本研究利用CMOS高速、高解析度攝影機配合雷射斷層掃描攝影術(laser tomography, LT)，首度提供預混紊流燃燒之散佈狀燃燒區域(distributed combustion regime)的火核及火焰影像證據，並定量量測最小引燃能量(minimum ignition energy, MIE)隨紊流強度增加之上升曲線，以了解MIE值在薄碎狀(flamelet)和散佈狀兩種截然不同燃燒區域之變化情形。實驗採用不同當量比(equivalence ratio, φ)之甲烷/空氣預混燃氣，並於十字型紊流燃燒器內執行。燃燒器內流場是由置於水平圓管兩端之一對反向旋轉特製風扇所產生，當對衝流場各通過一空孔板後，可在燃燒器中央觀測區產生一零平均速度之等向性紊流場，其紊流強度(u')最高約可達8 m/s，相對應之ReT = u'LI/nu? = 24,850，其中LI與nu分別為流場積分長度尺度和流體運動黏滯係數。引燃能量的控制採用Velonex 360高功率脈衝放電系統，而MIE值則透過Tektronix P6015A高電壓探針與Pearson電流感測器同步量測後計算而得。我們發現所謂的薄碎焰模式時，火核影像都呈超環面形狀，此時MIE值隨u'增加而呈線性增加。當在散佈狀燃燒模式時，火核則以破碎狀且不規則的形式呈現，其相對應之MIE值隨u'增加而呈指數型式增加。此結果顯示於紊流條件下之MIE值存有一轉變(transition)現象，並與薄碎焰過渡到散佈狀火焰相關。利用LT火焰面影像，沿著火焰傳遞方向進行灰階強度分析可以發現，在MIE值轉變前反應物與生成物之間呈明暗灰階快速變化，代表火焰厚度非常薄；但在MIE值轉變之後，反應物與生成物之灰階呈散佈狀分布，由此我們知道火焰已由原本之薄碎狀火焰轉變為散佈狀火焰，此結果可作為散佈狀火焰存在之實驗證據。另外，本研究也量測評估加氫效應對於火焰傳遞速度(flame propagation speed, SF)的影響與添加CO2對於MIE值的影響。前述實驗結果對预混紊流燃燒領域具有重要之學術價值，並可應用於諸如燃氣輪機和汽車引擎等燃燒設計。 This study aims to measure a transition on values of minimum ignition energy (MIE) divides two distinct combustion modes, namely flamelet and distributed regimes in premixed turbulent combustion particular interest is on the flame kernel formation and its subsequent flame propagation before and after the MIE transition. Lean methane/air premixtures at various equivalence ratios (φ) were studied and all experiments were carried out in a large cruciform burner. The burner consists of a pair of counter-rotating fans at each end of the horizontal vessel, capable of generating in tense isotropic turbulence with negligible mean velocities and the maximum value of turbulent intensities (u') as high as 8 m/s where the corresponding Reynolds number ReT = LIu'/nu? ? 24850. LI and nu?are the integral length scale of turbulence measured by LDV and PIV and the kinematic viscosity of reactants. The ignition source was controlled by the Velonex 360 high power pulse generator, where values of MIE were calculated from direct measurements of temporal variations of concurrent voltages and currents using a Tektronix high-voltage probe and a Pearson current transducer. We found that flame kernels remain regular and toroidal shape before the MIE transition, but after the MIE transition it is found that flame kernels become irregular and disrupted. Using the laser tomography (LT) to obtain developing turbulent flames, a drastic change of turbulent flame structures before and after MIE transition is observed. Before transition, the gray level distribution across the LT flame front image has vary sharp variation indicating that such flame front is very thin and is in the regime of turbulent-flamelet. After transition, the gray level distribution becomes distributed-like showing the existence of turbulent distributed flames. Finally, the effect of hydrogen addition on turbulent burning velocities and the effect of CO2 addition on values of MIE are also discussed. These results are important for the understanding of premixed turbulent combustion.
