| dc.description.abstract | This thesis investigates how exactly the ignition probability (Pig) of nanosecond repetitively pulsed discharges (NRPD) would vary with a change of pressure (1 ~ 5 atm) by using a pair of stainless-steel cantilevered electrodes with sharp ends. We apply the lean n-butane/air mixture at the equivalent ratio = 0.7 with on effective Lewis number Le ≈ 2.2 >> 1 using a fixed inter-electrode gap (dgap = 0.8 mm) over a wide range of pulsed repetitive frequency (PRF = 1-80 kHz) in a dual-chamber, fan-stirred explosion facility. First, we measure values of the laminar minimum ignition energy (MIEL) at 50% ignitability via the logistic regression method by using the conventional single-shot discharges (CSSD), where MIEL ≈ 23/10/6 mJ at 1/2/3 atm, respectively. Then we apply these values of MIEL measured by CSSD at three different pressures (1, 2, 3 atm) as a baseline for NRPD studies, in which the NRPD cumulative total energy Etot = 23.7 ± 1 mJ at 1 atm (Np = 11 pulses), 10.2 ± 0.4 mJ at 2 atm (Np = 5 pulses), 5.54 ± 0.2 mJ at 3 atm (Np = 3 pulses). Note that each NRPD pulse has 2.3 mJ except for the first pulse having 0.8 mJ. NRPD’s experimental results in quiescence are as follow. When Etot ≈ MIEL, Pig = 0 for PRF = 1-10 kHz even when Np = 100 pulses with Etot 230 mJ are used. The synergistic effect occurs at PRF = 40 kHz with the maximum Pig (i.e. Pig = 92%/70.7%/48% at 1/2/3 atm, respectively). As PRF > 40 kHz, Pig decreases for all three pressurs; too high PRFs are detrimental for ignition. The other data set of ignition experiments is obtained conducted at a fixed Etot ≈ 23 mJ for these three different pressures (1, 3, 5 atm). Results show that values of Pig increase with increasing pressure at any given values of PRF. Especially at 3 and 5 atm, Pig reaches 100% when PRF ≥ 20 kHz. The reason is because the decrease of MIEL with increasing pressure. As such, using the same Etot is easier to ignite at high pressure. Furthermore, the ignition delay times (τRmin) of both CSSD and NRPD decrease with increasing pressure, where τRmin is defined as the lapse of the flame kernel formation from ignition to a minimum flame radius (Rmin). Finally, we measure the laminar burning velocity (SL) that decreases with increasing pressure having a power law relation of SL ~ p-0.35, independent of ignition sources, energy, and PRF. These results may be of help for the ignition strategy when using NRPD in automobile engines and gas turbines in the future. | en_US |