| 摘要: | 本論文之研究是為了更進一步瞭解紊流預混燃燒引燃能量及紊流火焰速度，藉此提供工業意外爆炸依據和航空安全評估，並且促進以火花引燃燃燒之交通運輸工具發展。本論文著重於兩部份基礎研究：(1)高壓條件最小引燃能量(minimum ignition energy, MIE)之量測和(2)高溫高壓紊流預混火焰速度之相關性等二部份。此二者研究乃使用本實驗室已建立之大型高壓高溫雙腔體設計之十字型預混紊流燃燒器。第一部份是以貧油甲烷/空氣(當量比ϕ = 0.6)為預混燃氣，在1至5倍的大氣壓力條件下，使用圓柱平頭火花電極置於測試區中央處，以高電壓脈衝產生器定量控制火花放電電極之引燃能量，量測不同紊流強度(u′/SL = 0 ~ 50)條件下之MIE，SL是層流燃燒速度。藉由火花放電過程所量測之近似方波電壓和電流數據，用以精確的計算引燃能量，並且定義出50%的引燃機率。我們也利用高速Schlieren紋影顯像技術觀測高壓紊流火核影像，我們觀測到即便壓力在5倍大氣壓力下，當u′/SL大於某臨界值(u′/SL)c時，火核發展的形態仍會從紊流薄碎焰極急劇地改變成類似散佈狀火焰，並且伴隨有島塊焰的形成和局部火焰熄滅的現象。實驗結果顯示，MIET/MIEL會先隨正規化紊流強度u′/SL之增加而呈線性增加，但是當u′/SL大於某臨界值(u′/SL)c時會呈現指數型驟升之變化，此為引燃轉折，其中下標T和L分別代表層流與紊流時之值。當MIEL ≈ 6.84 mJ (1atm)、2.81 (3 atm)和2.11 (5atm)，引燃轉折分別發生在(u′/SL)c ≈ 12 (1atm)、 24 (3 atm)和34 (5 atm)。實驗結果也顯示，對於不同壓力和紊流強度下之MIET/MIEL數值，我們可以藉由壓力修正係數方式，修正火核反應區引燃轉折模式Péclet number，使之以Pe* 關係式整合在一曲線擬合上，經修正後的曲線關係式為Pe* = PeRZ(p/p0)-1/4，當Pe* ≈ 3.6時為引燃轉折發生之處。其中PeRZ = u′ηk/αRZ，即火核之紊流與化學反應間擴散強弱之指標，ηk為Kolmogorov紊流長度尺度，αRZ為反應區熱擴散係數，p0 = 1 atm。這些結果顯示自我相似性火花引燃現象。
第二部份是以Le = 1(當量比ϕ=0.9)之甲烷/空氣為預混燃氣，在高溫高壓條件下量測紊流火焰速度，並且瞭解及比較紊流火焰速度之間的相似性。高溫的實驗是在十字型燃燒器腔體外及兩片空孔板上安裝新型加熱器進行加熱，並且透過調整加熱的方式確保實驗中心區域的溫度差異小於1 oC。火焰傳播速率d/dt和SF(平均半徑的斜率)是經由球形擴張紊流火焰的Schlieren影像進行統計及計算平均火焰半徑而獲得，在此SF是d/dt對時間之平均值，當25 mm ≤ ≤ 45 mm。結果顯示，正規化紊流火焰傳播速率(SLb)-1d/dt與火焰紊流雷諾數ReT,flame = (u′/SL)(/δL)大約呈現1/2冪次關係，在溫度300 K時(SLb)-1d/dt ≈ (SLb)-1SF = 0.116ReT,flame0.54，在溫度423 K時為0.168ReT,flame0.46，在此u′是方均根紊流擾動速度，SL和SLb分別是層流燃燒速度和未作密度校正之層流燃燒速度，δL是層流火焰厚度。前者，300 K條件下所獲得的結果與Chaudhuri et al. (2012)的結果趨近一致。但是後者顯示，423 K條件下在高雷諾數(ReT,flame)時(SLb)-1d/dt會有向下彎折的現象。利用密度修正和Bradley’s所提的平均傳遞變數進行計算，在=0.5處，可以得到球狀紊流火焰速度ST,c=0.5 ≈ (ρb/ρu)SF(c=0.1/c=0.5)2。此外，在不同溫度條件下所獲得之散佈球狀火焰之數據相較於Kobayashi之Bunsen火焰，可用ST,c=0.5/SL = 2.9[(u′/SL)(p/p0)]0.38此關係式有更佳的呈現。再者，這些散佈球狀焰之數據也可被聚合成一曲線關係(ST,c=0.5-SL)/u′ = 0.16Da0.39 其Da = (LI/ u′)(SL/δL)為紊流Damköhler數，LI是積分長度尺度。
;This thesis includes two parts, (1) the measurement and scaling of high-pressure minimum ignition energies (MIE) and (2) the correlation of high-temperature/pressure turbulent burning velocities in premixed turbulent combustion. All experiments were conducted in a large dual-chamber, constant-pressure/temperature, fan-stirred 3D cruciform bomb capable of generating near-isotropic turbulence. In the first part of the study, the high-pressure MIE was measured from spark discharges in lean methane/air mixtures at the equivalence ratio ϕ = 0.6 using a pair of 2-mm cylindrical electrodes with flat ends over a wild range of turbulent intensities (u′/SL = 0~ 50), from 1 to 5 atm, where SL is the laminar burning velocity. Voltage and current waveforms of spark discharges with nearly square profiles were carefully generated for accurate determination of MIE, commonly defined as the 50% successful ignitability. Applying high-speed schlieren imaging, we observed a drastic change of kernel development from turbulent flamelet to distributed like with island formation and local