Darryl L. James

Thermally induced natural convection effects in Yucca Mountain drifts.

Thermally induced natural convection from the heat produced by emplaced waste packages is an important heat and mass transfer mechanism within the Yucca Mountain Project (YMP) drifts. Various models for analyzing natural convection have been employed. The equivalent porous medium approach using Darcys law has been used in many YMP applications. However, this approach has questionable fidelity, especially for turbulent flow conditions. Computational fluid dynamics (CFD), which is based on the fundamental Navier-Stokes equations, is currently being evaluated as a technique to calculate thermally induced natural convection in YMP. Data-model comparisons for turbulent flow conditions show good agreement of CFD predictions with existing experiments including YMP-specific data.

Numerical Modeling of Solar Thermo-Chemical Water-Splitting Reactor

Production of hydrogen using solar thermal energy has the potential to be a viable alternative to other hydrogen production methods, typically fossil-fuel driven processes. Thermochemical reactions for splitting water require high temperatures to operate effectively, for which solar is well-suited. Numerical modeling to investigate the concept of a solar-driven reactor for splitting water is presented in detail in this paper for an innovative reactor, known as the “counter-rotating-ring receiver/reactor/recuperator” (CR5) solar thermochemical heat engine that is presently under development. In this paper, details of numerical simulations predicting the thermal/fluid behavior of the innovative solar-driven thermo-chemical reactor are described in detail. These scoping calculations have been used to provide insight into the thermal behavior of the counter-rotating reactor rings and to assess the degree of flow control required for the CR5 concept.Copyright

CFD Calculation of Internal Natural Convection in the Annulus between Horizontal Concentric Cylinders

The objective of this heat transfer and fluid flow study is to assess the ability of a computational fluid dynamics (CFD) code to reproduce the experimental results, numerical simulation results, and heat transfer correlation equations developed in the literature for natural convection heat transfer within the annulus of horizontal concentric cylinders. In the literature, a variety of heat transfer expressions have been developed to compute average equivalent thermal conductivities. However, the expressions have been primarily developed for very small inner and outer cylinder radii and gap-widths. In this comparative study, interest is primarily focused on large gap widths (on the order of half meter or greater) and large radius ratios. From the steady-state CFD analysis it is found that the concentric cylinder models for the larger geometries compare favorably to the results of the Kuehn and Goldstein correlations in the Rayleigh number range of about 10{sup 5} to 10{sup 8} (a range that encompasses the laminar to turbulent transition). For Rayleigh numbers greater than 10{sup 8}, both numerical simulations and experimental data (from the literature) are consistent and result in slightly lower equivalent thermal conductivities than those obtained from the Kuehn and Goldstein correlations.

Internal Pressure Dynamics in Simulated Tornadoes

External and internal pressures on a model building with various porosities and dominant openings were obtained in a simulated tornado at a scale of ~ 1:100. Numerical simulations of internal pressures were obtained driven by external pressures collected in the tornado simulator. Comparisons are made between measured and simulated internal pressures. The significant role of porosity and dominant openings in tornado-like flows is discussed. Tornadoes are powerful phenomena that often have destructive and deadly implications for many societies around the world. Tornadoes commonly form in the presence of strong wind shear (vertical gradient of horizontal wind) which in the presence of high solar forcing that drives updrafts that twist the wind shear into circulation about a vertical axis. This rotation pulls in more warm air, which feeds the system with energy via evaporative cooling, and increases wind velocity and decreases pressure at the center, forming a tornado. Unlike regular atmospheric boundary layer winds, a structure’s response to a tornado is much more complex, due to the rapidly changing wind velocity and direction, as well as the large atmospheric pressure drop associated with the vortex core of the tornado. These complexities can combine to alter and magnify loads on a structure, or apply substantially different load patterns for which the structure was not designed to resist. Consequently, the risk of structural failure is heightened in a tornado. It is also possible that the highly turbulent flow near the core of the tornado can create fluctuations that lead to resonance of internal pressure. Any structure with an opening or potential opening (such as a damaged window or doorway) will experience a change of internal pressure. Internal pressures can often have a dramatic effect on the forces exerted on a structure under wind loads, and can easily become the governing loads for a structure. Tornadoes are no different in that sense, where it is possible for the internal pressure due to wind flow to combine with the external pressure drop at the core of the tornado vortex, to magnify the loads on exposed surfaces of the structure. While there is a growing body of research conducted on tornadoes and their interactions with buildings (Mishra et al. (2008), Haan et al. (2010), Haan et al. (2011), Hu et al. (2011), Sauer et al. (2011)), there are very few that have investigated internal pressures (Sabareesh et al., 2013). This paper seeks to study the effects that tornado-like flow has on the internal pressure of a scaled

