Study of low-to-moderate temperature oxidation of 1,2,4-trimethylbenzene/n-heptane blends

Single-pulse shock tube experiments were performed at a nominal pressure of 50 atm and a reaction time of 13 milliseconds (ms) over a temperature range of 700–1400 K to study the oxidation speciation of a 1,2,4-trimethylbenzene/n-heptane (HTMB) blend. The oxidation experiments were conducted across a range of equivalence ratios (φ = 2.0, 1.0, 0.50, and 0.25). Gas chromatography (GC) was used to quantitatively and qualitatively analyze the post-shock gases, providing speciation data on oxidation products as a function of temperature. The speciation results were simulated using the CRECK (CRECK_2003_TOT_HT_LT), LLNL, and Zeng_Xie mechanisms. All three mechanisms simulated the experimental speciation data well, with the CRECK mechanism demonstrating the best overall agreement. At lower temperatures (∼740 K) and under fuel-lean conditions (φ = 0.25), negative temperature coefficient (NTC) behavior was observed in both the reactants and the oxidation products, specifically in alkenes and aldehydes. Rate of production, sensitivity, and reaction path analyses were also conducted to investigate the reactions affecting the oxidation process. The primary reaction channel that initiates the oxidation process is H-abstraction by OH radicals, along with an additional H-abstraction reaction channel by HO₂ radicals, particularly under fuel-lean conditions. The formation of trimethylbenzyl radicals (RC₉H₁₁) from 1,2,4-trimethylbenzene decomposition inhibits the oxidation process, slowing down radical generation and, consequently, affecting the reactivity of n-heptane and the overall HTMB blend.