| 摘要: | 本研究定量量測貧油預混甲烷/空氣燃氣之最小引燃能量(minimum ignition energy, MIE)。MIE是一極重要的物理量，攸關著燃料之安全標準、引燃條件以及其後續之燃燒穩定性與效率。本研究特別針對預混燃氣於不同流場條件時，其所需引燃之MIE做深入地量測探討，即MIE與紊流強度(u'/S_L)和相關紊流特性之關係，S_L為層流燃燒速度。實驗方法採用不同貧油當量比之預混甲烷燃氣(equivalence ratio, phi= 0.6, 0.7, 0.8)，利用我們實驗室已發展多年之十字型預混紊流燃燒器，它在大水平圓管兩側各配備了一組反向旋轉風扇和空孔板，可產生強烈的近似等向性紊流場(u'可高達8 m/s)，並透過石英玻璃視窗，獲得火核成長與火焰傳遞影像。實驗開始前先將燃燒器內之空氣抽至近真空狀態，再將混合器內已預混之固定當量比的燃氣注入燃燒器內，並依據實驗所需之不同紊流強度條件來調節風扇轉頻。本研究以固定2.6 mm之不銹鋼電偶間距(約等同於甲烷最小熄滅距離)，以及使用Velonex公司所製造之高壓脈衝產生器與脈衝變壓器來產生放電火花，配合放電電路串聯電阻的方法，可精確地控制放電火花的引燃能量。我們直接於放電電路上，配置高壓探針與Pearson電流感測器，來定量量測兩電偶間之實際放電能量，同時以高速攝影機擷取引燃期間之火花放電、火核成長和火焰傳遞影像。本實驗在層流靜止流場的MIE量測結果與Lewis & von Elbe (1987)以及Ziegler et al. (1984)所得到之數據非常相近，誤差在7%內。MIE值會隨?值往化學計量比phi= 1方向增加而下降，此趨勢在紊流條件下亦同。另在所有不同phi值條件下(phi= 0.6, 0.7, 0.8)，MIE值均會隨著紊流強度(u'/S_L)增加而逐漸增加。針對接近貧油可燃極限phi= 0.6 (S_L = 9 cm/s)，我們發現MIE值有一重大轉變(transition)，當u'/S_L > 24，MIE值會驟昇。將其火核成長和火焰傳播影像與在適度紊流強度時之影像做比較，可以看出兩種相當不同的型態，即火焰由薄碎焰(flamelet)型態轉變為散佈狀火焰(distributed)型態，此一結果提供了預混燃燒狀態圖(phase diagram)中，散佈狀火燄區域存在的實驗證據，為一新發現。 This thesis aims to measure quantitatively the minimum ignition energy (MIE) of lean premixed methane/air mixtures over a very wide range of turbulent intensities (u'/S_L), where S_L is the laminar burning velocity. MIE is an extremely important parameter that is relevant to material safety standards, ignition conditions, and stability and efficiency of subsequent combustion processes. In the present study, lean combustible methane/air mixtures at various equivalence ratios, phi = 0.6, 0.7, and 0.8, are applied in the cruciform burner equipped with a pair of counter-rotating fans and perforated plates at each end of its horizontal vessel to generate intense near-isotropic turbulence (u' can be up to 8 m/s). Using a high-speed CMOS camera, the flame kernel development and its subsequent flame propagation are recorded. Before a run, the burner was first evacuated and then lean methane/air mixtures at a fixed phi mixed in a separate mixing chamber were injected into the burner to 1atm. The fan frequency can be varied from 0 Hz up to 172 Hz. This study uses two stainless steel electrodes with sharp ends separated by 2.6 mm gap, that is just about the minimum quenching distance for methane. The electrodes’ spark discharge was produced by a high power pulse generator and a transformer. The discharge energy across the electrodes can be controlled using variable resistances and can be directly measured by a high pressure voltage probe and a Pearson current monitor together with an oscilloscope. The MIE data for the case of u'/S_L = 0 are found to be very close to that obtained by Lewis & von Elbe (1987) and Ziegler et al. (1984) with no more than 7% difference. As phi increases toward the stoichiometry (phi = 1), values of MIE are significantly decreased. This trend is the same for both quiescent and turbulent cases. At any fixed values of phi(= 0.6, 0.7 or 0.8), values of MIE increase gradually with increasing values of u'/S_L. It is found that there is a transition on values of MIE when u'/S_L > 24. Across the transition, values of MIE increase abruptly and flame structures change from flamelet-like to distributed-like. This result provides the first experimental evidence that can be used to prove the existence of the distributed reaction zone regime in the well-known phase diagram for premixed turbulent combustion. This is a new finding. |
