Sandia National Laboratories

About: Sandia National Laboratories is a facility organization based out in Livermore, California, United States. It is known for research contribution in the topics: Laser & Thin film. The organization has 21501 authors who have published 46724 publications receiving 1484388 citations. The organization is also known as: SNL & Sandia National Labs.

Particle imaging of ignition and devolatilization of pulverized coal during oxy-fuel combustion

Abstract: Oxy-fuel combustion of coal is a promising technology for cost-effective power production with carbon capture and sequestration that has ancillary benefits of emission reductions and lower flue gas cleanup costs. To fully understand the results of pilot-scale tests of oxy-fuel combustion and to accurately predict scale-up performance through CFD modeling, fundamental data are needed concerning coal and coal char combustion properties under these unconventional conditions. In the work reported here, the ignition and devolatilization characteristics of both a high-volatile bituminous coal and a Powder River Basin subbituminous coal were analyzed in detail through single-particle imaging at a gas temperature of 1700 K over a range of 12–36 vol % O 2 in both N 2 and CO 2 diluent gases. The bituminous coal images show large, hot soot cloud radiation whose size and shape vary with oxygen concentration and, to a lesser extent, with the use of N 2 versus CO 2 diluent gas. Subbituminous coal images show cooler, smaller emission signals during devolatilization that have the same characteristic size as the coal particles introduced into the flow (nominally 100 μm). The measurements also demonstrate that the use of CO 2 diluent retards the onset of ignition and increases the duration of devolatilization, once initiated. For a given diluent gas, a higher oxygen concentration yields shorter ignition delay and devolatilization times. The effect of CO 2 on coal particle ignition is explained by its higher molar specific heat and its tendency to reduce the local radical pool. The effect of O 2 on coal particle ignition results from its effect on the local mixture reactivity. CO 2 decreases the rate of devolatilization because of the lower mass diffusivity of volatiles in CO 2 mixtures, whereas higher O 2 concentrations increase the mass flux of oxygen to the volatiles flame and thereby increase the rate of devolatilization.

Dense-fluid shear viscosity via nonequilibrium molecular dynamics

Abstract: A novel fluid-transport calculation by computer simulation, via nonequilibrium molecular dynamics, of laboratory methods of transport measurement is described. Shear viscosity of soft-sphere (${r}^{\ensuremath{-}12}$ potential) and Lennard-Jones particles (${r}^{\ensuremath{-}12}\ensuremath{-}{r}^{\ensuremath{-}6}$ potential) has been obtained from molecular dynamic modeling of Couette flow. Soft-sphere deviations from Enskog theory are similar to those found for hard spheres by Alder, Gass, and Wainwright, using time-correlations of equilibrium molecular dynamic system fluctuations. For the Lennard-Jones shear viscosity near the triple-point region, there is agreement between the equilibrium calculation of Levesque, Verlet, and Kurkijarvi and the nonequilibrium results using 108 atoms in a cube. However, systems two and three cubes wide give lower results, which, when extrapolated with inverse width, yield close agreement with the experimental argon shear viscosity. Comparison of the Lennard-Jones shear viscosity with experimental argon data along the saturated vapor-pressure line of argon confirms our successful simulation of macroscopic viscous flow with few-particle nonequilibrium molecular dynamic systems. A new result of the nonequilibrium molecular dynamics is the characterization of nonequilibrium distribution functions, which might provide the basis for a perturbation theory of transport. Since momentum transport is primarily accomplished by the repulsive potential core for high temperatures, the Lennard-Jones shear viscosity must behave like the soft-sphere system for high temperatures [viscosity divided by ${(\mathrm{t}\mathrm{e}\mathrm{m}\mathrm{p}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{e})}^{\frac{2}{3}}$ is a function of density divided by ${(\mathrm{temperature})}^{\mathrm{\textonequarter{}}}$]. In fact, the calculated excess shear viscosity (that part above the zero-density temperature dependence) has been successfully correlated in terms of the 12th-power scaling variables for temperatures as low as the critical value (along the freezing line). The utilization of soft-sphere scaling variables yields relatively simple functions for describing both the excess shear viscosity and the thermal-conductivity behavior throughout the fluid phase. The introduction of these scaling variables also clearly reveals two features: (i) weak temperature dependence, and (ii) the sign of the temperature derivative at constant density (negative for shear viscosity and positive for thermal conductivity). While both of these features have been experimentally observed in simple fluid experimental data, their cause has not been previously traced to the dominance of the core potential. Thus, the soft-sphere scaling variables should be useful for correlating experimental data.

CAM-SE: A scalable spectral element dynamical core for the Community Atmosphere Model

Abstract: The Community Atmosphere Model (CAM) version 5 includes a spectral element dynamical core option from NCAR's High-Order Method Modeling Environment. It is a continuous Galerkin spectral finite-element method designed for fully unstructured quadrilateral meshes. The current configurations in CAM are based on the cubed-sphere grid. The main motivation for including a spectral element dynamical core is to improve the scalability of CAM by allowing quasi-uniform grids for the sphere that do not require polar filters. In addition, the approach provides other state-of-the-art capabilities such as improved conservation properties. Spectral elements are used for the horizontal discretization, while most other aspects of the dynamical core are a hybrid of well-tested techniques from CAM's finite volume and global spectral dynamical core options. Here we first give an overview of the spectral element dynamical core as used in CAM. We then give scalability and performance results from CAM running with three different dynamical core options within the Community Earth System Model, using a pre-industrial time-slice configuration. We focus on high-resolution simulations, using 1/4 degree, 1/8 degree, and T341 spectral truncation horizontal grids.

Growth model interpretation of planet size distribution

Abstract: The radii and orbital periods of 4,000+ confirmed/candidate exoplanets have been precisely measured by the Kepler mission. The radii show a bimodal distribution, with two peaks corresponding to smaller planets (likely rocky) and larger intermediate-size planets, respectively. While only the masses of the planets orbiting the brightest stars can be determined by ground-based spectroscopic observations, these observations allow calculation of their average densities placing constraints on the bulk compositions and internal structures. However, an important question about the composition of planets ranging from 2 to 4 Earth radii (R⊕) still remains. They may either have a rocky core enveloped in a H2-He gaseous envelope (gas dwarfs) or contain a significant amount of multicomponent, H2O-dominated ices/fluids (water worlds). Planets in the mass range of 10-15 M⊕, if half-ice and half-rock by mass, have radii of 2.5 R⊕, which exactly match the second peak of the exoplanet radius bimodal distribution. Any planet in the 2- to 4-R⊕ range requires a gas envelope of at most a few mass percentage points, regardless of the core composition. To resolve the ambiguity of internal compositions, we use a growth model and conduct Monte Carlo simulations to demonstrate that many intermediate-size planets are "water worlds."