Effects of electrodes and imposed flow on forced ignition in laminar premixed hydrogen/air mixtures with large Lewis number

Abstract

Compared to quiescent premixed reactants, forced ignition of flowing/turbulent premixed reactants is expected to be more difficult because of increased dissipation of the deposited energy. However, the counterintuitive turbulence-facilitated ignition (TFI) has been observed in recent experiments for mixtures with large Lewis number, Le. The mechanisms behind TFI are still not well understood and this study aims to interpret a part of the TFI mechanisms through considering electrodes and imposed flow in the simulations of forced ignition in hydrogen/air mixtures. The imposed flow emulates the local turbulent effects around the electrodes which might blow the ignition kernel away from the electrodes. Since TFI occurs only for mixtures with large Le (e.g., lean hydrocarbon/air or rich H2/air mixture), a fuel-rich (ϕ=5.1) H2/air mixture with Le≈2.3 is investigated to reduce computational cost and consider more factors that may lead to TFI. Similar to TFI observed in experiments, the flow-facilitated ignition is observed for H2/air with ϕ=5.1 and Le≈2.3 when the electrodes have a small gap distance. The detailed effects of including electrodes on forced ignition of quiescent and flowing mixtures are explored. It is found that the existence of electrodes not only induces heat loss but also affects the shape and global curvature/stretch of the ignition kernel. The heat loss to the electrodes is demonstrated to play an important role for the ignition of mixtures with large Le. Compared to quiescent mixtures, considering an imposed flow normal to the electrodes can blow the flame kernel away from the cold electrodes. Such movement of the ignition kernel can greatly reduce both the heat loss to the electrodes and flame curvature/stretch, and thereby promote the ignition in mixtures with large Le. These results help to understand the underlying mechanisms for the TFI observed in experiments.