quench even at 5 atm, where u′/SL greater than some critical values depending on p. It is found that the scaling slopes of MIET/MIEL versus u′/SL change abruptly from a linear increase to an exponential increase when u′/SL > (u′/SL)c, showing ignition transition. The subscripts T and L represent turbulent and laminar properties, MIEL ≈ 6.84 mJ (1atm), 2.81 (3 atm), and 2.11 (5atm), and the transition occurs at (u′/SL)c ≈ 12 (1atm), 24 (3 atm), and 34 (5 atm). It is also found that above scattering MIET/MIEL data at different u′/SL and p can be merged together into a single curve when scaled with a pressure-corrected kernel (reaction zone, RZ) Péclet number, Pe* = PeRZ(p/p0)-1/4, showing the first and fourth power dependence before and after MIE transition at a critical Pe* ≈ 3.6. PeRZ = u′ηk/αRZ, ηk is the Kolmogorov length scale of turbulence, αRZ is the thermal diffusivity estimated at the instant of kernel formation, and p0 = 1 atm. These results reveal a self-similar spark ignition phenomenon.
The second part of the study, the high-temperature, high-pressure turbulent burning velocities and their correlation of expanding methane/air turbulent flames was reports at unity-Lewis-number. A novel heating method was used to ensure that the temperature variation in the domain of experimentation was less than 1℃. Schlieren images of statistically spherical expanding turbulent flames were recorded to evaluate the mean flame radius and the observed flame speeds, d/dt and SF (the slope of ), where SF is found to be equal to the average value of d/dt within 25 mm ≤ ≤ 45 mm. Results show that the normalized turbulent flame speed scales as a turbulent flame Reynolds number ReT,flame = (u′/SL)(/δL) roughly to the one-half power: (SLb)-1d/dt ≈ (SLb)-1SF = 0.116ReT,flame0.54 at 300 K and 0.168ReT,flame0.46 at 423 K, where SL and SLb are laminar flame speeds with respect to the unburned and burned gas, and δL is the laminar flame thickness. The former at 300 K agrees well with Chaudhuri et al. (2012) except that the present pre-factor of 0.116 and ReT,flame up to 10,000 are respectively 14% and four-fold higher. But the latter at 423 K shows that values of (SLb)-1d/dt bend down at larger ReT,flame. Using the density correction and Bradley’s mean progress variable converting factor for Schlieren spherical flames, the turbulent burning velocity at =0.5, ST,c=0.5 ≈ (ρb/ρu)SF(c=0.1/c=0.5)2, can be obtained, where the subscripts b and u indicate the burned and unburned gas. All scattering data at different temperatures for spherical flames can be better represented by ST,c=0.5/SL = 2.9[(u′/SL)(p/p0)]0.38, first proposed by Kobayashi for Bunsen flames. Also, these scattering data can be better represented by (ST,c=0.5-SL)/u′= 0.16Da0.39 with small variations, where the Damköhler number Da = (LI/ u′)(SL/δL) and LI is the integral length scale. |
