4,592 research outputs found

    Analysis of Granular Flow in a Pebble-Bed Nuclear Reactor

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    Pebble-bed nuclear reactor technology, which is currently being revived around the world, raises fundamental questions about dense granular flow in silos. A typical reactor core is composed of graphite fuel pebbles, which drain very slowly in a continuous refueling process. Pebble flow is poorly understood and not easily accessible to experiments, and yet it has a major impact on reactor physics. To address this problem, we perform full-scale, discrete-element simulations in realistic geometries, with up to 440,000 frictional, viscoelastic 6cm-diameter spheres draining in a cylindrical vessel of diameter 3.5m and height 10m with bottom funnels angled at 30 degrees or 60 degrees. We also simulate a bidisperse core with a dynamic central column of smaller graphite moderator pebbles and show that little mixing occurs down to a 1:2 diameter ratio. We analyze the mean velocity, diffusion and mixing, local ordering and porosity (from Voronoi volumes), the residence-time distribution, and the effects of wall friction and discuss implications for reactor design and the basic physics of granular flow.Comment: 18 pages, 21 figure

    Experimental study of the compaction dynamics for 2D anisotropic granular materials

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    We present an experimental study of the compaction dynamics for two-dimensional anisotropic granular systems. Compaction dynamics is measured at three different scales : (i) the macroscopic scale through the packing fraction ρ\rho, (ii) the mesoscopic scale through both fractions of aligned grains ϕa\phi_{a} and ideally ordered grains ϕio\phi_{io}, and (iii) the microscopic scale through both rotational and translational grain mobilities μr,t\mu_{r,t}. The effect of the grain rotations on the compaction dynamics has been measured. At the macroscopic scale, we have observed a discontinuity in the late stages of the compaction curve. At the mesoscopic scale, we have observed the formation and the growth of domains made of aligned grains. From a microscopic point of view, measurements reveal that the beginning of the compaction process is essentially related to translational motion of the grains. The grains rotations drive mainly the process during the latest stages of compaction.Comment: 8pages, 11 figure

    Pattern formation driven by cross--diffusion in a 2D domain

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    In this work we investigate the process of pattern formation in a two dimensional domain for a reaction-diffusion system with nonlinear diffusion terms and the competitive Lotka-Volterra kinetics. The linear stability analysis shows that cross-diffusion, through Turing bifurcation, is the key mechanism for the formation of spatial patterns. We show that the bifurcation can be regular, degenerate non-resonant and resonant. We use multiple scales expansions to derive the amplitude equations appropriate for each case and show that the system supports patterns like rolls, squares, mixed-mode patterns, supersquares, hexagonal patterns

    Shocks in sand flowing in a silo

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    We study the formation of shocks on the surface of a granular material draining through an orifice at the bottom of a quasi-two dimensional silo. At high flow rates, the surface is observed to deviate strongly from a smooth linear inclined profile giving way to a sharp discontinuity in the height of the surface near the bottom of the incline, the typical response of a choking flow such as encountered in a hydraulic jump in a Newtonian fluid like water. We present experimental results that characterize the conditions for the existence of such a jump, describe its structure and give an explanation for its occurrence.Comment: 5 pages, 7 figure

    Al-Pt MOCVD coatings for the protection of Ti6242 alloy against oxidation at elevated temperature

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    Results on isothermal oxidation at 873K for 90 h of Al-Pt coatings on Ti6242 coupons are reported. These coatings were obtained by low temperature, low pressure metalorganic chemical vapor deposition using Me3(MeCp)Pt(VI) and dimethylethylamine alane. Three coating architectures were investigated, namely pure Al, Pt and Al sequential sublayers, and co-deposited Al and Pt. Oxidation kinetics revealed a strong transient oxidation regime followed by a diffusion driven parabolic one. Such coatings allow to decrease oxidation kinetics more than one order of magnitude compared with those of the bare Ti6242. Scanning electron microscopy, second ion mass spectrometry, X-ray diffraction and transmission electron microscopy revealed that these coatings present a rough surface morphology. They are dense, they develop scales composed of γ-Al2O3 and δ-Al2O3 and they prevent titanium diffusion from the alloy to the surface. It is concluded that coatings produced by this process show promise for use as effective protection against oxidation of Ti6242 alloys and consequently they may raise the maximum operating temperature tolerated by corresponding parts in helicopter turboengines

    Growing non-equilibrium length in granular fluids: from experiment to fluctuating hydrodynamics

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    Velocity correlations in a 2D granular fluid are studied in experiments and numerical simulations. The transverse component of the velocity structure factor reveals two well defined energy scales, associated with the external "bath temperature" TbT_b and with the internal granular one, Tg<TbT_g<T_b, relevant at large and small wavelengths respectively. Experimental and numerical data are discussed within a fluctuating hydrodynamics model, which allows one to define and measure a non-equilibrium coherence length ξ\xi, growing with density, that characterizes order in the velocity field.Comment: 5 pages, 4 figure
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