OCR Text |
Show SNO = [ ( Kf \PWO\ + k2f [N][02] + k3f [N][OH\ )i " ( klb [N][NO] t ^ [0][ATO] + *3, [/fl[NO] )2 (2) - ( *8 [Ctf][WO] + k9 [CH2][NO] )3 ] * MNO The three terms on the right-hand-side represent respectively the formation of NO due to the forward reactions of Rl to R3, destruction due to backward reactions of Rl to R3 and reduction by R8 and R9. In each individual reaction, the contribution to the source is the product of an Arrhenius reaction rate coefficient, k (see Table 1), and the associated species concentrations. The molecular weight M N 0 in Eq. (2) converts S N 0 to units of kg/m3/s. To properly calculate N O formation from Eq. (2), the concentration of radicals like O, OH, H, C H , CH2, resulting from the combustion of natural gas, should be known. This is made possible by using the flamelet model for non-premixed combustion (see Peters, 1984 and 1986; Bray and Peters, 1994). Using a flamelet library specifically designed for the natural gas used in the combustion experiments, this model can provide the time-mean concentrations of 36 species everywhere in the computation domain including all of those needed in Eq. (2) with the exception of the N-atoms. The N-atoms, together with H C N , N C O and N H , are the main N O precursors (see Rl to RIO). Because reactions R 6 and R 7 are very fast (see their k values in Table 1), it is reasonable to assume that N C O and N H have negligible net generation rate and do not need to be evaluated. Hence, only two N O precursors, N and H C N , are to be calculated together with N O . As in Eq. (2) for N O , the source terms S N (for N-atoms) and SH C N (for H C N ) are computed based on their formation and destruction paths in Rl to RIO. Once these sources are integrated to yield the time-mean averages (see next section), they are substituted into the transport equations of N, H C N and N O (in the form of Eq. (1)). These equations are then solved to give their concentration distributions in the entire computation domain. This approach of N O modelling is different from the common practice (e.g. see Peters and Weber, 1995; Dearden et al., 1996) where the entire prompt-NO mechanism is reduced to a single empirical global reaction. Also, this approach does not follow studies like Peters and Weber (1995) and Carvalho et al. (1990) which ignore reaction R3. Instead, R3 is retained so that rich flames can be included in the study. Furthermore, the concentrations of O and O H are obtained from the flamelet library rather than being estimated from the concentration of major species using the partial equilibrium assumption as in Dearden et al. (1996). The choice to solve for N is a novel addition. Most studies typically assume that the N-atoms are in quasi-equilibrium and are hence eliminated from the rate equations. This assumption is perhaps justified within the thermal N O reactions; however, N participates actively in both the thermal- and prompt-NO mechanisms (as seen from Rl to RIO). It seems more appropriate not to make this assumption and evaluate N directly. 4 |