Competitive oxidation of methane and C2 hydrocarbons discerned by isotopic labeling and laser absorption spectroscopy of CO isotopologues in shock-heated mixtures

Abstract

The competitive oxidation of methane with C 2 hydrocarbons of differing functional groups (alkane, alkene , and alkyne) was examined experimentally via combustion of isotopically-labeled fuel mixtures and laser absorption spectroscopy of carbon monoxide isotopologues. Quantitative species time-histories of the 12 CO and 13 CO isotopologues were measured simultaneously and in situ using laser absorption spectroscopy behind reflected shock waves, used for near-instantaneous heating and auto-ignition of binary mixtures containing equal carbon fractions of the different fuels. A driver extension and gas tailoring were employed on the shock tube facility to extend test times up to 30 milliseconds, enabling dilute ignition of the fuel blends over a range of temperatures from 1100–1800 K. Tested fuel mixtures were primarily fuel-rich to force the competition of carbon oxidation between the fuel components. The novel dataset of multi-isotopologue species time-histories were compared to available chemical mechanisms, revealing insights on the influence of each C 2 fuel on methane ignition. The GRI-MECH 3.0 and Foundational Fuel Chemistry Model (FFCM-1) reaction models were modified to incorporate 13 C reactions and species. Detailed comparison of the measurement data with FFCM-1 simulations revealed generally good agreement at elevated temperatures ( > 1500 K), with increasing divergence at lower temperatures, particularly for mixtures involving ethane and acetylene . Reaction pathway and sensitivity analysis of the variance between data and the modified mechanisms reveal key reactions likely responsible for the disagreements.