Detailed knowledge of solids circulation, bubble motion, and frequencies of porosity oscillations is needed for a better understanding of tube erosion in fluidized bed combustors. A predictive two-phase flow model was derived starting with the Boltzman equation for velocity distribution of particles. The model is a generalization of the Navier-Stokes equations of the type proposed by R. Jackson, except that the solids viscosities and stresses are computed by simultaneously solving a fluctuating energy equation for the particulate phase. The model predictions agree with time-averaged and instantaneous porosities measured in two-dimensional fluidized beds. Observed flow patterns and bubbles were also predicted.

A bubbling fluidization model using kinetic theory of granular flow

This report describes the MFIX (Multiphase Flow with Interphase exchanges) computer model. MFIX is a general-purpose hydrodynamic model that describes chemical reactions and heat transfer in dense or dilute fluid-solids flows, flows typically occurring in energy conversion and chemical processing reactors. MFIX calculations give detailed information on pressure, temperature, composition, and velocity distributions in the reactors. With such information, the engineer can visualize the conditions in the reactor, conduct parametric studies and what-if experiments, and, thereby, assist in the design process. The MFIX model, developed at the Morgantown Energy Technology Center (METC), has the following capabilities: mass and momentum balance equations for gas and multiple solids phases; a gas phase and two solids phase energy equations; an arbitrary number of species balance equations for each of the phases; granular stress equations based on kinetic theory and frictional flow theory; a user-defined chemistry subroutine; three-dimensional Cartesian or cylindrical coordinate systems; nonuniform mesh size; impermeable and semi-permeable internal surfaces; user-friendly input data file; multiple, single-precision, binary, direct-access, output files that minimize disk storage and accelerate data retrieval; and extensive error reporting. This report, which is Volume 1 of the code documentation, describes the hydrodynamic theory used in the model: the conservation equations,more » constitutive relations, and the initial and boundary conditions. The literature on the hydrodynamic theory is briefly surveyed, and the bases for the different parts of the model are highlighted.« less

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MFIX documentation theory guide

Eulerian two-phase flow theory applied to fluidization

Hydrodynamics of gas-solid fluidization

Basic Remarks on Modeling and Simulation Solid Fuels Equipment and Processes Basic Calculations Zero-Dimensional Models Introduction to One-Dimensional Steady-State Models Moving-Bed Combustion and Gasification Model Chemical Reactions Heterogeneous Reactions Drying and Devolatilization Auxiliary Equations and Basic Calculations Moving-Bed Simulation Programs and Results Bubbling Fluidized-Bed Combustion and Gasification Model Fluidization Dynamics Auxiliary Parameters Related to Fluidized-Bed Processes Bubbling Fluidized-Bed Simulation Program and Results Circulating Fluidized-Bed Combustion and Gasification Model Circulating Fluidized-Bed Simulation Program and Results Appendices Index

Solid Fuels Combustion and Gasification: Modeling, Simulation, and Equipment Operations

The hydrodynamics of fluidization for two thin ''two-dimensional'' fluidized beds, one containing a centrally located rectangular obstacle and the other containing three square tubes roughly approximating a portion of a fluidized bed combustor (FBC), was solved using the FLUFIX/MOD2 computer program. The monolayer energy dissipation erosion model extended to include solids viscous stress was compared with the Finnie erosion model. Hydrodynamic and erosion convergence studies were performed for the first bed. Computed erosion rates from the monolayer energy dissipation erosion model compare more favorably for both beds with available erosion data than those from the Finnie erosion model. 27 refs., 15 figs.

Hydrodynamics of erosion of heat exchanger tubes in fluidized bed combustors

Hydrodynamics of fluidization: Fast-bubble simulation in a two-dimensional fluidized bed

Erosion in fluidized-bed combustors, a nearly commercial means of burning coal cleanly, has surfaced as a serious issue that may negatively affect their economics. The evidence suggests that the key to understanding this erosion is detailed knowledge of the coupled and complex phenomena of solids circulation and bubble motion. The FLUFIX computer code has been developed for this purpose. Computed hydrodynamic results compare well with limited experimental data (including the bubble frequency and size and the time-averaged porosity distribution) taken in a thin two-dimensional rectangular fluidized bed containing a rectangular obstacle. The monolayer energy dissipation erosion model is developed and shown to generalize the so-called power dissipation erosion model used successfully to analyze slurry jet pump erosion. The computed transient and time-averaged transient energy dissipation rates for the monolayer energy dissipation and Finnie erosion models are compared. The computed erosion rates are compared with each other and with available literature erosion data to validate the calculations. The results are in reasonable agreement. More erosion experiments are needed to further validate the computations. 29 refs.

Erosion calculations in a two-dimensional fluidized bed

Erosion in bubbling fluidized-bed combustors is a serious issue that may affect their reliability and economics. Available evidence suggests that the key to understanding this erosion is detailed knowledge of the coupled and complex phenomena of solids circulation and bubble motion. A thin ''two-dimensional'' rectangular fluidized bed with an obstacle served as a rough model for a fluidized-bed combustor. This model was studied experimentally and computationally using two hydrodynamic equation sets. The computed hydrodynamic results agree reasonably well with experimental data. Bubble frequencies compare well with those obtained from a high speed motion picture study. Time-averaged porosities computed from both models agree with time-averaged porosity distributions measured with a gamma-ray densitometer. 46 refs., 13 figs., 2 tabs.

Hydrodynamics of fluidization: Time-averaged and instantaneous porosity distributions in a fluidized bed with an immersed obstacle

Argonne National Laboratory's state-of-the-art hydrodynamic and erosion modeling capabilities are summarized in this paper. Recent improvements in the hydrodynamic model, and the monolayer energy dissipation and Finnie erosion models are explained as are the implementation of the Neilson-Gilchrist erosion model. Hydrodynamic results are presented and the erosion models are compared with each other for a generic two-dimensional few-tube approximation of a cold model fluidized bed combustor experiment as well as for single round and square tubes. Power spectral methods constituting means, autocorrelations, power spectral densities, variances ad cross-correlations are used to analyze the hydrodynamic results and to attempt correlating them with the erosion patterns and rates. All of the erosion models predict order of magnitude agreement with each other and with limited erosion data. However, the monolayer energy dissipation model produces more reliable trends such as the effect of fluidizing velocity on the overall erosion rate. 29 refs., 13 figs., 2 tabs.

D. Gidaspow

Papers

Hydrodynamics of erosion of heat exchanger tubes in fluidized bed combustors

Hydrodynamics of fluidization: Fast-bubble simulation in a two-dimensional fluidized bed

Erosion calculations in a two-dimensional fluidized bed

Hydrodynamics of fluidization: Time-averaged and instantaneous porosity distributions in a fluidized bed with an immersed obstacle

State-of-the-art computation of dynamics and erosion in fluidized bed tube banks