奈秒重覆脈衝放電之引燃機率量測於一預混貧油正丁烷/空氣燃氣: 電極間距、重覆脈衝頻率和紊流效應;Ignition Probabilities of Nanosecond Repetitively Pulsed Discharge in Randomly-Stirred Lean n-Butane/Air Mixture: Effect of Spark Gap, Pulsed Repetitive Frequency, and Turbulence

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Title:	奈秒重覆脈衝放電之引燃機率量測於一預混貧油正丁烷/空氣燃氣: 電極間距、重覆脈衝頻率和紊流效應;Ignition Probabilities of Nanosecond Repetitively Pulsed Discharge in Randomly-Stirred Lean n-Butane/Air Mixture: Effect of Spark Gap, Pulsed Repetitive Frequency, and Turbulence
Authors:	林柏良;Lin, Bo-Liang
Contributors:	機械工程學系
Keywords:	奈秒重覆脈衝放電;層流和紊流引燃機率;脈衝重複頻率;能量加乘效應;電極間距和紊流效應;Nanosecond repetitively pulsed discharge;pulsed repetitive frequency;synergistic effect;turbulence facilitated ignition;ignition probability;turbulent dissipation
Date:	2020-08-20
Issue Date:	2020-09-02 19:14:42 (UTC+8)
Publisher:	國立中央大學
Abstract:	本研究使用電漿輔助燃燒技術之一，即奈秒重覆脈衝放電(Nanosecond Repetitively Pulsed Discharge, NRPD)，針對貧油正丁烷/空氣在當量比 = 0.7(其有效Lewis數Le ≈ 2.1 >> 1)之預混燃氣，以固定脈衝總數(Number of Pulse, Np = 11 pulses)，即固定總引燃能量Etot ≈ 23 ± 1 mJ，此能量與傳統火花放電(Conventional Single Spark Discharge, CSSD)於電極間距dgap = 0.8 mm之層流最小引燃能量(MIEL，具50%引燃機率的能量)相同，探討以下三個效應對引燃機率 (Ignition Probability, Pig) 之影響: (1)重覆脈衝頻率(Pulsed Repetitive Frequency, PRF = 5~80 kHz)效應、(2)電極間距(dgap = 0.6、0.8和2.0 mm)效應和(3)方均根紊流擾動速度(u′ = 0~2.8 m/s)效應。實驗使用一對尖頭懸臂電極探針，於一大型雙腔體三維十字型風扇擾動爆炸設備中心處引燃，此設備可產生一近似等向性紊流場。本研究主要目標為探討NRPD的能量加乘效應(Synergistic Effect)，並與本實驗室先前CSSD引燃所得結果作一比較。在dgap = 0.8 mm和u′ = 0(層流)時，當PRF = 5 kHz，NRPD之Pig = 0，且即使提高Np = 5,000 (Etot 高達約10 J)，也完全無法引燃成功；但在PRF = 20 kHz，即有明顯的能量加乘效應，Pig = 90%遠高於50%；而當PRF = 60 kHz時，Pig = 34%小於50%。前述結果顯示能量加乘效應僅會發生於特定PRF範圍(約20 ~ 40 kHz在dgap = 0.8 mm時)，這是因為每一脈衝放電時，會產生震波(shock wave)，誘使電極周圍產生迴流渦流(Recirculation Vortices)，迴流渦流會捲入新鮮未燃燃氣於電極間距內，當迴流渦流頻率與PRF接近相同時，次個脈衝放電正好可有效引燃前個脈衝放電所引入之新鮮未燃燃氣，進而產生能量加乘效應，有效提高Pig。但於5 kHz條件下，脈衝放電時間間隔(1/PRF)過長，導致在下一個脈衝開始之前，初始火核與電極間的熱損失和受新鮮未燃燃氣對其自由基濃度的稀釋之影響，導致不僅無法產生能量加乘效應，且每個脈衝均無法有效引燃可燃氣，使得Pig = 0。在更小的dgap = 0.6 mm時，能量加乘效應會在更高重覆脈衝頻率下發生，約在PRF = 40~60 kHz間會有最高的引燃機率，顯示能量加乘效應會隨著dgap的減小而往更高頻之PRF移動。有關紊流效應，基本上紊流會使得引燃更困難發生，如同在dgap = 2 mm紊流條件下，於任一固定PRF，Pig均會隨著u的增加而下降，但當u′大於一臨界值時(u′c)，Pig下降斜率有一重大轉折，從平緩下降(u′ < u′c)到急遽下降(u′ > u′c)。在dgap = 2 mm，當PRF = 5 kHz時，臨界值u′c = 1.4 m/s；當PRF = 20~60 kHz，轉折發生於u′c = 2.1 m/s處。這現象與CSSD所發現之MIE轉折現象雷同。值得一提的是，我們也在NRPD引燃實驗中，發現有別於一般認知之紊流促進引燃(Turbulence Facilitated Ignition, TFI)現象，但它只會發生在足夠小dgap (= 0.6 mm和0.8mm)和足夠大Le >> 1條件下。例如:在dgap = 0.6 mm，當PRF = 60/80 kHz時，層流Pig = 78%/61%，但在u′ = 0.5 m/s之紊流Pig竟反而增加至92%/80%；在dgap = 0.8 mm，也有類似現象。當u′ > u′c = 0.5 m/s，紊流效應重新主導引燃機率，Pig會隨u之增加而下降。前述NRPD結果對了解貧油預混紊流燃燒之引燃增強有重要的助益。