Ignition diagrams for selected hydrocarbons

For fuels of different carbon atom numbers, or different C H ratios, some relationship to the respective mixture at J = 1 would seem preferable. For example, a comparison is made in Fig. 6.15 of the ignition and cool-flame boundaries of m-CsHm and n-C4Hio in air obtained at I = 1 in a nickel steel cylindrical 190 cm vessel [138]. The considerably enhanced reactivity of w-hexane over that of -butane is apparent throughout almost the entire range of conditions. However, that the reactivity of n-hexane approaches or may even be lower than that of n-butane at about 650 K may be attributed to an inversion created by the underlying chemistry in the ntc region (see Section 6.5). 

A complementary ignition diagram to that for n-C4Hio in an unstirred closed vessel was obtained in a jet-stirred flow reactor by Proudler et al. [58]. The reactor was a 500 cm stainless steel cylinder, operated at a mean residence time of 9.4 0.4 s with a reactant mixture [n-C4Hio] [O2] = 1.13 1 (Fig. 6.16). Oscillatory cool-flames and ignitions were detected within narrow temperature ranges, but comparable with those of the closed 

A marked contrast is observed in the behaviour of the simplest aromatic hydrocarbon-air mixtures at high pressures. No cool-flame phenomena or an ignition peninsula in the (p-Ta) diagram are observed. These are found only when sufficiently reactive aliphatic side-chains are associated with the aromatic ring. Burgoyne et al. [129] showed this to be the case for n-propylbenzene in a closed vessel (Fig. 6.18). The ortho- and meta-isomers of the xylenes also showed a similar reactivity. Benzene, toluene and ethylbenzene were found to undergo spontaneous ignition at temperatures only above 700 K. 

Experimental studies of relationships between fuel structure and the ease with which hydrocarbons undergo spontaneous ignition have received considerable attention from engineers through the characterization of 

(p-Ta) ignition diagrams for (a) benzene, (b) toluene, (c) ethyl benzene, and (d) n-propyl benzene. (After Burgoyne et al. [35].) 

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