Numerical modeling of gas-particle flows inside fluidized beds

Abstract

Fluidized beds have widespread application in industry due to their increased rate of heat, mass, and momentum transfer. In order to effectively design fluidized beds at the industrial scale, it is essential to have an understanding of the complex hydrodynamic behavior of the dense gas-particle flows inside them. This thesis is focused on the bubbling fluidization of Geldart B particles. The Eulerian–Eulerian “Two-fluid model” (TFM) approach was used to simulate dense gas-particle flows inside two different three-dimensional (3D) bubbling beds. The numerical code Multiphase Flow with Interphase eXchanges (MFIX) was used to perform all the 3D simulations. The results were validated against published experimental data. This manuscript-based thesis documents four different studies. The first study, Chapter 2, reports an in-depth investigation of two different models for the particle stress tensor in the elastic-inertial regime and assesses their ability to predict the hydrodynamics of a 3D cylindrical fluidized bed. Contours of inertial number, defined as the ratio of the inertial forces to the frictional forces, were used to visualize the flow properties. Analysis of the flow properties for a range of gas-particle regimes based on the inertial number enhances our insight into the flow behavior in such a complex system. Chapter 3 reports a comprehensive study to assess the effect of three different particle-wall boundary conditions (BCs) on the structural features of a dense gas-particle flow inside a 3D thin bubbling bed. Accordingly, the effect of each wall model on the velocity field, 3D bubble statistics, gas-pressure fluctuations, and particle resolved-scale Reynolds stress were investigated. Also, the dominant mixing regions inside the bed were identified in order to quantitatively describe the bed performance. Chapter 4 performs an in-depth systematic study that uses a particle energy budget analysis to investigate the dynamics of the bubbling bed discussed in Chapter 3. The budget analysis helps not only to quantify the relative importance of various terms contributing to the energy cascade, but also to identify the regions in the bed where most of the energy transfer takes place. Chapter 5 applies state-of-the-art post-processing methodologies, namely, the Proper Orthogonal Decom- position (POD) and the swirling strength criterion to the fluctuating particle flow fields predicted by the TFM of a bubbling bed to identify and analyze the dominant spatio-temporal patterns of the particulate phase. The variation of the POD temporal coefficients associated with the particle volume fraction fluctu- ation field suggested the existence of a low-dimensional attractor and irregular periodicity in the flow. The particle vortical motions were characterized by their flat structure. POD was used to obtain a reduced-order reconstruction of the particle velocity and volume fraction fields using a subset of eigenmodes. In summary, this thesis attempts to quantitatively describe some important features of bubbling beds dynamics that have received relatively little attention in the literature. To this end, it was observed that the use of inertial number, investigation of the energy cascade process, and studying particle vortical structures were helpful to quantitatively explore the underlying physics of bubbling beds. A major objective was also to identify a set of proper TFM parameters and particle-wall BC for high-fidelity simulation of bubbling beds.