| 摘要: | 本論文針對壓力和紊流耦合效應對最小引燃能量(minimum ignition energy, MIE)之影響，進行定量量測和分析研究，進而提出一物理模式來解釋實驗結果。以貧油甲烷/空氣(當量比? = 0.6) 為預混燃氣，利用本實驗室已建立之大型高壓雙腔體設計之十字型預混紊流燃燒爐，使用平頭火花電極置於測試區中央處，以高功率脈衝產生器，定量控制中央火花放電電極之引燃能量，量測在層流和不同紊流條件下之MIE值。有關紊流之控制，乃由一十字型風扇擾動式內爐所提供，它置於一可吸收爆炸壓力之大型安全外爐內，在十字型內爐水平圓管底端兩側配置一對反向旋轉風扇與空孔板，可於十字型內爐中央區域產生一近似等向性紊流場，其平均速度幾可忽略，而方均根紊流擾動速度(u?)最高可達約8.42 m/s。我們也針對兩種計算MIE之統計方法，進行介紹和比較。實驗結果顯示，MIEL和MIET值均會隨壓力之增加而顯著下降，其中下標L與T分別代表層流與紊流時之值。如同在常壓條件下，我們發現引燃轉折現象也存在高壓條件下，即MIET/MIEL = ?值會先隨正規化紊流強度u?/SL值之增加而呈線性增加，但當u?/SL值大於某臨界值時，?值會呈現大幅驟升之變化，SL為層流燃燒速度。另外，我們也利用Schlieren紋影顯像技術觀測高壓紊流火核影像，用以瞭解引燃轉折前後之火核結構轉變。經引入一壓力修正因子，對先前在常壓時所獲得用以解釋常壓引燃轉折現象之物理模式，即火核反應區Péclet數(Pe)小於某臨界值Pec時?1 = 1 + c1Pe，而Pe > Pec時?2 = 1 + c2 (Pe4 –c3)進行修正，其中Pe = u??K/?RZ，即火核之紊流與化學反應間擴散強弱之指標，?K為Kolmogorov長度尺度，?RZ為以平均溫度Tm所估算之反應區熱擴散係數；所修正後之物理模式，可表述為當Pe* < Pe*c時，?1 = 1 + c1Pe*，Pe* > Pe*c時?2 = 1 + c2 (Pe*4 –c3)，其中經壓力修正之Pe* = Pe(p/p0)-1/4，p0 = 0.1 MPa。此修正後之物理模式可合理地解釋高壓引燃轉折之結果，此研究結果將對許多燃燒相關應用 (如內燃機等)有所助益。This thesis quantitatively measures the coupling effects of pressure and turbulence on minimum ignition energies (MIE) following by a physical model to explain these results. Lean methane-air mixtures at the equivalence ratio ? = 0.6 are used because much higher MIE is required to ignite such lean mixtures. Experiments are carried out in an already-established high-pressure, double-chamber explosion facility and a high-power pulse generator is used to control ignition energies of a pair of spark-electrodes with flat ends having a gap of 1 mm to increase required MIE for high pressure measurements positioned at the centre of a large inner cruciform burner. The inner burner lodged in a huge high-pressure absorbing outer chamber is equipped with a pair of counter-rotating fans and perforated plates capable of generating intense near-isotropic turbulence with negligible mean velocities and roughly equal magnitudes of turbulent fluctuation velocities in all three directions where the root-mean-square turbulent fluctuating velocities (u?) can be up to 8.42 m/s. Two statistical methods used to estimate MIE are reviewed and compared. Results show that values of MIEL and MIET noticeably as pressure (p) increases, where the subscripts L and T represent laminar and turbulent values. It is found that, similar to obtained at p = 0.1 MPa, the increasing slopes of MIET/MIEL = ? curves under elevated pressure conditions (p = 0.1 and 0.3 MPa) change drastically from linear to exponential when values of u?/SL are greater than some critical values depending on p showing ignition transition, where SL is the laminar burning velocity. Moreover, the Schlieren imaging technique is used to acquire flame kernel images at high pressure turbulent conditions in attempt to distinguish the structure difference of flame kernels before and after ignition transition. Finally, by introducing a pressure correction, we can modify the previous model at normal pressure condition, such that all data curves obtained at different pressure conditions with different critical values of u?/SL can be collapsed roughly into a single curve. Our previous model (Shy et al. 2010 [10]) based on a reaction zone (ignition kernel) Péclet number, Pe = u??K/?RZ, is used to explain ignition transition, where ?K is Kolmogorov length scale and ?RZ is the thermal diffusivity at the surface of the ignition kernel estimated at the mean temperature between flame adiabatic and reactant temperatures. In it when Pe < Pec for the pre-transition, ? = 1 + a1Pe, while ? = 1 + a2 (Pe4 –b2) for the post-transition when Pe > Pec, where a1, a2 and b2 are experimental constants. The present pressure modified correction is: Pe* = Pe(p/p0)-1/4, where p0 = 0.1 MPa. Using Pe* to take the pressure effect into consideration, all MIET/MIEL data at various values of u?/SL up to 50 and under different pressure conditions (p = 0.1, 0.3, 0.5 MPa) can be represented by a single curve having two drastically different increasing slopes with increasing Pe*: ? = 1 + a1Pe*,before transition and ? = 1 + a2 (Pe*4 –b3) after transition. These results are useful in many industrial devices such as spark-ignition-engines and internal combustion engines. |
