Heat release peculiarities of polyoxymethylene dimethyl ether 1 – Part I: Effect of initial thermochemical conditions

Abstract

Polyoxymethylene dimethyl ethers are strongly discussed as a promising oxygenated substitutes and potentially carbon-neutral fuels for compression-ignition engines. Polyoxymethylene dimethyl ether 1, also known as methylal or dimethoxymethane is the simplest representative of this family. Detailed knowledge of methylal combustion characteristics and underlying chemical kinetics are required to fully assess its combustion potential. In this study a numerical investigation of combustion heat release peculiarities of methylal/oxidizer propellant subjected to spontaneous auto-ignition was conducted in a constant-volume adiabatic reactor. The domain of initial conditions (temperature, pressure, equivalence ratio) promoting a discovered three-stage methylal heat release was outlined and analyzed for the first time. Fourteen detailed methylal kinetic mechanisms were validated against experimental ignition delay data. Three mechanisms (among fourteen) predicting in the most accurate manner the experimental data were selected to simulate the auto-ignition behavior of methylal/air mixtures. The overall range of examined initial in-reactor conditions spanned temperatures of 600–1400 K, pressures of 10–40 bar, and equivalence ratios of 0.1–2.0. The unknown previously three-stage methylal heat release phenomenon was revealed to predominantly occur within low-to-intermediate initial temperature range (below 900 K) at ultra-lean conditions (equivalence ratios below 0.5). The third-stage was found to exhibit prolonged CO2 and H2O formation supposedly due to lessened temperature gain prior to CO-to-CO2 chemistry and competing H2O formation pathways. At stoichiometric and rich conditions, an evidence of the three-stage ignition was found with high proximities between the latter stages. A comparative analysis with the highly-reactive non-oxygenated n-alkane representative, n-heptane, was conducted.