Flow Structure Investigation And Computational Fluid Dynamics Simulation For Baffled Bubbling Fluidized Beds

Baffled bubbling fluidized beds are widely applied in chemical industries due to high gas-solid contacting efficiency as well as low particle back-mixing. Traditionally, the scale-up of baffled bubbling fluidized beds are mainly based on trial and error approaches, which are time and cost consuming. With the development of computational fluid dynamics (CFD) method, it is now possible to study the scale-up effect via numerical simulation. In CFD simulations, drag models play an important role to determine the flow characteristics in fluidized beds. Conventional drag models, derived from the homogeneous assumption in a control volume, overestimate the drag force between gas and solid in fluidized beds, resulting in low simulation accuracy. Baffles have significant influence on the hydrodynamics of fluidized beds. Hence, equations in the drag models developed for baffle-free fluidized beds like the force balance equations and empirical corrections are not directly applicable to local regions of fluidized beds due to the addition of baffles and need to be modified. So far, there is no general method to conduct the modification. It is therefore of vital importance to develop a reliable drag model for baffled fluidized beds. In this work, an approach suitable for CFD simulations of bubbling fluidized beds with horizontal baffles was explored, and a structure-based drag model was modified to predict the flow characteristics of baffled bubbling fluidized beds.The present thesis investigates bubbling fluidized beds with perforated plates and louver baffles, where the solid volume fraction distributions in both radial and axial directions and the axial pressure distribution were measured by an optical fiber probe and a U-type tube pressure meter. In addition, the averaged bubble diameter was measured by double optical fiber probe for the bubbling fluidized bed with the louver baffles. It was found that the whole fluidized bed was divided into several fluidized sections by horizontal baffles. Bubbles were broke up to form smaller bubbles after passing baffles. Consequently, the baffle actually redistributes the gas like a gas distributor and the averaged bubble size after passing a baffle is quite close to characteristic open size of the baffle, e.g. the two adjacent vane distance of the louver baffle. In addition, the axial solid volume fraction reaches the lowest value at dilute regions below the baffles and the axial solid volume fraction decreases with increasing the superficial gas velocity. These experimental results provide useful information for later CFD simulations.To study the baffle effect by the CFD simulations, it is proposed to treat the horizontal baffles as gas distributors. The CFD simulations of the baffled bubbling fluidized beds were therefore transformed to several baffle-free bubbling fluidized beds in series with the baffles as the gas distributors. The baffle-free bubbling fluidized bed was simulated by the structure-based drag model. Based on the above assumption, the modified structure-based drag model was coupled into the two fluid model and the commercial software Fluent was employed to conduct the numerical simulations in three dimensions. Simulated results indicate that the modified structure-based drag model showed higher accuracy. The bed expansion, axial and solid volume fraction distributions as well as the axial pressure distribution can be better predicted, as compared with the Gidaspow drag model.The present thesis has also explored to couple the modified structure-based drag model into the discrete particle model using the commercial software Barracuda, and compared the simulation results with other drag models like the Wen&Yu-Ergun drag model and the Parker drag model. The modified structure-based drag model has advantages over other drag models in terms of the calculation accuracy.