A computational study on flame propagation in mixtures with non-zero reaction progress 2019-01-0946
Most of the literature reported flame speed data are acquired in conventional apparatus such as the spherical combustion bomb and counterflow configurations and are limited under atmospheric pressure and ambient or slightly elevated unburnt temperatures. As such, these data are hardly relevant to internal combustion engines, with typical pressures of 10-50 bar and unburnt temperature up to 900K. These elevated temperature and pressure not only modifies dominant flame chemistry, but more importantly, they inevitably facilitates pre-ignition reactions and hence can change the upstream conditions of a regular hot flame, leading to modified flame properties. This study focuses on how auto-ignition chemistry affects flame propagation, especially in the negative-temperature coefficient (NTC) regime, where Dimethyl ether (DME), n-heptane and iso-octane are chosen as the typical fuels exhibiting low temperature chemistry (LTC). The computation of laminar flame speed of lean and stoichiometric mixture of fuel/air was performed at different ignition reaction progress, by selecting the thermal chemical states corresponding to different residence time during auto-ignition process as the flame upstream condition. Using scaling and budget analysis, it is shown that a well-defined flame speed for such partially reactive mixture in the classical diffusion-reaction limit could still be feasible with appropriate computational domain, especially with sufficiently reduced induction length. The comparison of flame speed against different types of progress variables indicates a nearly linear relationship between the flame speed and progress variables based on the fuel mass fraction and temperature. Thermal and chemical effect of a cool-flame upstream has been isolated by comparing the flame speed of the initial mixture and that of the instantaneous mixture under the same thermodynamic conditions. It is found that the enhanced propagation is shown to be largely a thermodynamic effect, while chemistry nevertheless plays a retarding role. Sensitivity analysis has been performed to identify the key species which mostly influence flame propagation at different reaction progress and a general scheme of simplified mixture was constructed to describe flame propagation in a partially reactive mixture, for both lean and stoichiometric, as well as high pressures conditions in general.
Han Lin, Peng Zhao, Haiwen Ge
Oakland University, Texas Tech University