Numerical simulations of supercritical CH4/O2 flame propagation in inhomogeneous mixtures following ignition

Abstract

The propagation of flames in non-homogeneous medium is studied for the particular case of the combustion of methane with oxygen at high pressure, in supercritical regime. A reduced chemistry containing 17 species and 44 elementary reactions is used in two configurations: 1/ a pocket of methane in a steady environment of oxygen at 10 MPa, and 2/ a double transcritical injection in a splitter plate configuration at 5.4 MPa. High fidelity simulations are carried out with the real gas version of the SiTCom-B numerical code. The addition of a hot spot to the reactive mixtures resulted in the formation of triple flames propagating along the stoichiometric isoline whatever the configuration chosen: with or without turbulence, cryogenic temperatures, random distribution of methane or not. In the case of a high density ratio between the two fluids (CH4 and O2), the flame naturally chooses to move towards that of lower density. In the case of the combustion of one pocket of methane, the addition of a homogeneous isotropic turbulence promotes combustion by increasing the length of the flame as well as the lean premixed combustion mode compared to the rich or non-premixed combustion. A similar behavior is observed when the pocket of CH4 is replaced by a set of random pockets. In the case of the splitter plate, tribachial flames propagating along the stoichiometric line are observed again. Combustion develops in finger-like structures mostly with a non-premixed mode of combustion. However, the lean and rich premixed combustion remains and the flame spreads towards the injector. The tip of the flame then attempts to pull up the flow into a reduced space stuck between dense oxygen and a rapid flow of methane.