Numerical study of cluster and particle rebound effects in a circulating fluidised bed |
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Authors: | E. Helland R. Occelli |
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Affiliation: | a ABB Turbo Systems AG, Thermal Machinery Laboratory, Bruggerstrasse 71a, CH-5401 Baden, Switzerland b IUSTI-CNRS (UMR 6595), University of Provence, Technopole de Chateau Gombert 5, rue Enrico Fermi-F, 13453 Marseille Cedex 13, France |
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Abstract: | Gas-particle flows in a vertical two-dimensional configuration appropriate for circulating fluidised bed applications were investigated numerically. In the computational study presented herein the motion of particles was calculated based on a Lagrangian approach and particles were assumed to interact through binary, instantaneous, non-frontal, inelastic collisions including friction. The model for the interstitial gas phase is based on the Navier-Stokes equations for two-phase flows. The numerical study of cluster structures has been validated with experimental results from literature in a previous investigation. Numerical experiments were performed in order to study the effects of different cluster and particle rebound characteristics on the gas-particle flow behaviour.Firstly, we investigated the hard sphere collision model and its effect on gas-particle flow behaviour. The coefficient of restitution in an impact depends not only on the material properties of the colliding objects, but also on their relative impact velocity. We compared the effect of a variable restitution coefficient, dependent on the relative impact velocity, with the classical approach, which supposes the coefficient of restitution to be constant and independent of the relative impact velocity.Secondly, we studied the effects of different cluster properties on the gas-particle flow behaviour. Opposing clustering effects have been observed for different particle concentrations: within a range of low concentrations, groups of particles fall faster than individual particles due to cluster formation, and within a well-defined higher concentration range, return flow predominates and hindered settling characterises the suspension. We propose herein a drag law, which takes into account both opposing effects and have compared the resulting flow behaviour with that predicted by a classical drag law, which takes into account only the hindered settling effect. |
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Keywords: | Circulating fluidised bed Clusters Drag law Gas-particle flow Numerical simulation Particle rebound characteristics |
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