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Dispersion thermique dans des milieux granulaires : caractérisation à coeur et en proche paroi

Abstract : Heat transfer in a granular media through which a fluid is flowing is called thermal dispersion. It results from a combination of diffusive and convective heat transfer. The control of thermal dispersion is a critical point each time strongly endo- or exothermic chemical reaction in heterogeneous catalysis occurs. In this type of situation the control of the chemical conversion and/or of the lifespan of the catalyst is of prime importance. This work relates to the experimental characterization of thermal dispersion in granular media first in the core region and then in the near-wall region. In the first part of this work, a bed made up of granular medium (glass spheres or catalyst support) has been constructed. An air flow produced by a downstream fan goes through this bed. The bed is heated by Joule effect (time step) thanks to a metal wire perpendicular to the flow. The transient temperature responses are measured by several thermocouples. A specific inversion technique makes the estimation of the two thermal dispersion coefficients possible. Experimental correlations explaining the variation of the lateral and longitudinal dispersion coefficients with fluid velocity have been constructed on dimensional analysis considerations. They derive from the measurements on the glass beads/air and support of catalyst/nitrogen systems (use of dry nitrogen allows to get rid of the hygrophilic character of the micro-porous grains). The last part of this work relates to study of the thermal effects in the near wall region (effects caused by the hydrodynamic channelling phenomena). A new experimental set-up, with heating by a plane foil resistance on the wall and thermocouples inserted in the granular medium, has been built. Temperature measurements highlights the wall-effects. A new analytical model, that will make simulations of a two layers porous medium (core and near-wall regions) has been developed
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Amélie Testu. Dispersion thermique dans des milieux granulaires : caractérisation à coeur et en proche paroi. Autre. Institut National Polytechnique de Lorraine, 2005. Français. ⟨NNT : 2005INPL100N⟩. ⟨tel-01752511⟩



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