High Pressure and Temperature Study of Cyclohexane, Methylcyclohexane, and 6-Bromo-1-hexene Pyrolysis

An experimental shock tube study was completed investigating the pyrolysis of cyclohexane, methylcyclohexane, and hex-5-en-1-yl radical through a precursor, 6-bromo-1-hexene. Speciation data was obtained at high pressures (nominal pressures of 40, 100, and 200 bar) and high temperatures in the 900 to 1700 Kelvin range. The high pressure environment was used in this work for two reasons. First, to provide pyrolysis speciation data at pressures relevant to current combustors since speciation data available in the literature was only obtained at or near atmospheric pressures. Secondly, to determine whether pressure alone may be enough to drive the formation of alkylcyclopentanes from alkylcyclohexanes. This was observed to not be the case as no alkylcyclopentanes were found. An alkenylcyclopentane, methylenecyclopentane, was observed in the cyclohexane and methylcyclohexane work in only trace amounts and it was not quantifiable in the 6-bromo-1-hexene experiments. The initial fuel concentration was observed to have the greatest effect on the distributions of product species detected. Matching product profiles were obtained at different experimental pressures when the initial fuel concentration in the reaction zone was kept equivalent. In a single exploratory set of experiments at 200 bar and 3195 ppm of cyclohexane as the fuel, it was found that the normalized amount of all the cyclic products quantified increased compared to the experiments conducted with more dilute mixtures. Following the successfully completion of the experimental work, a comparative chemical kinetic modeling study was undertaken. A literature mechanism was compared with a computer generated mechanism. Both mechanisms successfully predicted the major products of cyclohexane and methylcyclohexane pyrolysis. Only the computer generated mechanism was compared against the 6-bromo-1-hexene experimental results, where the experiments revealed the hex-5-en-1-yl radical to be a benzene precursor. However, the model failed to predict the large amounts of benzene that were observed experimentally. This modeling result illustrated that the hex-5-en-1-yl, or 5-hexenyl, radical dissociation pathways are more complex than currently modeled.