| 摘要: | 本論文探討壓力效應(1 ~ 5 atm)對於奈秒重覆脈衝放電(NRPD)之引燃機率(Pig)的影響。實驗在一個大型雙腔體風扇擾動十字型燃燒爐中進行，其中心處配置了一對固定電極間距(dgap = 0.8 mm)之不鏽鋼尖端探針，搭配NRPD以脈衝重覆頻率(PRF = 1 ~ 80 kHz)，引燃預混貧油正丁烷/空氣之混合物( = 0.7，有效Lewis數Le ≈ 2.2 >> 1)。首先，我們使用傳統火花放電引燃(CSSD)系統，透過邏輯回歸方法計算出50%引燃機率時的層流最小引燃能量(MIEL)，其中MIEL在1、2、3 atm條件下，分別為23、10、6 mJ，隨壓力增加，MIEL值會下降。我們以CSSD所得之MIEL值作為NRPD之基準，以累積總能量Etot = 23.7 ± 1 (NP = 11個脈衝波於1 atm)、10.2 ± 0.4 mJ (Np = 5個脈衝波於2 atm)、5.5 ± 0.2 mJ (Np = 3個脈衝波於3 atm)進行NRPD引燃機率量測實驗。經量測後得知，NRPD的第一個脈衝波能量約為0.8 mJ，而從第二個脈衝波開始，能量均約為2.3 mJ。結果顯示:當以Etot ≈ MIEL進行實驗，在PRF = 1 ~ 10 kHz時，Pig = 0，即使是使用NP = 100個脈衝波(Etot ≈ 230 mJ)，引燃仍為0。最高的Pig值，發生在PRF = 40 kHz，其相對應之Pig = 92%/70%/48%，當p = 1/2/3 atm。而當PRF > 40 kHz時，三個壓力的Pig值都會隨著PRF增加而降低，顯示NRPD能量加乘效應僅會發生在特定PRF = 40 kHz，太低或太高PRF均不利於引燃。若以固定Etot ≈ 23 mJ於1、3、5 atm條件下進行NRPD實驗，結果顯示:Pig在給定的PRF條件下，皆會隨著壓力上升而增加，且於高壓條件(p = 3、5 atm)，當PRF ≥ 20 kHz時，Pig皆為100%，這是因為MIEL值會隨壓力升高而降低，故同樣Etot在高壓時，較易引燃。此外，CSSD與NRPD兩個不同引燃系統之引燃延遲時間τRmin皆隨著壓力的升高而減少。其中τRmin定義為在火核發展過程中，從引燃至最小火焰半徑(Rmin)所需的時間。最後，我們測量了層流燃燒速度(SL)，其值隨著壓力增加而降低，且SL ~ p-0.35，SL與引燃系統和PRF無關。本研究對未來使用NRPD於高壓環境之引燃，如汽車引擎和燃氣輪機應有所助益。;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. |
