| Summary: | 碩士 === 國立中央大學 === 機械工程學系 === 102 === The present study investigates the hydrodynamics and heat transfer phenomena that occur during the biomass fast pyrolysis process. A numerical approach that combines an Eulerian multifluid model and the kinetic theory of granular flow applied to simulate the gas-solid flow in a bubbling fluidized bed reactor. In this study, rice husk and quartz with specified properties have been used as biomass and inert material, respectively. Since the dynamics of these systems are often chaotic, the model was first validated the feasibility using previous findings, then an extensive parametric study was conducted to determine the effects of the major variables on the flow distribution and the heat transfer between the phases. The set of governing and constitutive equations was solved by the commercial computational fluid dynamics code, ANSYS FLUENT 13.0.0. Herein, various aspects such as solid volume fraction, velocities, bed mixing, granular temperature, thermal temperature, pressure, heat transfer mechanisms excluding radiation were studied in detail. A comparison of results from two different drag models, namely, Syamlal-O’Brien and Gidaspow, was also included in this thesis.
It was found that there was no significant difference between the predicted pressure drops for different drag models, whereas it slightly increased with the increasing inlet gas velocity. The pressure drop increased with decreasing the initial volume fraction of biomass.
The distribution of solid volume fraction and velocity profiles were obtained for different cases. Although the results revealed the inherently chaotic nature of fluidized bed, the general trends were still obtained which agree good with previous studies.
The results also indicated that the solids mixing was strongly influenced by the inlet gas velocity and the initial volume fraction of biomass. Better particle mixing could be obtained by increasing the inlet gas velocity.
The granular temperatures of both solids were found to be higher near the bottom and the top portion of the bed. It increased with the increasing inlet gas velocity but with the decreasing initial percentage of biomass.
The heat transfer happening inside the fluidized bed was described by the distribution of solids temperature, the variation of heat transfer coefficients and the surface heat fluxes. Both heat transfer quantities were observed to be dominant in the dense bed regions as they strongly depend on the solids concentration in the fluidized bed. Due to the strong mixing, the distribution of biomass temperature was fairly homogeneous in the bed region. The increasing inlet gas velocity promoted the mixing of solid particles, thus resulted in effective heat transfer between the particles and wall to particles. There was no significant difference between the simulated total surface heat fluxes for different drag models.
Overall, the current results were based on some assumptions and in two-dimensions only due to the limit of time and computational effort during this work. Therefore, those results could not describe the adequately thermo-physical processes occurring in the fluidized bed reactor. A full three-dimensions model with less assumptions should be taken into account in future work. Furthermore, a quantitative comparison of the simulations with the experimental results can be done to further validate the utility of the suggested setup.
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