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Show the average moisture content for flue gas of the modeled unit. The k-epsilon model(9) has been widely used as an estimate of the effects of turbulent dispersion. This model has been incorporated in some NOxOUT studies, but in others the increase in computation time and data storage requirements as well as reduction in convergence rate were restrictive. Consequently, a simpler approach has also been used. The primary effect of turbulence is to greatly increase the rate of mass and energy dispersion, resulting in much larger transfer coefficients than in non-turbulent situations. When expediency was required, the turbulence was included by increasing the transfer coefficients to values comparable to those for k-epsilon predictions in similar projects. The heat released during combustion reactions has been modeled in several ways. In the most simple case, the heat was added as an enthalpy source in a boundary cell containing the mass inflow. Alternately, this heat was released in a set of cells covering the expected combustion zone. This latter method had the advantage that the predicted temperatures were more realistic due to the greater distribution volume. For a few cases, the combustion process was modeled as a simple combustion reaction with a fixed rate constant and heat of reaction. The chemical reaction model gave more realistic combustion zone predictions and temperature estimates, but was very costly in terms of convergence, data storage, and total computational time. Consequently, combustion was usually approximated as occurring in a specified zone with the sources of heat and combustion products distributed throughout the volume. Radiation is a primary heat transfer mechanism in furnaces, but is also very difficult to treat computationally. Because of the complexity of numerical treatment, radiation has not yet been specifically included in the model. Instead, the rate of thermal conduction was increased in order to accelerate the process of heat loss to the surroundings, and heat transfer rates to the unit boundaries were also increased to account for the additional heat transfer. This approximation is admittedly crude, and the treatment of turbulence will be improved as process development continues. Heat transfer to internal tube bundles was modeled as a heat loss per unit volume over the cells corresponding to the bundle locations. Some development work is in progress to improve the accuracy of heat transfer coefficient estimates. Despite the simple estimates for physical properties and transport mechanisms, the model has provided surprisingly good estimates of field performance and has been useful in explaining observed behavior. Modeling of Spray Injectors The major portion of CFD model development work at Nalco Fuel Tech has been the treatment of spray injectors. Typical sprays produce droplets with a wide range of sizes traveling at different velocities and directions. These drops interact with the flue gas and quickly evaporate. It is imperative to avoid impingement of -5- |