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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Chemical and Biochemical Engineering

Supervisor

Zhu, Jesse

2nd Supervisor

Zhang, Chao

Joint Supervisor

Abstract

Various circulating fluidized bed (CFB) systems including gas-solid fluidization, liquid-solid fluidization, and gas-liquid-solid three-phase fluidization are numerically studied. With a comprehensive knowledge from the experiments, improved computational fluid dynamic (CFD) models are developed for detailed investigations on a wide operating range in the gas-solid CFB (GSCFB) system. The CFD model developed is also extended to study two new types of fluidized beds, an inverse liquid-solid circulating fluidized bed (ILSCFB) and a bubble induced fluidized bed (BIFB), as a supplement to the experimental work.

Flow structures and transitions from low-density operations to high-density operations in both GSCFB riser and downer are characterized based on numerical results and validated by experimental data. Correlations on the overall bed density in the GSCFB riser and downer under different operating conditions are developed respectively. The solid inlet geometry is found to have profound impacts on the flow structure in the GSCFB riser, which leads to the modifications on the inlet boundary conditions in the CFD model.

A cluster-driven drag model, which includes the information of clusters, is proposed for the simulation of the GSCFB riser. With more realistic physical meanings of the gas-solid interactions provided, a good agreement with the experimental results is also achieved. The cluster effects on the flow development and solids distribution are discussed based on the numerical results.

The CFD approach is also extended to study an ILSCFB system where light particles are used and validated by experimental results. The flow structures from the CFD simulations in the ILSCFB riser and downer are compared. CFD results show that the flow structure in the ILSCFB is more uniform compared with the GSCFB system. Numerical results also show that the binary particle system in the ILSCFB shares many similarities with the single-particle system.

A three-phase Eulerian-Eulerian CFD model is developed and validated by the experimental results for a newly invented BIFB. Three flow regimes and the corresponding transition gas velocities in the BIFB are defined based on the experimental and numerical results. Effects form the particle density, solids loading, and superficial gas velocity are also studied.

Summary for Lay Audience

Several types of circulating fluidized bed (CFB) systems are studied via computational fluid dynamics (CFD) approach in this work. CFB is a kind of chemical reactor to continuously handle granular materials. By introducing a fluid, such as gas, liquid, or even both gas and liquid, particles will be suspended, resulting in multiphase flows in a CFB. Except for the commonly seen gas-solid CFB systems, new types of CFBs, such as the inverse liquid-solid CFB and the bubble-induced inverse gas-liquid-solid three-phase fluidized bed, have been developed recently by changing the flow directions or the particle properties.

CFD approach is a numerical method that solves a set of governing equations, which describe the velocity and pressure fields of the multiphase flows, to simulate the flow mechanisms in the CFB systems. Due to the fast development of computer technology, CFD modelling has become an effective and economical tool to investigate the flow structures in various CFB systems. Different CFD models have been developed in this work for the gas-solid CFB riser and downer reactors, an inverse liquid-solid CFB, and a bubble-induced three-phase fluidized bed, respectively. The flow structures, such as profiles of solids concentration and velocity, flow development, and the interactions between particles and fluid are investigated. .A cluster-driven drag model for the simulation of gas-solid CFB risers is proposed, which includes the characteristics of particle clusters based on the data obtained from experiments.

The expansion of fluidization technology relies on both experimental and numerical works. Experimental work can help improve the numerical theories by providing more accurate descriptions of the underlying physics with a comprehensive knowledge of the fluidized bed systems. CFD modelling can supplement to the experimental study by carrying out the simulations under a wider operating window and provide more information in the micro or meso scale. The fulfillment of the fluidization map can be achieved by the co-work from experimental studies and numerical simulations.

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