| 摘要: | 近年來，使用奈秒級重複脈衝放電(Nanosecond Repetitively Pulsed, NRP)的貧油預
混燃燒技術引起了廣泛的關注，因為它具有提高熱效率和減少火花引燃(Spark Ignition,
SI)引擎廢氣排放的巨大潛力。然而，貧油預混燃燒的挑戰之一是失效點火的問題，尤
其是火花引擎在極端條件下進行引燃時，即高壓(p)、高溫(T)和高均方根紊流擾動速度
(u′)條件下越容易發生失效點火的問題。此外，大多數 NRP 文獻屬於探討層流條件之研
究，僅少數文獻探討紊流條件之研究。而 NRP 過往的文獻中，缺乏探討近似等向性紊
流場條件的相關研究，致使我們無法充分了解 NRP 放電的全部潛力。因此，對貧油燃
氣的引燃過程及其隨後的火焰傳播過程有更好的理解，是開發可靠且高效率的引燃系
統不可或缺的關鍵技術，進而於高熱效率 SI 引擎中實現穩定的貧油燃燒。因此，本論
文將使用兩種不同的放電方法，來探討碳氫燃料之燃燒現象：含傳統單次火花放電
(Conventional Single Spark Discharge, CSSD)和 NRP 放電。
實驗在高溫高壓雙腔體預混燃燒設備中執行，可透過一對 10 匹馬力馬達所驅動之
反向旋轉風扇，於十字型燃燒器中心引燃區產生一近似等向性紊流場。實驗條件如下：
共有三種不同的燃氣混合物，包括貧油和富油氫氣(分別為當量比 = 0.18；有效 Lewis
數 Le ≈ 0.3 << 1 以及 = 5.1；Le ≈ 2.3 >> 1)，貧油異辛烷（ = 0.8；Le ≈ 2.98 >> 1） 以
及貧油正丁烷（ = 0.7；Le ≈ 2.1 >> 1）。上述燃料使用 CSSD 於不同探針間距(dgap)以及
不同 u′值條件下，進行引燃實驗，探討偏好擴散效應(Le 效應)、dgap 以及紊流消散對兩
種不同引燃現象的耦合效應(coupling effects)，分別為最小引燃能量(Minimum Ignition
Energy, MIE)轉折以及紊流促進引燃(Turbulent Facilitated Ignition, TFI)現象。為了進行比
較，在 dgap = 0.8 mm 條件下，引燃貧油正丁烷燃氣，使用 NRP 放電在一定範圍的重複
脈衝頻率（Pulse Repetitive Frequency, PRF = 5〜70 kHz），並固定 11 個連續脈衝，使其
累積總引燃能量 Etot約等於 23 mJ，其中 Etot約等同於使用 CSSD 在 dgap = 0.8 mm 於正丁
烷燃氣中所獲得的 50%引燃機率之層流 MIE。
CSSD 結果顯示，由於火核的幾何形狀和偏好擴散之間的耦合效應，TFI 現象僅出
現在當 dgap < 1 mm 時且 Le >> 1 的混合燃氣中。此外，紊流 MIE 會隨 u′的增加先呈現出
下降趨勢而後上升，呈現一非單調性的曲線。結果顯示，當 u′大於某些取決於燃料性
質的臨界值時，紊流效應會重新確立其主導地位，並使引燃更加困難。然而，在 dgap >iii
1 mm 的條件下，火核呈似棒狀且火核之正曲率很小或可忽略不計，故偏好擴散對火核
之影響可忽略不計，即使對於 Le >> 1 的混合物，亦沒有發現 TFI 之現象。本論文發現
除了當在 dgap > 1 mm 時，沒有 TFI 現象，亦發現隨著 u′的增加，且在 Le >> 1、dgap > 1
mm 條件下，亦會產生 MIE 轉折之現象。此外，對於 Le << 1 的貧油氫燃氣混合物，在
任何 dgap條件下皆沒有發現 TFI 現象，但當 dgap = 0.3 mm << 1 mm 時，有發現 MIE 轉折
現象之存在。
CSSD 和 NRP 放電在 dgap = 0.8 mm、Le ≈ 2.1 >> 1 的相同貧油正丁烷燃氣條件下進
行比較，透過使用相同的放電能量 Etot = MIEL ≈ 23 mJ，本論文發現 CSSD 的層流引燃
概率(Laminar Ignition Probability, Pig,L)為 50％，而對於 NRP 放電，Pig,L為 0% ~ 90%，
其值取決於 PRF，其中在 PRF = 20 kHz 時有最高的 Pig,L = 90%。當 PRF = 20 kHz，即最
高的 Pig,L 之發生。可歸因於放電頻率的增益效應(synergistic effect)，其與放電時產生於
電極尖端之反應物向內流循環頻率(inward reactant flow recirculation frequency)之同步效
應。此外，本論文發現紊流引燃在大部分的 PRFs 中均使引燃較困難，除了在 PRF = 60
kHz 在 u′ = 0.5 m/s 時，有最高的 Pig,L為 37 %大於在 u′ = 0 時的 Pig,L = 34 %，顯示與
CSSD 相似之 TFI 現象。;Recently, premixed lean combustion using the nanosecond repetitively pulsed (NRP)
discharge (plasma-assisted combustion) has attracted many attentions because of its great
potential for increasing thermal efficiency and reducing the exhaust emissions of spark ignition
(SI) engines. However, one of the challenges for fuel-lean premixed combustion is the misfire
problem, especially when spark ignition takes place at extreme conditions relevant to SI engine
conditions, i.e. high pressure (p), high temperature (T), and high r.m.s turbulent fluctuation
velocity (u′). In addition, most NRP studies are in quiescence mixture condition with a few in
flowing mixtures having large mean velocity. The lack of NRP results in near-isotropic
turbulence with negligible mean velocities prevents us to fully understand the full potential of
the NRP discharge. Thus, a better understanding of ignition processes of lean fuel/air mixtures
and their subsequent flame propagation is indispensable for the development of a reliable and
efficient ignition system, so that stable lean-burn combustion in high-thermal efficiency SI
engines could be achieved. This motivates us to study the ignition of lean hydrocarbon fuels
