Numerical modeling of gas-particle flows inside fluidized beds
Date
2018-06-11
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Type
Thesis
Degree Level
Doctoral
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.
Description
Keywords
Numerical modelling, Fluidized bed, Frictional stress, Wall boundary condition, Energy cascade, Particle phase, POD
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Mechanical Engineering
Program
Mechanical Engineering