OCR Text |
Show N 0 X MODELLING APPROACH The objective of N O x modelling is to obtain a prediction of the N O field within the calculation domain. It is assumed that the predominant contribution to N O x comes from N O in this study. Mathematically the task involves solving the steady state balance equation for N O which has the form a,--- , a N0 dXj + sNO m where the left-hand-side represents the convection of N O species ((pN0) by fluid flow velocity tij, and the right-hand-side is the diffusion of cpNO with effective diffusion coefficient Dno plus a source term SN0. The essence of N O modelling is to determine how the time-mean source term SN0 should be represented based on the rates of N O reactions in natural gas flames. In natural gas combustion, there are two major sources of NO: the oxidation of atmospheric nitrogen by the Zeldovich thermal N O mechanism (Zeldovich et al., 1947) and the reaction sequence initiated by hydrocarbon radicals with molecular nitrogen via the Fenimore prompt-NO mechanism (Fenimore, 1971). There is a third mechanism that involves the reaction of O atoms with N 2 to form N 2 0 which subsequently transforms to N O (Malte and Pratt, 1974). The N 2 0 mechanism is typically important only in premixed, pressurized combustion (Bowman, 1992). The current study focuses primarily on atmospheric diffusion flames and hence the N 20 mechanism is not included. Thermal NO Reactions The formation of thermal NO is governed by the extended Zeldovich reactions (see Miller and Bowman, 1989): O + N2 ~ NO + N (Rl) N + 02 ~ NO + O (R2) N + OH ** NO + H (R3) This reaction path is called the thermal NO mechanism because the initial step, i.e. the forward reaction of Rl, has a very high activation energy and is therefore very temperature sensitive. Prompt NO Reactions Following Fenimore's suggestion (Fenimore, 1971) and more recent studies by others (see Drake and Blint, 1991, Glagborg et al. 1992, Miller, 1996), the prompt N O mechanism is assumed to be initiated by 2 |