96,817 research outputs found
Scaled Particle Theory for Hard Sphere Pairs. II. Numerical Analysis
We use the extension of scaled particle theory (ESPT) presented in the
accompanying paper [Stillinger et al. J. Chem. Phys. xxx, xxx (2007)] to
calculate numerically pair correlation function of the hard sphere fluid over
the density range . Comparison with computer
simulation results reveals that the new theory is able to capture accurately
the fluid's structure across the entire density range examined. The pressure
predicted via the virial route is systematically lower than simulation results,
while that obtained using the compressibility route is lower than simulation
predictions for and higher than simulation predictions
for . Numerical predictions are also presented for the
surface tension and Tolman length of the hard sphere fluid
Vacuum Technology for Ion Sources
The basic notions of vacuum technology for ion sources are presented, with
emphasis on pressure profile calculation and choice of pumping technique. A
Monte Carlo code (Molflow+) for the evaluation of conductances and the
vacuum-electrical analogy for the calculation of time-dependent pressure
variations are introduced. The specific case of the Linac4 H- source is
reviewed.Comment: 40 pages, contribution to the CAS-CERN Accelerator School: Ion
Sources, Senec, Slovakia, 29 May - 8 June 2012, edited by R. Bailey,
CERN-2013-00
Mass transfer accompanied with complete reversible chemical reactions in gas-liquid systems: an overview
In many processes in the chemical industry mass transfer accompanied with reversible, complex chemical reactions in gas-liquid systems are frequently encountered. In point of view of design purposes it is very important that the absorption rates of the transferred reactants can estimated sufficiently accurate. Moreover, mass transfer phenomena can also affect substantially important process variables like selectivity and yield. Therefore large amounts of research effort has been invested in describing these processes and in the development of models that can be used for the calculation of the mass transfer rates and other parameters.\ud
\ud
The description of the absorption of a gas followed by a single first order irreversible reaction is simple and straightforward. For all mass transfer models, e.g. film, penetration and surface renewal respectively, this process can be analytically solved. For other processes however, only for a limited number of special cases analytical solutions are possible and therefore numerical techniques must be used for the description of these phenomena. Besides numerically solved absorption models the mass transfer rates often can be calculated satisfactory accurate by simplifying the actual process by means of approximations and/or linearizations. In this paper an overview will be given of the absorption models that are available for the calculation of the mass transfer rates in gas-liquid systems with (complex) reversible reactions. Both numerically solved and approximated models respectively will be treated and conclusions on the applicability and restrictions will be presented. Also perspectives and white spots will be indicated
Computational fluid dynamics
An overview of computational fluid dynamics (CFD) activities at the Langley Research Center is given. The role of supercomputers in CFD research, algorithm development, multigrid approaches to computational fluid flows, aerodynamics computer programs, computational grid generation, turbulence research, and studies of rarefied gas flows are among the topics that are briefly surveyed
Relaxation of a dewetting contact line Part 1: A full-scale hydrodynamic calculation
The relaxation of a dewetting contact line is investigated theoretically in
the so-called "Landau-Levich" geometry in which a vertical solid plate is
withdrawn from a bath of partially wetting liquid. The study is performed in
the framework of lubrication theory, in which the hydrodynamics is resolved at
all length scales (from molecular to macroscopic). We investigate the
bifurcation diagram for unperturbed contact lines, which turns out to be more
complex than expected from simplified 'quasi-static' theories based upon an
apparent contact angle. Linear stability analysis reveals that below the
critical capillary number of entrainment, Ca_c, the contact line is linearly
stable at all wavenumbers. Away from the critical point the dispersion relation
has an asymptotic behaviour sigma~|q| and compares well to a quasi-static
approach. Approaching Ca_c, however, a different mechanism takes over and the
dispersion evolves from |q| to the more common q^2. These findings imply that
contact lines can not be treated as universal objects governed by some
effective law for the macroscopic contact angle, but viscous effects have to be
treated explicitly.Comment: 21 pages, 9 figure
LDEF microenvironments, observed and predicted
A computer model for prediction of atomic oxygen exposure of spacecraft in low earth orbit, referred to as the primary atomic oxygen model, was originally described at the First Long Duration Exposure Facility (LDEF) Post-Retrieval Symposium. The primary atomic oxygen model accounts for variations in orbit parameters, the condition of the atmosphere, and for the orientation of exposed surfaces relative to the direction of spacecraft motion. The use of the primary atomic oxygen model to define average atomic oxygen exposure conditions for a spacecraft is discussed and a second microenvironments computer model is described that accounts for shadowing and scattering of atomic oxygen by complex surface protrusions and indentations. Comparisons of observed and predicted erosion of fluorinated ethylene propylene (FEP) thermal control blankets using the models are presented. Experimental and theoretical results are in excellent agreement. Work is in progress to expand modeling capability to include ultraviolet radiation exposure and to obtain more detailed information on reflecting and scattering characteristics of material surfaces
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