| OCR Text |
Show velocity is computed, and from that the mean residence time profile for a gas molecule traveling down a single slice. For the purposes of this work, and within the calculation time/storage limits imposed by the computation approach, an empirical fit to the radial temperature profile is adequate. The empirical method provides excellent accuracy for furnaces whose thermal environment is well characterized. The radial temperature profile is divided into two regimes, the boundary layer and the bulk gas. The user specifies a boundary layer thickness (the dividing line) and the program applies a polynomial curve of up to sixth order to each regime. In the simplest version the bulk temperature is constant across the radius and the boundary layer temperature varies linearly between the bulk gas and the wall temperature. If data are available (from experiment or from three-dimensional modeling), more detail in the form of a polynomial curve can be added. The destruction of organic wastes can be arbitrarily separated into two processes: in-flame oxidation and pyrolysis and postflame oxidation and pyrolysis. Although the better part of destruction may occur within the flame region, the current combustion knowledge is not sufficient to quantify the complex interactions in real combustors. The complexity of chemical and physical processes within the flame forces consideration only of the oxidation and pyrolysis in the postflame region. Therefore the model will tend to over estimate the actual waste output for furnaces with highly efficient burners and atomizers. Even in the postflame gas phase, the chemical mechanism at the molecular level for typical large waste molecules is not well known. An alternative engineering approach is to describe the reaction scheme in terms of a small set of semi-global reaction rates of the Arrhenius form: •^- = A.C^CQ exp(-Ei/RT) i=l,n (3) where Ei, the activation energy; Aj, the pre-exponential factor; and the exponents a and b are empirically determined (for pseudo-first-order rates a and b equal unity). CO2 is the local oxygen concentration, R is the universal gas constant T is the local temperature, and C-j is the component 5.5.11 |