using two different discharge methods: conventional single spark discharge (CSSD) and NRP
discharge.
Experiments were performed in a large dual-chamber, constant temperature/pressure, and
fan-stirred explosion facility capable of generating near-isotropic turbulence using three
different fuel/air mixtures, including lean and rich hydrogen (equivalence ratio f = 0.18 and f
= 5.1 with the effective Lewis number Le ~ 0.3 << 1 and Le ~ 2.3 >> 1, respectively), lean isooctane (f
= 0.8 with Le ~ 2.98 >> 1), and lean n-butane (f = 0.7 with Le ~ 2.1 >> 1). These
mixtures were ignited by CSSD at different spark gaps (dgap) over a range of u′ to understand
the coupling effects of differential diffusion (Le effect), dgap, and turbulent dissipation on two
distinct ignition phenomena: Minimum Ignition Energy (MIE) transition versus Turbulent
Facilitated Ignition (TFI). For comparison, the same lean n-butane/air mixture was ignited at
dgap = 0.8 mm using the NRP discharge over a range of pulse repetitive frequency (PRF = 5~70
kHz) with a fixed train of 11 pulses having a constant total ignition energy Etot ~ 23 mJ, where
Etot equals to the laminar MIE at 50% ignitability of the same lean n-butane/air obtained by
using CSSD at the same dgap = 0.8 mm.
The CSSD results show that TFI only occurs at Le >> 1 mixtures and at sufficiently small
dgap < 1 mm owing to the coupling effects between the embryonic kernel geometry and
differential diffusion. Further, turbulent MIE exhibits a first decrease and then increase non-v
monotonicity with increasing u. This reveals that turbulent dissipation re-asserts its dominance
and renders the ignition more difficult when u is greater than some critical values depending
on fuel types. At modest dgap > 1 mm where the embryonic kernel is a rod-like geometry with
very small or negligible positive curvature, the effect of differential diffusion is negligible and
TFI disappears even for Le >> 1 mixtures. Not only there is no TFI when dgap > 1 mm, but also
we find MIE transition for Le >> 1 and at dgap > 1 mm when increasing u. MIE transition means
that values of MIE first gradually increase with increasing u and then rapidly increase with
increasing u when u is greater than some critical value. Moreover, there is no TFI for the lean
hydrogen/air mixture with Le << 1, regardless of dgap, where MIE transition is found when dgap
= 0.3 mm << 1 mm.
From the comparison between CSSD and NRP discharge using the same energy Etot = MIEL
~ 23 mJ for the same lean n-butane/air mixture with Le ~ 2.1 >> 1 at dgap = 0.8 mm we find that
the laminar ignition probability (Pig,L) is 50% for CSSD, while Pig,L varies from 0% to 90% for
NRP discharge, depending on PRF, where the highest Pig,L = 90% occurs at PRF = 20 kHz,
being detrimental with higher PRFs. The highest Pig,L at PRF = 20 kHz could be attributed to
the coupling effects from the synergistic effect and the inward reactant flow recirculation
frequency near the electrode tips. Furthermore, we find that turbulence renders ignition more
difficult at most PRFs except at PRF = 60 kHz where the highest Pig = 37% occurs at u′= 0.5
m/s, Pig = 34% at u′= 0, showing similar TFI as that found by using CSSD. |
