Chemical kinetic modeling of jet fuel surrogates [electronic resource]
Keywords: Mechanical Engineering.

Jet fuels, like typical transportation fuels, are mixtures of several hundreds of compounds belonging to different hydrocarbon classes. Their composition varies from one source to another, and only average fuel properties are known at best. In order to understand the combustion characteristics of the real fuels, and to address the problem of combustion control, computational studies using a detailed kinetic model to represent the real fuel, serves as a highly useful tool. However, the complexity of the real fuels makes it infeasible to simulate their combustion characteristics directly, requiring a simplified fuel representation to circumvent this difficulty. Typically, the real fuels are modeled using a representative surrogate mixture, i.e. a well-defined mixture comprised of a few components chosen to mimic the desired physical and chemical properties of the real fuel under consideration. Surrogates have been proposed for transportation fuels, including aviation fuels, and several kinetic modeling attempts for the proposed surrogates have also been made. However, (i) the fundamental kinetics of individual fuels, which make up the surrogate mixtures is not understood well, (ii) their combustion behavior at low through high temperatures has not been comprehensively validated, and this directly impacts the (iii) reliability of the multi-component reaction mechanism for a surrogate made up of these individual components. The present work is aimed at addressing the afore-mentioned concerns. The objective of this work is to develop a single, reliable kinetic model that can describe the oxidation of a few representative fuels, which are important components of transportation fuel surrogates, and thereby capture the specificities of the simpler, but still multi-component surrogates. The reaction mechanism is intended to well-represent the individual components as well as a multi-component surrogate for jet fuel made up of these fuel components. Further, this reaction mechanism is desired to be applicable at low through high temperatures, and be compact enough that chemical kinetic analysis is feasible. First, a representative compound for each of the major hydrocarbon classes found in the real jet fuel is identified. A surrogate for jet fuels is chosen to be comprised of n-dodecane (to represent normal alkanes), methylcyclohexane (to represent cyclic alkanes), and m-xylene (to represent aromatics). A Component Library approach is invoked for the development of a single, consistent, and reliable chemical scheme to accurately model this multi-component surrogate mixture. The chemical model is assembled in stages, starting with a base model and adding to it sub-mechanisms for the individual components of the surrogate, namely m- xylene, n-dodecane, and methylcyclohexane. The chemical model is validated comprehensively every time the oxidation pathways of a new component are incorporated into it and the experimental data is well captured by the simulations. In addition to the jet fuel surrogate, with the number of fuels described in the proposed reaction mechanism, a surrogate for the alternative Fischer-Tropsch fuels is also considered. Surrogates are defined for jet fuels and Fischer-Tropsch fuels by matching target properties important for combustion applications between the surrogate and the real fuel. The simulations performed using the proposed reaction mechanism, with the surrogates defined as fuels, are compared against global targets, such as ignition delays, flow reactor profiles, and flame speed measurements for representative jet fuels and Fischer-Tropsch fuels. The computations show promising agreement with these experimental data sets. The proposed reaction mechanism is well-suited to be used in real flow simulations of jet fuels. The proposed reaction mechanism has the ability to describe the kinetics of n- heptane, iso-octane, substituted aromatics, n-dodecane, and methylcyclohexane, all of which are important components of transportation fuel surrogates. Considering the large number of hydrocarbons whose kinetics are well described by this reaction mechanism, there are avenues for this reaction mechanism to be used to model other transportation fuels, such as gasoline, diesel, and alternative fuels, in addition to the jet and Fischer-Tropsch fuels discussed in the present study.