Experimental study of fuel decomposition and hydrocarbon growth processes for cyclohexane and related compounds in nonpremixed flames

Abstract

C1 to C12 stable hydrocarbons, soot volume fraction, several major species, and gas temperature have been measured in a series of methane/air coflowing nonpremixed flames whose fuel was separately doped with 2000 ppm of cyclohexane, cyclohexene, 1,3-cyclohexadiene, 1,4-cyclohexadiene, and benzene. The cyclohexadienes mostly dehydrogenated to benzene, while cyclohexane and cyclohexene mostly decomposed to C2, C3, and C4 hydrocarbons. Addition reactions were the main benzene source in the cyclohexane-doped flame, whereas both addition and dehydrogenation were important benzene sources in the cyclohexene-doped flame. Reaction pathways were identified in each flame from the hydrocarbon product distributions and from calculated rate constants of relevant reactions. Dehydrogenation was a minor pathway in the cyclohexane- and cyclohexene-doped flames because unimolecular dissociation, not H-atom abstraction, consumed the dopants and their conjugate radicals in these nonpremixed flames. The maximum soot volume fraction correlated poorly with the maximum benzene mole fraction in the cyclohexadiene-doped flames, which indicates that benzene formation was not a rate-determining step to soot production in those flames.