Effect of Nanosecond Repetitively Pulsed Frequency on Laminar Flame Propagation of Lean n-Butane/Air Mixture with Le >> 1

This study aims to present the effect of pulse repetitive frequency (PRF) from nanosecond repetitively pulsed discharge (NRPD) on laminar flame propagation of lean premixed n-butane/air at an equivalence ratio ϕ of 0.7. The lean n-butane/air with Lewis number Le ≈ 2.1 >> 1 is ignited in the constant pressure combustion chamber at the atmospheric condition and 0.8-mm spark gap (dgap), where Le is determined as the ratio of thermal diffusivity and mass diffusivity. A fixed train of 11 pulses varying PRFs from 5 to 60 kHz, but the same total ignition energy Etot = 23 mJ is applied. The result indicates that the embryonic kernel is quenched at lower PRF = 5 kHz while it becomes the successful flame propagation at the other higher PRFs (10–60 kHz). The fastest combustion process is at PRF = 20 kHz and being detrimental with other PRFs. The non-monotonic effect of PRFs on the combustion process of lean n-butane/air mixture could be attributed to the synergistic effect that strongly depends on the competition between PRF and recirculation frequency of inward reactant flow near the electrode tips. Although NRPD could enhance the combustion process within a specific PRF range, the laminar flame propagation rate after self-sustained propagating is almost constant and independent of PRF values. The unstretched laminar flame speed approximates 25–26 cm/s. The NRPD results are also compared with that of conventional single spark discharge (CSSD). The schlieren flame images are presented to understand the flame structure under the effect of discharge methods.