;One of plasma-assisted combustion technologies, namely nanosecond repetitively pulsed discharge (NRPD), is a promising method to enhance and improve the ignition and combustion performance of spark engines. In this thesis, NRPD is used to measure the ignition probability (Pig) of a lean n-butane/air mixture at an equivalence ratio = 0.7 with an effective Lewis number Le ≈ 2.1 >> 1. NPRD experiments are conducted in the dual-chamber fan-stirred cruciform burner capable of generating near-isotropic turbulence. All NRPD experiments apply a total ignition energy (Etot) of 23 ± 1 mJ via a fixed train of 11 pulses through a pair of pin to pin electrodes which are cantilevered at an angle 45o to the horizon. Note that Etot is the same as the laminar minimum ignition energy (MIEL) measured by the conventional single spark discharge (CSSD) having 50% ignition probability at the distance between electrodes dgap = 0.8 mm. To explore the NRPD synergistic effect and compare it with previous CSSD results, this thesis investigates three effects on Pig: (1) The pulsed repetitive frequency (PRF = 5~80 kHz) effect, (2) the gap effect (dgap = 0.6, 0.8, 2.0 mm), and (3) the r.m.s turbulent fluctuating velocity effect (u′ = 0~2.8 m/s). NRPD results show that the synergistic effect depends on the coherence of the frequency of flow recirculating zone within the inter-electrodes (fRC) and PRF. When PRF is approximately coupling with fRC at u′ = 0 (laminar), the synergistic effect is most profound at PRF= 20 kHz having a highest Pig = 90% > 50% at dgap = 0.8 mm, while Pig = 34% < 50% at PRF = 60 kHz. Interestingly, it is found that Pig = 0% at PRF= 5 kHz even when 5,000 pulses (Etot = 10 J) are applied. At the smaller dgap = 0.6 mm, the synergistic effect occurs at higher PRF (40~60 kHz), suggesting that the optimal PRF for the ignition enhancement increases with decreasing dgap. At dgap = 2 mm, we find that turbulence renders ignition more difficult, which is in line with the common notion of the turbulence effect on ignition where Pig decreases as u′ increases at any given PRFs. Furthermore, we find that there is a Pig transition when u′ is greater than a critical u′c depending on PRF; the decreasing slope of Pig changes from gradually to abruptly. Such Pig transition is inversely similar to MIE transition found by previous CSSD experiments. It is worthy noting that the turbulence facilitated ignition (TFI) phenomenon also exists in the NRPD experiments. In contrast to dgap = 2 mm, a non-monotonic transition is observed at smaller dgap = 0.6 mm and 0.8 mm. At dgap = 0.6 mm and PRF = 60 kHz/80 kHz, the laminar Pig,L is 78%/61%, whereas the turbulent Pig,T at u′ = 0.5 m/s is increasing to 92%/80% > Pig,L, showing TFI. Similarly, TFI is also observed at dgap = 0.8 mm. When u′ > u′c = 0.5 m/s, turbulence re-claims its dominance, where Pig,T decreases with increasing u′. These results are important to our understanding of ignition enhancement by using NRPD for lean turbulent premixed combustion.
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