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Show Computational Fluid Dynamics Methods NOxOUT projects were modeled using a computational fluid dynamics software package called "PHOENICS" (Cham, Ltd.)l8) running on a Sun 4/110 Workstation. This program solved the set of conservation equations in order to predict fluid flow patterns, temperature distributions, and chemical concentrations within cells representing the geometry of the physical unit. In addition to the standard PHOENICS features, a set of FORTRAN subroutines were written to describe flue gas properties and injector characteristics which were needed by PHOENICS in the solution of the equations. The process units were approximated as a set of space-filling cells that adequately resembled their physical geometry. The number of cells was chosen to be great enough to provide the necessary details of the unit, but not so great as to require unacceptable data storage space or computational time. Anywhere from 4,000 to 30,000 cells were used, depending on the number of conserved quantities solved. The intricacies of the physical unit were included by setting the porosities of individual cells or cell faces to values between 0 and 1. In this way it was possible to closely approximate the geometry of the process unit being modeled. Cells corresponding to the locations of inlets or exits on the unit were assigned net mass sources which were positive for inflow or negative for outflow. Energy sources such as heat loss to a tube bundle or heat released during combustion were also specified for cells where appropriate. Chemical concentrations of different species were specified for mass entering a cell or for compositional changes due to reactions. Numerical approximations for the conserved quantities were found by integrating the governing equations over each of the individual cells, resulting in a set of algebraic equations relating the average values within each cell to the fluxes between adjacent cells. The conserved quantities were the total mass, the mass of each independent chemical species, the total momentum, and the total energy. Special sources such as reactions or heat transfer were added to the flows through the cell faces to determine the total flow into or out of each cell. Once boundary and initial approximations for each variable were assigned, the total amount of conserved quantities flowing into and out of a cell from adjacent cells (using both convective and diffusive transport mechanisms) were determined. In a steady state solution, the net flow for a given cell was very close to zero; that is, the amount of a quantity flowing into a cell exactly equaled the amount flowing out. If the solution was not at steady state, a net imbalance existed which caused an accumulation of mass, energy, or momentum in a cell. This accumulation produced a change in the flow and physical properties of the cell, and the new values were used as initial values for the next iteration. Iterations were performed until the total changes in properties were sufficiently small compared to their absolute values. An ideal gas equation of state was used to estimate flue gas density, and the thermal properties and viscosity of flue gas were estimated from published data. The heat capacity of flue gas was assumed to be constant, but was adjusted depending on -4- |