OCR Text |
Show The chemistry module quantifies the effect of homogeneous gas phase chemical reactions within the furnace. A common assumption in combustion modeling is that the chemical reaction rates are controlled by the rate of turbulent mixing. This "fast kinetics" assumption, valid when the reaction time scales are much less than the mixing time scales, greatly simplifies the model equations and provides reasonably accurate predictions for the major chemical components. Temperature and velocity field calculations are also predicted adequately. For the NOx chemistry, where reaction time scales are similar in magni tude to the time scales for mixing, the "fast kinetics" hypothesis is not valid. A full kinetic description is used in the present effort. To meet the objectives of this work, the react10n set employed must contain detailed descriptions of the hydrocarbon combustion, thermal NOx, and prompt NOx kinetic mechanisms. Westbrook and Dryer have reported a large number of potential reactions in a typical combustion process4 • A kinetic sensitivity analysis was performed to determine a realistic but manageable reaction set to describe the combustion/NOx chemistryS . The analysis was performed ·by simulating a premixed gas-oxidant flame under adiabatic conditions. Table 1 displays the resulting set of 115 reactions chosen to represent the complex chemistry. Thermodynamic properties for the reacting species are evaluated based on the JANF databanks. The control volumes are treated as perfectly. stirred reactors (PSR) in the solution procedure. For initialization purposes only, each control volume is assumed to be at chemical equilibrium. During subsequent iterations, the steady state solutions to the fully kinetic component material balance equations are determined via a Newton-Raphson convergence. If the Newton-Raphson method fails, which is possible for near-equilibrium control volumes, the transient PSR equations are integrated to their steady state solution via a stiff ODE solver7 • The heat transfer module calculates the radiative transport in the furnace using the method of discrete ordinates8 • The resul ting equations wi th the appropriate radiati ve boundary conditions are integrated over each control volume and solved by a point-by-point iterative scheme. The model geometry represents a cylindrical furnace wi th a burner at one end and an exi t flue at the other end. The grid can be adjusted to prov~de more nodes at a given location to increase accuracy without severely penalizing computational time. Typically there should be more node points in the vicini ty of the flame and fewer node points near the flue end of the furnace. GRREK currently operates on a VAX 11-780 minicomputer. Solution time varies greatly depending on the nature of the problem, but is on the order of several minutes to several -4- |