| OCR Text |
Show implies relative rates of 1, 3 and 7 for these rate expressions respectively with consequential effects on any detailed modelling of these reactions. Equation (3) is similar to the expression obtained experimentally by De Soete [9] but we [10] have modified it to the form given below, namely (4) where f = 4.75 + C1 n - C2 9 + C3 92 - C4 93 where C1 - C4 are constants, n is the number of hydrocarbon fuel and 9 the fuel/air equivalence ratio. Values of C1 to C4 are 8.19 x 10-2 , 23.2, 32, 12.2 respectively. These values are valid for aliphatic alkane hydrocarbon fuels and for 9 between 0.62 - 1.43. The form of equation 4 is consistent with equation 3 with the addition of the [02]a term which represents the efficiency of conversion of HCN . to NO (as opposed to N2 under ri ch condi t ions) . However equat ion 4 only applies to high temperature flames and a modification of this based on reaction orders calculated from experimental composition data must be applied in the lower temperature slower reacting regime. We have found [2] that Ea can take two values corresponding to a low temperature range and a high temperature one, namely for T < 1920 K Ea = 42.5 kcal/mol and for T > 1920 K Ea = 72.5 kcal/mol, where the values of Ea are based on experimental data (11, 12). It should be noted that equation 4 can be cast in a non-Arrhenius form with ~ = 1 (as in equation 3 but with a variable power of T and a constant activation energy, namely (5) where c is a constant, although we have used equation 4 for the calculations in this paper. In practice the methane and oxygen exponents, a and ~, are variable and can take values respectively from 0 to 1 for 02 and 0.2 to 1 for CH4 . At low temperatures (i.e. < 1300 K) the reactivity of CH4 with 02 is very 5 |
