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Abstract
The oxidation of iso-octane was experimentally studied in a jet stirred flow reactor operating at high pressures (7 and 9 bar) and the compositional changes of the reaction mixture were followed, at a fixed residence time, in the low and intermediate temperature range, from 600 to 760 K, by means of gas sampling and chromatographic analysis of the combustion products. Significant changes in the concentration of the main reaction products were found in relation to the different phenomenologies, i.e., slow combustion, cool flames, jump, which iso-octane exhibited by changing the temperature. Typical slow combustion behavior associated with a very low heat release was analyzed in the range 600-620K. In this regime CO2, 2,2,4,4-tetrumethyltetrahydrofuran, propanone, and formalydehyde were the main detected reaction products, By increasing the operating temperature above 620 K the concentration of all reaction products, in particular CO, and C7 olefins, drastically increased in correspondence of the sudden increase of the heat release rate and the appearance of cool flame phenomenology. A further increase in the temperature caused a progressive reduction of the heat release rate, typical of the region of negative temperature coefficient (NTC). The decrease of the oxygen containing species concentration along with the increase of the olefinic product concentration, observed in the NTC region, were attributed to the effect of the temperature on the competitive reactions involving, at low temperature, the formation of alkylperoxy radicals and at intermediate temperature, the formation of conjugate olefins and HO2 radicals. The pressure increase shifted the low temperature oxidation regime, controlled by the peroxy radical formation, to higher temperatures indicating that the low temperature oxidation chemistry should play an important role, also at the higher temperatures typical of internal combustion engines.