| dc.description.abstract | 本論文有兩大重點：(1)研究預混紊流燃燒之火花引燃機制，即定量量測最小引燃能量(minimum ignition energy, MIE)、火核成長和火焰傳播；(2)探討加氫效應對MIE值及火焰傳播速度(flame propagation speed, SF)之影響。MIE值攸關氣爆、火災預防和燃料安全準則，並可直接應用於火花引燃(spark ignition, SI)引擎之設計和提昇引燃效率。本研究使用低碳燃料甲烷和空氣之預混燃氣，針對不同當量比(equivalence ratio) = 0.6 ~ 1.3，從層流到具極高強度之紊流場，從薄碎焰(flamelet)到散佈狀燃燒(distributed combustion)區域，進行一系列完整之MIE值定量量測，找出MIE值和正規化紊流強度 (u’’/SL)和Karlovitz (Ka)數之函數關係，其中u’’是方均根紊流擾動強度，SL為層流燃燒速度，Ka為一化學反應和紊流特性時間比，Ka = (u’’/SL)2(ReT)-0.5，紊流雷諾數ReT = u’’LI/nu，LI為紊流積分長度尺度而nu為反應物之運動黏滯係數。所有引燃實驗均在一大十字型預混紊流燃燒器執行，它配置一對由10匹馬力三相馬達驅動之特製反向旋轉風扇和空孔板，可產生一近似等向性紊流場，其u’’值可高達約8 m/s於中央均勻區，也是本實驗尖端電極所置放之處。使用高功率脈衝引燃系統和高速CMOS攝影機(每秒5000張，512 x 512 pixels)，進行MIE值定量量測和火核及火焰傳播影像之擷取。結果顯示，在任一固定φ值下，MIE值均會先隨u’’/SL增大而漸增，但當u’’/SL或Ka超過一臨界值時，原漸增之MIE值變為驟升，此一MIE值轉折(transition)，為一新發現。藉由比較MIE值轉折前之火焰影像，我們發現火焰由原本薄碎燄模式轉變為散佈狀模式。轉折發生時所對應之 u’’/SL值為16 ~ 27，而相對應之臨界Ka (Kac)均大於1且約在φ=1附近有一最小值，如Ka≒9, 7 和4.2 當φ= 0.6, 0.7 和0.8而Kac≒5 和2.2 當φ= 1.3和1.2，這顯示傳統Klimov和Williams規範(Kac ~ 1)需要作修正。有關加氫效應(φ = 0.6)，加入少量氫氣即可有效降低MIE值和增加SF，如在層流條件(u’’/SL = 0)，加氫量由0%增加至20%，MIE值會由2.14 mJ驟降至0.27 mJ。而當u’’/SL = 5.1，同樣0%到20%的加氫量，會使MIE值由3.7 mJ降至約0.313 mJ。加氫20%之SF會比無加氫時，快約42%。加氫效應在紊流條件下會更加明顯，如在u’’/SL = 5.13，同樣的加氫量，並使SF值增加了86%。是故加氫於貧油預混甲烷燃氣，不僅可有效降低貧油脂失效點火(misfire)機率並可提升SF。本研究成果可應用至SI引擎和燃氣渦輪機，並深具學術價值。
| zh_TW |
| dc.description.abstract | This thesis has two objectives: (1) The study of spark ignition mechanism for premixed turbulent combustion via quantitative measurements of minimum ignition energy (MIE), kernel formation and growth, and subsequent flame propagation; (2) the study of the influence of hydrogen addition on MIE and flame propagation speed (SF). MIE is an extremely important parameter for atmospheric explosion, fire prevention, and fuel safety standards. MIE data can be directly applied to the design of the spark ignition (SI) engine and thus increase the ignition efficiency. This study uses low carbon fuel, methane (CH4) and air premixtures, at various equivalence ratios varying from 0.6 to 1.3. At each value of equivalence, a complete dataset of MIE is measured under both laminar and turbulent conditions covering different combustion modes from laminar to flamelet to distributed regimes. Thus, MIE curves as a function of normalized turbulence intensities (u’’/SL) and/or turbulence Karlovitz number (Ka) at various values of equivalence ratio can be established, where u’’, SL and Ka are root-mean-square turbulence intensity, laminar burning velocity and a time ratio of chemical reaction to turbulent defined as Ka = (u’’/SL)2(ReT)^-0.5. Here the turbulent Reynolds number ReT = u’’LI/nu, where LI and nu are the integral length scale and the kinematic viscosity of reactants. All ignition experiments are carried out in a large cruciform premixed turbulent combustor which is equipped with a pair of counter-rotating fans and perforated plates at each end of its horizontal vessel, capable of generating intense isotropic turbulence with values of u’’ up to 8 m/s in the central uniform region where the spark electrodes with needle ends are located. Using a high-power pulse ignition system with Pearson current and high-voltage probes, values of MIE can be measured. Furthermore, ignited kernel and subsequent flame propagation are recorded via a high-speed imaging system with 5000 frames/s and 512 x 512 pixels resolution. Results show that at any fixed values of equivalence ratio, there is a transition on values of MIE. Before the transition, values of MIE only increase gradually with increasing values of u’’/SL or Ka. Above the transition, MIE values increase abruptly. This transition is a new finding. By comparing radially-expanding flame front images before and after the transition, it is found that combustion modes may change from turbulent-flamelet to turbulent-distributed. The critical values of u’’/SL for the occurrence of transition are varying with different values of equivalence ratio ranging from 16 to 27, and their corresponding values of Kac are all greater than unity indicating the failure of Klimov & Williams’ criterion that predicted Kac = 1. The minimum value of Kac (> 1) is expected to occur near equivalence ratio = 1, since values of Kac are decreasing from both lean and rich sides. For examples, Kac ~ 9, 7 and 4.2 when equivalence ratio = 0.6, 0.7 and 0.8, while Kac ~ 5 and 2.2 when equivalence ratio = 1.3 and 1.2, respectively. When hydrogen is doped (equivalence ratio = 0.6), values of MIE and/or SF can be effectively decreasing and/or increasing. For instances, MIE values decrease from 2.14 mJ to 0.27 mJ in quiescent condition when the hydrogen addition increases from 0% to 20%, while under the same amount of hydrogen addition, MIE values decrease form 3.7 mJ to 0.313mJ when u’’/SL = 5.13. Similarly, values of SF with 20% hydrogen addition are about 42% higher than that of no hydrogen addition in quiescent conditions, but up to 86% increase is found when u’’/SL = 5.13. Therefore, the usage of hydrogen addition in lean premixed turbulent methane combustion not only decreases effectively the probability of misfire, but also increases values of SF significantly. These findings are important for fundamental understanding of premixed turbulent combustion and for practical applications to SI engines and gas turbines. | en_US |