OCR Text |
Show 2.0 BACKGROUND 2.1 Process Heater Model Overview The model used specifically for the simulation of process heaters provides full coupling between the "fireside" and "process side" computations. Heat transfer between the process coils and the hot radiant firebox depends on the process fluid temperature, the overall heat transfer coefficient from the process fluid to the hot coil surface, the fireside temperature distribution, and the local flow field. T he heat transferred to the process fluid affects the local chemical composition and temperature which is then fed back to the fireside computations. Further details concerning how this is accomplished in the model are discussed in the following two sections. 2.2 Fireside Modeling 2.2.1 Gas Phase Combustion The computational approach involves numerical discretization of the partial differential equation set which describes the physics of the system. Typically 105 - 10 discrete computational nodes are needed to resolve the most relevant features of a three-dimensional combustion process. Around 60 variables (representing, e.g., gas velocity, temperature, concentration of various chemical species) are tracked at each node. Accurate simulation of the combustion processes requires accurate modeling of the dominant or controlling physical mechanisms in the process. Simulation of process temperatures in a process heater requires modeling of the flow patterns, reaction chemistry, gas and wall temperatures, heat transfer in the furnace (fireside), and heat transfer and temperature change in the process fluid (process side). In the computer model used here, coupled equations of chemical reaction, turbulent fluid flow, and convective and radiative heat transfer are solved to give a realistic and detailed model of the processes taking place within the fired zone. Turbulence is modeled using traditional methods of moment closure including Prandtl's mixing length model, the two-equation k-e model (Launder and Spalding, 1972) and the nonlinear k-e model (Speziale, 1987). In all simulations discussed in this report, the nonlinear k-e model was used due to its more accurate prediction of normal Reynolds stress effects, allowing the prediction of secondary flows as occur in non-circular ducts. Within the model, the rate at which the combustion reactions occur is assumed to be limited by the rate of mixing between the fuel and the oxidizer. That is, the rate of chemical reactions is assumed to be fast compared to the rate of mixing (i.e. full chemical equilibrium is assumed), which is a reasonable assumption for the chemical reactions governing heat release. The thermochemical state at each spatial position is a function of the degree of mixing (parametrized by the mixture fraction,/), the mass fraction of particle off-gas (r\), and the enthalpy (parametrized by the degree of heat loss, HL). The effect of turbulence on mean chemical composition is incorporated by assuming that the mixture fraction at each spatial position is described by a "clipped-gaussian" probability density function (PDF) having spatially varying mean and variance. M e a n species concentrations are obtained by convolution over this assumed PDF. 2 |