Preflame studies

Flame speeds were measured on a nozzle burner. Fuel and air were metered and mixed in the same manner as in the preflame studies. The entire gas supply line and burner assembly were electrically heated to avoid condensation of fuels and additives. The burner was surrounded by a wider tube to prevent ambient air from influencing the flame. The flame speed was determined from the height of the flame cone. 

Preflame and Ignition Studies. The studies were carried out with twice stoichiometric fuel-air mixtures. When a halogen compound was present in a fuel, the flow rate of the fuel stream was adjusted to maintain the hydrocarbon-to-air ratio at twice stoichiometric. 

These studies provide comprehensive information on the effects of many halogen compounds on the combustion behavior of various hydrocarbons. The combustion behavior included preflame oxidation, ignition over a wide range of temperature, and the flame process. The effect of iodine compounds on preflame combustion, which had not been previously investigated, is now reported. 

Seakins (16) has reported that the low temperature oxidation of propane is promoted by chloroform but not by carbon tetrachloride. Our studies, however, show that chloroform and carbon tetrachloride have generally similar effects on all preflame stages (Figure 3) and that their patterns of oxidative degradation are also similar (Figure 8). Under the conditions of Seakins experiments the following reaction, which he suggested, probably initiates the sequence of reactions responsible for promotion. 

Sampling Studies. To obtain a better understanding of the chemistry of preknock reactions, a number of investigators have sampled and analyzed hydrocarbon-air mixtures undergoing preflame oxidation in engines. The types and concentrations of some of the more stable intermediates taking part in these reactions have been determined. While... 

Emission of cool flame radiation is associated with an early stage in the preflame reactions of most hydrocarbons, as evidenced by the fact that the appearance of cool flames occurs at the same time as the initial pressure development. At a later time in the cycle a second radiation phenomena, described as a blue flame, has been observed under certain conditions (36, 124). A second cool flame also occurring late in the cycle may be the same phenomenon (81, 103, 131). While the importance of cool flame and blue flame phenomena in the over-all reaction mechanism is not fully understood, their occurrences can be used to mark certain stages in the course of the preflame reactions. This principle has been used extensively in studying the effect of hydrocarbon structure, physical variables, and additives on engine preknock reactions. 

The promotion of the decay of active particles (H atoms and OH-radicals) in the presence of various metal compounds has been repeatedly verified Which metal particles do actually react with the atoms and radicals propagating the chain reactions in a flame Hastie studied flames of SbBrj-inhibited methane and discovered CHjBr and HBr in the preflame zone, and Sb and SbO in the high-temperature reaction zone. Thus, halogen as well as metal are supplied to the gas phase. Both are capable of reacting with the active particles in the combustion reaction, embracing extensive flame areas. According to Ref. the metal reacts via the following route ... 

There is no direct experimental evidence for this complex decomposition and it may well occur by several steps [107]. However, substantial yields of unsaturated carbonyl compounds are formed particularly at high pressures [78] under initial reaction conditions where cool flames propagate. For example, the cool-flame oxidation of 2-methylpentane at 525 °C and 19.7 atm in a rapid compression machine [78] yields no less than 14 unsaturated carbonyl compounds viz acrolein, methacrolein, but-l-en-3-one, pent-2-enal, pent-l-en-3-one, pent-l-en-4-one, trans-pent-2-en-4r one, 2-methylbut-l-en-3-one, 2-methylpent-l-en-3-one, 4-methylpent-l-en-3-one, 2-methylpent-l-en-4-one, 2-methylpent-2-en-4-one, 2-methyl-pent-2-enal and 4-methylpent-2-enal. Spectroscopic studies of the preflame reactions [78] have shown that the unsaturated ketones account for ca. 90 % of the absorption which, occurs at 2600 A. At lower initial temperatures and pressures acrolein and crotonaldehyde are formed from n-pentane [69, 70] and n-heptane [82], and acrolein is also formed from isobutane [68]. 

Reich, L. and Stivala, S. S. Elements of Polymer Degradation. McGraw-Hill, New York 1971 Tkac, A. Radical processes in polymer burning and its retardation. I. ESR methods for studying the thermal decomposition of polymers in the preflame and flame zones. J. Polym. Sci. Polym. Chem. Ed., 19, 1475 (1981)... 

The diffusion combustion of polymer materials is being studied in the same way as in the above model for solid fuel combustion in this case five zones are distinguished in the combustion wave heat transport and reaction zones in the condensed phase, preflame and reaction zones inside the flame, and a combustion products zone [15]. 

A study of the concentration probes of a methane-air flame with the addition of SbCl3 and SbBr3 demonstrated that RX and HX are formed essentially in a preflaming zone and Sb and SbO in the reaction zone [109]. 

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