Simulation of hydrodynamics and heat transfer in bubbling fluidized beds

This report presents the results of computational fluid dynamics (CFD) simulations carried out to model the hydrodynamics and surface-to-bed heat transfer in bubbling fluidized beds. A commercial CFD software, FLUENT V6.2, was used for the simulations. The multiphase flow model was a granular two-fluid (Eulerian-Eulerian) approach coupled with kinetic theory for the particle phase. Glass bead particles with two different average diameters (150 mum and 750 mum) were considered in the simulations. To study the effects of model parameters on bed hydrodynamics, a single jet was introduced at the bottom of a 2-D fluidized bed (0.17 cm x 25 cm) kept at minimum fluidization conditions. It was found that the gas-solid drag function used had a significant effect on solids volume fraction distribution, bubble size and shape. The coefficient of restitution and kinetic theory model (Syamlal et al. , 1993 vs. Gidaspow et al., 1994 models) changed the granular temperature and solids phase viscosity distribution but had limited effect on the solids volume fraction distribution. Furthermore, numerical simulations of heat transfer between a fluidized bed and a heated immersed heater were carried out at different superficial gas velocities and results were compared with experimental data. The mesh size and solids phase thermal conductivity models were found to affect the time averaged heat transfer coefficient significantly. Although, significant discrepancies were obtained between the simulation and experimental heat transfer coefficients quantitatively, the solids phase thermal conductivity model of Kuipers et al. (1992) overall provided better agreement with the experimental data compared to kinetic theory approach of Hsiau and Hunt (1993).