Experimental and fuel-surrogates modeling study of the oxidation of specialty cetane number fuels

Abstract

Single pulse shock tube experiments were performed at 50 atm nominal pressure and 4 milliseconds nominal reaction time over a temperature range of 900–1800 K, to study the oxidation speciation of a multicomponent jet fuel, F-24, and six cetane number (CN) specialty fuels - CN30, CN35, CN40, CN45, CN50, and CN55. The oxidation experiments were carried out at an equivalence ratio of approximately 1.0. Gas chromatography (GC) was used to quantitatively and qualitatively analyze the post shock gases. The correlation between the formation of critical oxidation species and the chemically controlled combustion propensity as reflected by the cetane number of each fuel was investigated. The species were simulated using a surrogate-based mechanism from the CRECK Modelling Group. The species produced from the oxidation of the CN fuels were initially modeled using optimized chemical composition surrogates, but with less than satisfactory agreement. Efforts to enhance the agreement of experiment with model results by increasing the iso-paraffinic content in the surrogates did not yield significant improvements. Subsequently, the aromatic content of the surrogates was adjusted, resulting in surrogates whose model predicted oxidation species better matched the experimental data. Rate of production, sensitivity and reaction path analyses using the surrogate model were performed to obtain the important reactions responsible for the formation of key species and to examine the chemistry of complex multicomponent fuel systems. The primary reactions responsible for driving the oxidation chemistry were largely influenced by the chemical functional groups present in the fuels. In addition, the study highlights the effectiveness of the fuel-surrogate approach where surrogates representing the chemical functional group composition of the parent fuel serve as a valuable tool for predicting the combustion chemistry of unknown fuels.