Convective effects on head-on quenching of premixed flames in a stagnation flow

Head-on quenching (HOQ) is one of the canonical configurations of flame-wall interaction (FWI), where a premixed flame propagates at a normal angle to the wall and quenches. However, few studies have investigated the effect of convective flow on the HOQ process, which is relevant to the turbulent FWI in practical combustors. Therefore, the effect of convective flow on the HOQ of a laminar premixed flame was numerically investigated in an inert, isothermal wall-stagnating flow. The magnitude of convection was controlled by the nozzle velocity. Two kinds of fuels, namely n-heptane and a methane/hydrogen mixture, were considered. It was found that the quenching distance and the maximum wall heat flux were influenced by two kinds of effects induced by the flow field. The first effect is the convective effect, which transports the flame closer to the wall with increasing nozzle velocity. The second effect is the flame stretch effect, which strengthens (weakens) the flame for mixtures with the Lewis number Le less (greater) than unity. For mixtures with Le < 1 (e.g., lean methane/hydrogen/air), the two effects cooperate to decrease the quenching distance monotonically with increasing nozzle velocity. Meanwhile, for mixtures with Le > 1 (e.g., n-heptane/air), the two effects compete, and the quenching distance first increases and then decreases with increasing nozzle velocity. Meanwhile, for n-heptane flames, the HOQ process in a stagnation flow is governed by the Damköhler number and the flame power. For methane/hydrogen flames, the maximum wall heat flux did not scale with the flame power. It was implied that the quenching mode may transition away from thermal quenching for mixtures with high hydrogen addition ratios, which requires further investigation.