The 12 th International Conference on Fluidization-New Horizons in Fluidization Engineering Engineering Conferences International Year 2007 Numerical Study of the Intrinsic and Feedback Dynamics of a Gas-Solid Fluidized Bed

The aim of this paper is to understand the complex spatio-temporal patterning of the dense bed when the inlet conditions can be modified by the bed itself. In this study, the inlet conditions (fluid pressure and velocity upstream the bed) take into account resistive effects from the distributor and capacitive ones from the air-supply system (plenum). The present work addresses particularly the issue of the transition between multiple and single bubble regimes that occurs for some particular inlet conditions. INTRODUCTION Nowadays the industrial fluidized bed reactors have low pressure-drop air distributor to reduce the cost of blower power: consequently the air supply system (especially the plenum) has an important relation with the movements of the bed. The influence of boundary conditions is a rather new issue in the literature where most of the time numerical simulations assume that the superficial velocity is constant. However it has been shown that this assumption is not always relevant. In Johnsson et al. (1) the pressure drop across the air distributor modifies the bed dynamics and in Kage et al (2), Borodulya et al. (3), Baird and Klein (4) the volume of the plenum appears important also. The variations of these two parameters can induce two particular kinds of behaviour described in Johnsson et al. (1): the single bubble regime and the multiple bubbles regime. When the pressure drop of the air distributor is low enough the bed is in the single bubble regime characterized by the eruption of a unique large bubble at a very definite frequency. In this case the pressure fluctuations are directly transmitted to the plenum without attenuation: this is the “coupled case”. On the other hand, for very high pressure drop distributors the bed is in the multiple bubbles regime characterized by a large-band fluctuation spectrum and no interaction with the plenum (“uncoupled case”). To describe these behaviors, the numerical simulation can play an important role. The Eulerian approach to describe both the particles and the gas phase is the most developed (Peirano et al. (5), Sasic et al. (6)) but with the increase of computational capacity, Eulerian-Lagrangian simulations (Helland et al. (7), Hoomans et al. (8)) gain interest because of their ability to describe micro-scale mechanism. 1 Bonniol et al.: Intrinsic and Feedback Dynamics of a Fluidized Bed Published by ECI Digital Archives, 2007