Large eddy simulation of hot jet ignition in moderate and high-reactivity mixtures

Abstract

Hot jet ignition technology is a promising improvement to conventional spark ignition for reducing emissions in premixed combustion engines as it can extend the lean-burn limits. In addition, the rapidity of the ignition via hot jet makes this technology appropriate for deflagrative pressure-gain combustors using a wave rotor, which have high-frequency operation and fast-burn requirements. In order to improve the controllability of the ignition process, the thermofluidic and chemical mechanisms by which the ignition is promoted need to be rigorously elaborated. In the present work, the chemical thermofluid dynamics of a transient turbulent hot jet in a reactive environment is studied via large eddy simulation (LES) with detailed kinetic chemistry. Computational model of the reacting and non-reacting jets are presented, analyzed and compared to the available experimental data. Various mesh sizes in conjunction with adaptive mesh refinement were tested to ensure the insensitivity of the solution to the grid. To quantify the effect of main chamber fuel reactivity on the ignition process, two different blends of methane-hydrogen under the same equivalence ratios are tested and the effective thermo-chemical mechanisms of ignition are investigated. It is understood that the ignition kernels emerge at the regions with minimal scalar dissipation rate, mainly at the trailing shear layer of the jet, and are partially transported to or formed at the head vortex. The temporal variation of the heat release rate and formaldehyde distribution at the fuel mixture with lower hydrogen content suggests that auto-ignition is responsible for the subsequent flame formation. However, for higher hydrogen content, it is observed that auto-ignition is not as prolonged beyond the initial kernels, while the propagating premixed flame that emanates from the shear layer mostly comprises the overall heat release.