| OCR Text |
Show 1.6.8 is the partial pressure in atmosphere of H20, estimated as 0.15. It is found that [OH] is smaller than [OH]eq over the entire temperature range of interest. The difference is even more pronounced if [OH] is calculated using the value of k3 recently measured by Fujii et al. C29], and by Silver and Kolb [301 Their combined measurements covered the temperature range 294-1250 K, and yielded k3 = 3.2 x 1012 exp(-1067/T) cmSgmole'is"1, where T again is in kelvin. For the temperature range of present interest (1076-1287 K), this yields k3 si 1.3 x 1012 cm3gmole"is"i. The large difference between [OH] and COH]eq suggests that [OH] is determined by a kinetic process involving reactions (3) and (2) as sinks for the hydroxyl radicals. Various consequences of such a kinetic process are discussed in the next section. The model of Sec. II can also be applied to the experimental results of Lucas and Brown [141 These results were obtained in a combustion tunnel, as were the results of [41 However, the conditions of the two experiments were rather different. The residence time in [14] was on the order of 35 ms, which is about 20 times shorter than that in [4], The diameter of the test section used in [14] was 5 cm, of that used in [4] 122 cm; the length of this section was 60 cm in [14] and 122 cm in [41 The fuel used in [14] was #1 diesel oil, that used in [4] was natural gas. The data shown in Fig. 6 of [14] are reproduced in the present Fig. 6. They represent the values of [NO] measured at the end of the test section, at various values of A and of temperature. Similar results are shown in Figs. 7, 8 and 11-13 of [141 The data again show a pronounced minimum near 1200 K. The curves of Fig. 6 represent the fit that could be obtained using the model of Sec. II. They are based on the following values: k1k3/k2 = 3.5 x 1010 cm3gmole'1s"1 k3[OH] = 3.0 s"1, [NO]c = 8.5 ppm at 1110 K k3[OH] = 26 s"1, [NO]c = 72 ppm at 1263 K. The first of these values is in reasonable agreement with Eq. (17), while the values obtained for k3[OH] and [NO]c are appreciably higher than those given by Eqs. (18) and (19). Apparently, the experimental OH-concentration in [14] was considerably larger than in [41 In fact, the curves shown in Fig. 6 correspond to [OH] = [OH3gq, provided k3 = 5.6 x 10** cm3gmole~1s"1. The quantity [OH]eq was calculated using the values of the 02- and H20-concentrations corresponding to the experimental equivalence ratio (C021 si 3.4%, CH20] s: 11.5%). The larger values of [OH] correspond to the smaller values of the characteristic reaction time tc obtained in [14] as compared with [41 As pointed out in [14], comparably short reaction times were obtained by Fenimore [12]. In two important respects, the curves of Fig. 6 are similar |
