Importance of a Cycloalkane Functionality in the Oxidation of a Real Fuel

Typically, real jet fuels are composed primarily of normal-, iso-, and cycloalkanes, along with significant fractions of alkyl aromatics. This study elucidates fundamentally the influence of the cycloalkane functionality on the fully prevaporized global combustion kinetic behavior of a fuel mixture composed of normal- and isoparaffin and alkyl aromatic fractions. Methylcyclohexane is chosen as a representative of this generic functional class. A n-decane/iso-octane/toluene mixture previously applied in surrogate fuel research is used as the reference functional class composition for this study. A second fuel mixture is derived by incorporating ∼25 mol % methylcyclohexane into this reference component mixture in such a manner that four selected combustion property targets, derived cetane number, hydrogen/carbon ratio, molecular weight, and threshold sooting index, of both the reference fuel and the new mixture are the same. In so doing, the intention is to regulate the gas-phase oxidative reactivity of both fuels while singularly perturbing only the molecular class composition. The specific influence of the cycloalkane functionality on the global gas-phase reaction is then tested by experimentally analyzing the low-, intermediate-, and hot-ignition reactivity of both fuels and also by analysis of laminar diffusion flame extinction limits measured at 1 atm. These observations indicate that the cycloalkane functionality imparts no distinctive influence on the low-temperature alkyl-peroxy-radical-governed global reactivity of complex liquid transportation fuel types, nor do the extinction limit observations indicate any significant effects on global behaviors at high temperatures. However, the cycloalkane functionality does differentially influence the hot-ignition transition, by accelerating the global reactivity equivalent to an increase in reaction temperature of ∼20–30 K at 800–900 K (12.5 atm). A kinetic modeling analysis suggests that this modest difference is due to the differential intermediate species population provided by the molecular structures of a branched alkane (of near equal carbon number, iso-octane) versus that provided by methylcyclohexane. These observations are discussed in the context of the existing literature in terms of fundamental fuel formulations, surrogate fuels for the replication of global combustion behaviors, and numerical combustion models for real fuels.