Characteristics of Tornado-Like Vortices Simulated in a Large-Scale Ward-Type Simulator

Tornado-like vortices are simulated in a large-scale Ward-type simulator to further advance the understanding of such flows, and to facilitate future studies of tornado wind loading on structures. Measurements of the velocity fields near the simulator floor and the resulting floor surface pressures are interpreted to reveal the mean and fluctuating characteristics of the flow as well as the characteristics of the static-pressure deficit. We focus on the manner in which the swirl ratio and the radial Reynolds number affect these characteristics. The transition of the tornado-like flow from a single-celled vortex to a dual-celled vortex with increasing swirl ratio and the impact of this transition on the flow field and the surface-pressure deficit are closely examined. The mean characteristics of the surface-pressure deficit caused by tornado-like vortices simulated at a number of swirl ratios compare well with the corresponding characteristics recorded during full-scale tornadoes.

MODELING CHEMICAL AND THERMAL STATES OF REACTIVE METAL OXIDES IN A CR5 SOLAR THERMOCHEMICAL HEAT ENGINE.

“Sunshine to Petrol” is a grand-challenge research project at Sandia National Laboratories with the objective of creating a technology for producing feedstocks for making liquid fuels by splitting carbon dioxide (and water) using concentrated solar energy [1]. A reactor-level performance model is described for computing the solar-driven thermochemical splitting of carbon dioxide via a two-step metal-oxide cycle. The model simulates the thermochemical performance of the Counter-Rotating-Ring Receiver/Reactor/Recuperator (CR5). The numerical model for computing the reactor thermochemical performance is formulated as a system of coupled first-order ordinary differential equations describing the energy and mass transfer within each reactive ring and radiative energy transfer between adjacent rings. In this formulation, each of the counter-rotating rings is treated in a one-dimensional sense in the circumferential direction; supporting circumferential temperature and species gradients with assumed negligible gradients in both the radial and axial directions. The model includes radiative heat transfer between adjacent counter-rotating rings, variations in the incident solar flux distribution, heat losses to the reactor housing, and energy of reaction associated with the reduction and oxidation reactions. An overview of the physics included in this first-generation numerical model will be presented. Preliminary results include the circumferential distributions of temperature and species within each of the reactive rings. The computed overall chemical conversion efficiency will be presented for a range of design and operating parameters; including ring speed, carrier ring mass, reactive material loading, radiative emissivity, and differing incident flux distributions.Copyright

A ONE-DIMENSIONAL MODEL CAPTURING SELECTIVE ION TRANSPORT EFFECTS IN NANOFLUIDIC DEVICES

This paper presents a numerical model of one-dimensional, steady-state, multi-species, ion transport along a channel of variable width and depth. It is intended for computationally efficient simulation of devices with large variations in characteristic length scale—for example those incorporating both micro- and nanochannels. The model represents both volume charge in the fluid and surface charge on the channel walls as equivalent linear charge densities. The relative importance of the surface terms is captured by a so-called “overlap parameter” that accounts for electric double-layer effects, such as selective ion transport. Scale transitions are implemented using position-dependent area and perimeter functions. The model is validated against experimental results previously reported in the literature. In particular, model predictions are compared to measurements of fluorescent tracer species in nanochannels, of nanochannel conductivity, and of the relative enhancement and depletion of negatively and positively charged tracer species in a device combining microand nanochannels. Surface charge density is a critical model parameter, but in practice it is often poorly known. Therefore it is also shown how the model may be used to estimate surface charge density based on measurements. In two of the three experiments studied the externally applied voltage is low, and excellent results are achieved with electroosmotic terms neglected. In the remaining case a large external potential (~ 1 kV) is applied, necessitating an additional adjustable parameter to capture convective transport. With this addition, model performance is excellent.

