Exploring the low-temperature oxidation chemistry with ozone addition in an RCM: A case study on ethanol

Abstract

The low reactivity of some fuels restricts their oxidation experiments at temperatures lower than a specific threshold value. The oxidation of fuel exhibiting low reactivity can be initiated in practical combustors by other highly reactive components at lower temperatures beyond this threshold. To meet the lack of corresponding experimental study, ozone (O3) was utilized as the active species to extend experimental conditions towards lower temperatures. A new experimental facility combining an O3 system and a rapid compression machine (RCM) was developed, with which the generation and qualification of O3 in the RCM was achieved. The RCM-O3 facility can be used to directly investigate the ignition behavior and provide time-resolved species information in the presence of O3 under high-pressure conditions. As a case study, the oxidation and ignition behavior of ethanol were investigated under low-temperature conditions (<820 K) where the auto-ignition of pure ethanol cannot be initiated. Ignition delay times for stoichiometric ethanol/O2 mixtures containing varying concentrations of O3 (0 ppm, 1000 ppm, and 2000 ppm) were determined in the temperature region of 769–1036 K under the pressures of 15 and 25 bar. Time-resolved species profiles were obtained during the ignition process. A combination of a fast sampling system and gas chromatography (GC) technique was used to record the species profiles at 840 K and 802 K in the presence of O3. Model analysis revealed that the addition of O3 promoted the production of OH radicals. The reactivity of the system was not restricted by the decomposition of H2O2 in the presence of O3. The hydrogen-abstraction reactions of ethanol by OH radicals were highlighted to a great extent. The reaction network was not significantly affected when O3 was used as the additive, which makes the experimental results can well characterize ethanol-based chemistry. The H-abstractions at α- and β-site of ethanol by OH radicals showed opposite effects on ignition, and the branching ratio of the reaction pair was constrained according to experimental results in this work.