Interplay of alkyl-substituted aromatics in ternary aromatic/n-heptane blends: Shock tube study toward SAK surrogates

Abstract

The oxidation behavior of multi-aromatic/n-alkane blends was investigated to elucidate cross-interactions among alkyl-substituted aromatics and their collective influence on fuel reactivity and synthetic aromatic kerosene (SAK) surrogate development. Shock tube speciation experiments at high pressure (∼50 atm) were performed for ternary blends of 1,2,4-trimethylbenzene (TMB124), 3-ethyltoluene (ET), 3-n-propyltoluene (PT), and n-heptane, namely HETTMB (TMB124/ET/n-heptane), HPTTMB (TMB124/PT/n-heptane), and HETPT (ET/PT/n-heptane). Gas chromatographic analysis quantified over 30 products, enabling detailed comparisons with kinetic modeling results. Simulations using the CRECK_ETPT mechanism and its extended version, CRECK_ETPT_Ext_Crs, captured major speciation trends and clarified the role of resonance-stabilized aromatic radicals (RSRs). ET- and PT-derived radicals were found to enhance reactivity through H-abstraction from n-heptane and additional radical generation from decomposition pathways, whereas the TMB124-derived radical RC₉H₁₁ behaved primarily as a radical sink. The analysis also showed that meta-substitution affects key aromatic pathways, particularly those associated with benzene formation, highlighting limitations of lumped ortho-derived chemistry for these systems. These findings provide a mechanistic basis for developing chemically consistent multi-aromatic SAK surrogate mechanisms that explicitly account for inter-aromatic radical interactions.

Novelty and significance statement

This study presents the first comprehensive and systematic examination of interactions among alkyl-substituted aromatics during oxidation in multicomponent mixtures representative of synthetic aromatic kerosene (SAK)-like fuel systems. High-pressure shock tube speciation experiments, coupled with kinetic modeling, show that cross-reactions among resonance-stabilized aromatic radicals (RSRs) can subtly yet mechanistically influence fuel reactivity and product formation. The results demonstrate that the oxidation behavior of multi-aromatic blends cannot be inferred from binary systems alone and highlight the importance of including aromatic–aromatic radical cross-reaction pathways in detailed kinetic mechanisms. Understanding these interactions is essential for developing predictive kinetic models of practical fuels and synthetic aviation fuel surrogates, as they provide deeper mechanistic insight into alkyl-aromatic oxidation chemistry.