| 摘要: | 本論文量測甲苯汽油替代燃料TRF85/空氣 (Toluene Reference Fuel 85; 由77.4%異辛烷、17.6%正庚烷以及5%甲苯所組成)的層紊流最小引燃能量(Minimum Ignition Energy, MIE)。以貧油的TRF85 (當量比為 = 0.8; 有效Lewis數Le ? 2.97) 加熱汽化加入空氣進行預混，利用本實驗室已建立之大型高壓雙腔體設計之十字型紊流燃燒爐，將一對尖頭火花探針安置於中央測試區，再以高壓單一脈衝產生器以選定之火花能量來引燃。藉由十字型內燃燒爐水平管兩側的對轉風扇，可以在中心區域產生近似等向性紊流場，而方均根擾動速度(u′) 的實驗條件控制在0 ~ 4.9 m/s。此研究主要探討兩種引燃現象，一個是最近被發現的紊流促進引燃(Turbulence Facilitated Ignition, TFI)，也就是在探針間距(dgap)小於1 mm且Le >> 1時，一定程度的紊流可以增加引燃機率且降低MIE。而本實驗在dgap = 0.8 mm時發現層流最小引燃能量MIEL = 4.76 mJ ，而當u′ = 1.4 m/s時紊流最小引燃能量MIET = 2.39 mJ，顯示TFI現象亦適用於液態燃料。另一個現象則是Shy團隊在2007年時所發現的引燃能量轉折現象，MIE會先隨著紊流強度的提升而緩慢的增長，當紊流強度超過臨界方均根擾動速度u′c時，MIE會與紊流呈指數性成長。結果顯示壓力與MIE在層流以及u′ = 1.4 m/s下呈現負次冪的關係 MIEL ~ p-1.7，MIET ~ p-1.6。隨著正規化紊流強度u′/SL的提升，正規化最小引燃能量 Γ = MIET/MIEL會先緩慢的增長，而當臨界正規化紊流強度 (u′/SL)c > 5.5 (0.7-atm), 7.0 (1-atm) and 9.7 (3-atm) 時，Γ會大幅上升，SL為層流火焰燃燒速度。為了進一步解釋上述的現象，此研究引入了Shy團隊在2010及2013所提出的P?clet數(Pe)壓力模型，火核反應區Pe* = Pe(p/p0)-1/4(u′k/RZ)。在引燃能量轉折現象發生前，Pe*會小於臨界P?clet數 Pe*c ? 4，Γ = 1 + 0.45Pe*，而在轉折發生後，Pe* > Pe*c，Γ = 1 + 0.04(Pe*4 - 400)。此研究對火花引燃引擎汽油替代燃料之引燃機制應有火助益。;This thesis investigates experimentally the effects of pressure and turbulence on laminar and turbulent minimum ignition energies (MIEL and MIET) of a lean gasoline surrogate fuel (TRF85)/air mixture with an effective Lewis number Le ? 2.97 >> 1. TRF is the toluene reference fuel, 85 is the research octane number, and thus TRF85 consists of 77.4% i-octane, 17.6% n-heptane, and 5% toluene. Ignition experiments are conducted by a pair of electrodes with sharp ends at the center of the experimentation domain in an already-established dual-chamber cruciform burner capable of generating near-isotropic turbulence. A wide range of ignition energy (Eig) varying from 0.47 mJ to 45.97 mJ can be discharged by a high-voltage ignition system and controlled by a home-made ignition circuit. The r.m.s. fluctuating velocity (u′) can be varied from 0 to 4.9 m/s. Two phenomena are investigated. The first concerns a turbulence facilitated ignition (TFI) phenomenon, where MIEL can be larger than MIET under two restrictions, i.e. sufficiently small spark gap (dgap; typically smaller than 1 mm) and sufficiently large Le >> 1. At dgap = 0.8 mm, MIEL ? 4.76 mJ > MIET ? 2.39 mJ at u′ = 1.4 m/s. The second phenomenon is a MIE transition, similar to that discovered by Shy et al. (2007), where the increase slopes of MIET versus u′ change from gradually to exponentially when u′ is greater than some critical value (u′c). Laminar results show that MIEL decreases with increasing pressure having a power law relation of MIEL ~ p-1.7. At u′ = 1.4 m/s, MIET ~ p-1.6. As to turbulent results, the increasing slopes of the normalized MIE = MIET/MIEL = Γ with the turbulent intensity (u′/SL) change from gradually to exponentially when (u′/SL)c > 5.5 (0.7-atm), 7.0 (1-atm) and 9.7 (3-atm), where SL is the unstretched laminar flame speed. Finally, a modified reaction zone P?clet number Pe* = Pe(p/p0)-1/4(u′k/RZ) with pressure correlation based on the model proposed by Shy et al. (2010; 2013) is used to examine the aforesaid MIE transition, where k is the Kolmogorov length scale and RZ is the reaction zone thermal diffusivity estimated by the average temperature between the adiabatic flame temperature and the reactant temperature. In the pre-transition where Pe* is less than a critical value of Pe*c ? 4, Γ = 1 + 0.45Pe*. In the post-transition where Pe* > Pe*c, Γ = 1 + 0.04(Pe*4 - 400). These results should be important to spark ignition dynamics in lean-burn spark ignition engines. |
