Characteristics of rotating gliding arc induced thermal ignition of lean methane-air mixtures

Abstract

The rotating gliding arc discharge (RGA) induced methane-air ignition process under the lean conditions is investigated by the high-speed videography and the numerical simulation, with the focus on the effects of dynamic discharge characteristics. The experimental and numerical results both indicate that the ignition process in the burner can be divided into two stages, including the waiting stage and the rapid flame propagation stage. The effective ignition time (τburner) including both stages decreases nonlinearly as the effective input ignition power (i.e. Pig/Pcomb) increases, following the exponential law, i.e. τburner=2.98 × 10−9(Pig/Pcomb)−7.299+81.75. It is because of that as the input power is relatively low τburner is governed by the duration of the waiting stage to decrease notably with the increment of the input power. However, as the input power is too large τburner becomes determined by the rapid flame propagation stage to be insensitive to the input power. The rotation frequency of the plasma sources can impact the τburner with that too low or too high frequency is detrimental to the ignition. The flame quenching is less likely to occur with the increase of the plasma volume. Based upon the experimental and numerical results, an RGA-induced ignition mechanism is proposed. It indicates that the two-staged ignition process is largely determined by the waiting stage, where multiple flame kernels are frequently initialized by the RGA to be locally quenched or propagate downstream/upstream with flow. Due to the recirculating flow, the energy released by the plasma and the ignited flame kernels can be accumulated upstream to reduce the local Ka number and enhance the flame propagation at the waiting stage. As Ka is lower than a critical value, the flame kernels can propagate rapidly to complete the ignition.