provided Envelope D material for the contractors supporting Tank Waste Remediation System (TWRS) Privatization. Pretreatment of the sludge included enhanced sludge washing and gravity settling tests and providing scale-up data for both these unit operations. Initial and final solids as well as decanted supernatants from each step of the process were analyzed chemically and radiochemically. The results of this work were compared to those of Lumetta et al. (1996a) who performed a similar experiment with 15 grams of C-106, sludge. A summary of the results are shown in Table S.1. Of the major nonradioactive components, those that were significantly removed with enhanced sludge washing included aluminum (31%), chromium (49%), sodium (57%), and phosphorus (35%). Of the radioactive components, a significant amount of {sup 137}Cs (49%) were removed during the enhanced sludge wash. Only a very small fraction of the remaining radionuclides were removed, including {sup 90}Sr (0.4%) and TRU elements (1.5%). These results are consistent with those of the screening test. All of the supernatants (both individually and as a blend) removed from these washing steps, once vitrified as LLW glasses (at 20 wt% Na{sub 2}O), would be less than NRC Class C in TRU elements and less than NRC Class B in {sup 90}Sr.

Colloidal agglomerates in tank sludge and their impact on waste processing

Disposal of millions of gallons of existing radioactive wastes in underground storage tanks is a major remediation activity for the United States Department of Energy. These wastes include a substantial volume of insoluble sludges consisting of submicron colloidal particles. Processing these sludges under the proposed processing conditions presents unique challenges in retrieval transport, separation, and solidification of these waste streams. Depending on processing conditions, these colloidal particles can form agglomerated networks having high viscosities that could clog transfer lines or produce high volumes of low-density sediments that interfere with solid-liquid separations. Under different conditions, these particles can be dispersed to form very fine suspended particles that do not settle. Given the wide range of waste chemistries present at Department of Energy sites, it is impractical to measure the properties of all treatment procedures. Under the current research activities, the underlying principles of colloid chemistry and physics are being studied to predict and eventually control the physical properties of sludge suspensions and sediment layers in tank wastes and other waste processing streams. Proposed tank processing strategies include retrieval transport, and solid-liquid separations in basic (pH 10 to 14), high ionic strength (0.1 to 1.0 M) salt solutions. The effect of salt concentration, ionic strength, and salt composition on the physical properties such as viscosity, agglomerate size, and sedimentation of model suspensions containing mixtures of one or two of the major components found in actual wastes have been measured to understand how agglomeration influences processing. Property models developed from theory and experiment on these simple suspensions are then applied to explain the results obtained on actual wastes.

EMSL Pore Scale Modeling Challenge/Workshop

Report covers the background for the workshop, objectives, important research directions, necessary capabilities and overall recommendations.

Microstructural and Rheological Characterization of Colloidal Aggregates of Nuclear Waste Slurries

Abstract Characterization of tank waste suspensions contained at the Hanford Site is predicated on the identification of colloidal, rheological and microstructural parameters, crucial for the planned retrieval, transport and process operations necessary for remediation. The current study demonstrates the first successful characterization of a waste stream, SY-102, and the resultant relationship between rheology and microstructure. To develop the understanding, the waste was characterized by transmission electron microscopy, light scattering, electrophoresis, viscometry and sedimentation. The characterization shows that the microstructure and deformation behavior of SY-102 spans length scales ranging from nanometers to microns. The particle size distribution data, viscometry and sedimentation shows that the system is comprised of deformable aggregates. Finally, consideration of the isoelectric point of SY-102, pH ∼3.2, suggested that the rheopexy observed in SY-102 was caused by the presence of amorphous silica.

GRAVITY SETTLING OF HANFORD SINGLE-SHELL TANK SLUDGES

ABSTRACT The U.S. Department of Energy plans to use gravity settling in million-gallon storage tanks while pretreating sludge on the Hanford site. To be considered viable in these large tanks, the supernatant must become clear, and the sludge must be concentrated in an acceptable time. These separations must occur over the wide range of conditions associated with sludge pretreatment. In the work reported here, gravity settling was studied with liter quantities of actual single-shell tank sludge from Hanford Tank 241-C-107. Because of limited sludge availability, an approach was developed using the results of these liter-scale tests to predict full-scale operation. Samples were centrifuged at various g-forces to simulate compaction with higher layers of sludge. A semi-empirical settling model was then developed incorporating both the liter-scale settling data and the centrifuge compression results to describe the sludge behavior in a million-gallon tank. The settling model predicted that the compacted slud...

Methods for Quantifying the Uncertainties of LSIT Test Parameters, Test Results, and Full-Scale Mixing Performance Using Models Developed from Scaled Test Data

This report discusses the statistical methods for quantifying uncertainties in 1) test responses and other parameters in the Large Scale Integrated Testing (LSIT), and 2) estimates of coefficients and predictions of mixing performance from models that relate test responses to test parameters. Testing at a larger scale has been committed to by Bechtel National, Inc. and the U.S. Department of Energy (DOE) to “address uncertainties and increase confidence in the projected, full-scale mixing performance and operations” in the Waste Treatment and Immobilization Plant (WTP).