Both experimental and computational studies of the fluidization of high-density polyethylene (HDPE) particles in a small-scale full-loop circulating fluidized bed (CFB) are conducted. Experimental measurements of pressure drop are taken at various locations along the bed. The solids circulation rate is measured with an advanced particle image velocimetry (PIV) technique. Bed height of the quasi-static region in the standpipe is also measured. Comparative numerical simulations are performed with a computational fluid dynamics solver utilizing a discrete element method (CFD-DEM). This paper examines the effect of different drag laws used in the CFD simulations through a detailed and direct comparison with experimental data from a small-scale, full-loop circulating fluidized bed. The Hill−Koch−Ladd drag correlation was shown to have good agreement with respect to system component pressure drop and inventory height in the standpipe.

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Modeling of heat transfer and reactive chemistry for particles in gas-solid flow utilizing continuum-discrete methodology (CDM) by Jordan M. H. Musser A comprehensive multi-phase flow model requires coupled hydrodynamics, boundary conditions, heat and mass transfer, and chemical reaction kinetics. A model must also capture the multi-scale nature of these problems. Computational fluid dynamics-discrete element method (CFD-DEM) provides an accurate description of chemical reactions and heat and mass transfer at the particle scale. Currently, MFIX-DEM, the existing CFD-DEM used as the foundation for this work, can only model coupled hydrodynamics. This dissertation extends the functionality of MFIX-DEM by addressing the remaining deficiencies in three separate efforts. The first effort outlined in this dissertation focuses on the algorithmic development of discrete mass inflow and outflow boundary conditions. This approach allows for the construction of more dynamic models of gas-solid systems. It permits the amount and type of particles to fluctuate during a simulation. Examples illustrating the added functionality are provided. The second investigation explores the three modes of heat transfer in gas-solids systems. Models for particle-particle contact conduction, particle-fluid-particle conduction, particle-gas convection, and particle-particle radiation are selected. Model selection is based on model simplicity, acceptance in existing CFD-DEM heat transfer models, extendibility to particles of different sizes, and computational expense. Modifications are made to selected models before implementing them into MFIX-DEM. The implementation of each model is verified for simple two particle test cases, or in the case of gas-particle convection, a single fixed particle in a flowing gas. Strong agreement is observed between the simulation data and the analytic or numerical solution. Finally, a mathematical interface for managing user-defined particle-gas chemical reactions is developed. The shrinking, unreacted core model is selected as the particle reaction model for its accurate physical account of particle-gas reactions and ability to allow particles to initially contain inert material. The implementation of the reactive chemistry interface is verified for a single reacting particle. Strong agreement is observed between simulation data and the analytic solutions for the particle's mass, species mass fraction, and internal energy equations. Agreement between the simulation data and analytic solution for the shrinking, unreacted core is considered acceptable. ! Update one over the particle's moment of inertia OMOI(NP) = 5.0d0 / (2.0d0 * PMASS(NP) * DES_RADIUS(NP)**2) ! Clear the necessary variables.

In this paper, the limitations of axisymmetric and Cartesian two-dimensional (2D) simulations of cylindrical gas-solid fluidized beds are discussed. A new method has been proposed to carry out pseudo-two-dimensional (2.5D) simulations of a cylindrical fluidized bed by appropriately combining the benefits of Cartesian 2D and axisymmetric assumptions. This is done by constructing a computational domain consisting of a central thin plate and two wedges. The proposed method was implemented in the open-source code MFIX and applied to the simulation of a lab-scale bubbling fluidized bed with necessary sensitivity study. After a careful grid study to ensure the numerical results are grid independent, detailed comparisons of the flow hydrodynamics were presented against axisymmetric and Cartesian 2D simulations. Furthermore, the 2.5D simulation results have been compared to the three-dimensional (3D) simulation for evaluation. This new approach yields better agreement with the 3D simulation results than with axisymmetric and Cartesian 2D simulations.

To improve quality of numerical models used in simulations of a fluidized bed gasifier at any scale, the sources of uncertainty in the simulation have to be identified and quantified. There are several sources of uncertainty that can affect any simulation result and scale up process such as uncertainty in the model input values, uncertainty in the reaction models and kinetic rates, uncertainty in selection of the appropriate numerical models affecting the hydrodynamics, uncertainty in selection of adequate computational grid resolution (uncertainty due to discretization error), uncertainty in the selection of proper numerical techniques required for solution of the discretized conservation equations, and many more. The current study aims to investigate the effect that reaction models for gasification, char oxidation, carbon monoxide oxidation, and water gas shift will have on the syngas composition at different grid resolution, along with bed temperature, which affects the reactions. The global sensitivity analysis conducted showed that among various reaction models employed for water gas shift, gasification, char oxidation, the choice of reaction model for water gas shift has the greatest influence on syngas composition, with gasification reaction model being second. Syngas composition also shows a small sensitivity to temperature of the bed. The hydrodynamic behavior of the bed did not change beyond grid spacing of 18 times the particle diameter. However, the syngas concentration continued to be affected by the grid resolution as low as 9 times the particle diameter. This is due to a better resolution of the phasic interface between the gas and solid that leads to stronger heterogeneous reactions.

Uncertainty quantification (UQ) analysis is increasingly becoming one of the major requirements of simulationbased engineering to assess the confidence in the results and make better-informed decisions based on the insight derived from the simulations. In an earlier study, Bayesian UQ analysis was applied to existing bench-scale fluidized-bed gasifier experiment results. In the current study, a series of simulations were carried over with the open-source computational fluid dynamics software MFiX to reproduce the experimental conditions, where three operating factors, i.e., coal flow rate, coal particle diameter, and steam-to-oxygen ratio, were systematically varied to understand their effect on the syngas composition. Bayesian UQ analysis was this time performed on the numerical results for comparison purposes. This is part of ongoing research efforts to explore the applicability of advanced UQ methods and processes such as Bayesian methods for large-scale complex multiphase flow simulations. As part of Bayesian UQ analysis, a global sensitivity analysis was performed based on the simulation results, which shows that the predicted syngas composition is strongly affected not only by the steam-to-oxygen ratio (which was observed in experiments as well) but also by variation in the coal flow rate and particle diameter (which was not observed in experiments). The carbon monoxide mole fraction is underpredicted at lower steam-to-oxygen ratios and overpredicted at higher steam-tooxygen ratios. The opposite trend is observed for the carbon dioxide mole fraction. These discrepancies are attributed to either excessive segregation of the phases that leads to the fuel-rich or -lean regions or alternatively the selection of reaction models, where different reaction models and kinetics can lead to different syngas compositions throughout the gasifier.

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MFIX-Exa is a computational fluid dynamics–discrete element model (CFD-DEM) code designed to run efficiently on current and next-generation supercomputing architectures. MFIX-Exa combines the CFD-DEM expertise embodied in the MFIX code—which was developed at NETL and is used widely in academia and industry—with the modern software framework, AMReX, developed at LBNL. The fundamental physics models follow those of the original MFIX, but the combination of new algorithmic approaches and a new software infrastructure will enable MFIX-Exa to leverage future exascale machines to optimize the modeling and design of multiphase chemical reactors.