Thermal Recuperation Modeling of a Solar Thermochemical Reactor

The United States spends over 700 billion dollars on foreign oil every year. A promising method to reduce this dependence, and be carbon-neutral as well, is concentrated solar thermochemical technology. Concentrated solar thermochemical technology has the potential to directly convert sunlight into a useable, carbon-neutral fuel that can be easily stored and integrated into our existing forms of energy demand such as transportation and heating fuels. Research is being performed by several groups at Sandia National Laboratories to fundamentally understand the complex physics and chemistry occurring within a solar thermochemical reactor prototype named the CR5 [counter-rotating-ring receiver/reactor/recuperator]. The objective of the work presented in this paper is to understand recuperative heat transfer within the CR5 as a function of reactor geometry and operational conditions. The CR5 vessel utilizes counter-rotating disks to provide for thermal energy recuperation, which is a necessity for an efficient reactor. Initially a simplified steady-state two-dimensional recuperation analysis was made to evaluate the relationship between the reactive material fin height and recuperation. The results from the simplified two-dimensional model indicate that recuperation is a strong function of fin height. Next, a more detailed, but still simplified, transient, three-dimensional model was developed. The initial three-dimensional simulations presented in this paper were performed to determine recuperator effectiveness and mesh density requirements for a generic case that had 2.5 kW energy input, fin height to gap ratio of 0.9, and finned reactor disks rotating at 1 rpm. A recuperator effectiveness of 88% and 85% was calculated from the finer and coarser meshes, suggesting that the coarser mesh (lumped in the fin thickness direction) is adequate for future parametric analysis simulations. This analysis will lead to a better understanding of recuperation as a function of reactor geometry, energy input, rotational speed, and thermophysical properties.Copyright

Analysis of micromixers and biocidal coatings on water-treatment membranes to minimize biofouling.

Biofouling, the unwanted growth of biofilms on a surface, of water-treatment membranes negatively impacts in desalination and water treatment. With biofouling there is a decrease in permeate production, degradation of permeate water quality, and an increase in energy expenditure due to increased cross-flow pressure needed. To date, a universal successful and cost-effect method for controlling biofouling has not been implemented. The overall goal of the work described in this report was to use high-performance computing to direct polymer, material, and biological research to create the next generation of water-treatment membranes. Both physical (micromixers - UV-curable epoxy traces printed on the surface of a water-treatment membrane that promote chaotic mixing) and chemical (quaternary ammonium groups) modifications of the membranes for the purpose of increasing resistance to biofouling were evaluated. Creation of low-cost, efficient water-treatment membranes helps assure the availability of fresh water for human use, a growing need in both the U. S. and the world.

In-drift natural convection analysis of the low temperature operating mode design.

Abstract Yucca Mountain has been designated as the nation’s high-level radioactive waste repository, and the U.S. Department of Energy has been approved to apply to the U.S. Nuclear Regulatory Commission for a license to construct a repository. The temperature and humidity inside the emplacement drift will affect the degradation rate of the waste packages and waste forms as well as the quantity of water available to transport dissolved radionuclides out of the waste canister. Thermal radiation and turbulent natural convection are the main modes of heat transfer inside the drift. This paper presents the result of three-dimensional computational fluid dynamics simulations of a segment of emplacement drift. The model contained the three main types of waste packages and was run at the time that the peak waste package temperatures are expected. Results show that thermal radiation is the dominant mode of heat transfer inside the drift. Natural convection affects the variation in surface temperature on the hot waste packages and can account for a large fraction of the heat transfer for the colder waste packages. The paper also presents the sensitivity of model results to uncertainties in several input parameters. The sensitivity study shows that the uncertainty in peak waste package temperatures due to in-drift parameters is <3°C.