在不同電極火花間距之貧油汽油主要參考燃料的層、紊流最小引燃能量量測;Measurements of Laminar and Turbulent Minimum Ignition Energies on A Lean Gasoline Primary Reference Fuel at Various Electrode-Spark Gaps

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题名:	在不同電極火花間距之貧油汽油主要參考燃料的層、紊流最小引燃能量量測;Measurements of Laminar and Turbulent Minimum Ignition Energies on A Lean Gasoline Primary Reference Fuel at Various Electrode-Spark Gaps
作者:	廖御超;LIAO, YU-CHAO
贡献者:	機械工程學系
关键词:	層流和紊流最小引燃能量;紊流促進引燃現象;最小引燃能量轉變;火花電極間距效應;路易斯數效應;Laminar and turbulent minimum ignition energies;turbulent facilitated ignition;minimum ignition energy transition;electrode gap distance effects;Lewis number effect
日期:	2020-10-05
上传时间:	2021-03-18 17:41:34 (UTC+8)
出版者:	國立中央大學
摘要:	本研究使用高溫高壓雙腔體三維十字型風扇擾動紊流預混燃燒設備，配合電極火花引燃及其能量量測系統，量測汽油主要替代燃料(Primary reference fuel, PRF;即異辛烷加正庚烷)之層流與紊流最小引燃能量(Laminar and turbulent minimum ignition energy, MIET and MIEL)。此外，本實驗利用燃油預蒸發系統與加熱系統以確保PRF能夠被完全汽化。實驗條件為初始溫度T = 373K、初始壓力P = 1atm、當量比(equivalence ratio)  = 0.8，其有效Lewis數為Le ? 2.95 > 1，方均根紊流擾動速度u′ = 0 ~ 3.68 m/s。實驗結果主要包含三個部分：(1)量測PRF於不同混合比例下即不同辛烷值(Research octane number, RON = 0 - 100)之MIEL。實驗結果發現MIEL首先會隨著RON的上升而緩慢上升，當經過一臨界值約RON = 90時，MIEL會劇烈上升，整體呈現一非線性曲線。 (2)選定PRF95於不同電極探針間距(Electrode gap distance, dgap = 0.8 ~ 2.0 mm)條件下，進行層流(u′ = 0 m/s)與紊流(u′ = 2.76 m/s)之MIE量測，探討MIET與MIEL隨dgap之變化，並嘗試找出發生紊流促進引燃現象(Turbulent facilitate ignition, TFI)即MIET < MIEL之臨界dgap。實驗結果發現TFI僅存在於dgap = 0.8 mm，此時MIET = 30.1 mJ < MIEL = 26.8 mJ。在dgap = 1.0 mm時MIET = 24.4 mJ ? MIEL = 24.9 mJ，而在dgap = 1.5 與2.0 mm情況下，MIET >> MIEL，由以上結果可推測發生TFI的臨界電極探針間距為dgap = 1.0 mm。此外，根據MIET與MIEL隨dgap變化之關係，我們發現MIEL約正比於dgap-3，而MIET則正比於dgap-0.3，可知MIEL對於dgap的變化較MIET敏感。(3)選定dgap = 0.8與2.0 mm條件下，分別量測其紊流效應。在固定dgap = 0.8 mm條件下，我們發現MIET值隨著u′值之增加呈現非單調曲線，即當u′ < 2.76 m/s有TFI (MIET < MIEL)，最小MIET值發生於u′ = 1.84 m/s，而當u′ > 2.76 m/s時，MIE會大幅度地上升，此時MIET > MIEL，情節又回到傳統認知，即紊流會使引燃更加困難。而於2.0 mm情況下，我們發現一MIE轉變(MIE Transition)現象，MIET值會先隨著u′值增加而呈線型上升，於u′ = 2.3 m/s後，MIET值會隨u′值之增加而呈指數性急遽地增加。此外，計算發生MIE轉變臨界點之Karlovitz數(Karlovitz number)，即臨界Karlovitz數(Kac)，將之與先前文獻所量測之不同燃料：貧油氫氣( = 0.18, Le = 0.3)、富油氫氣( = 5.1, Le = 2.3)、甲烷( = 0.6-1.3, Le ? 1.0)與貧油異辛烷( = 0.8, Le = 2.98)進行比較，觀測Kac隨Le的變化。結果發現Kac會隨著Le的上升而下降，呈現一關係式：Kac ~ Le-2，顯示在預混紊流火燄區域圖(Regime diagram of premix turbulent flames)中發生MIE轉變之標準需考量Le之變化。;This thesis measures laminar and turbulent minimum ignition energies (MIET and MIEL) for binary blends of iso-octane and n-heptane, referred to as Primary Reference Fuel (PRF), at initial condition of T = 373K, P = 1 atm, equivalence ratio  = 0.8 with effective Lewis number Le = 2.95 >> 1 over wide ranges of r.m.s. turbulent fluctuation velocity u′ = 0 ~ 3.68 m/s. Spark ignition experiments are conducted in a high pressure/temperature, fan-stirred, large dual-chamber, premixed turbulent 3D cruciform combustion facility capable of generating isotropic turbulence with electrical spark ignitor and energy measurement system. A heating system and a pre-vaporized system are applied to make sure that PRF can be well evaporated. The results mainly consist of three parts: (1) MIEL is measured on the blends of iso-octane and n-heptane showing the effect of research octane number (RON). Result shows that the MIEL firstly increases gradually with RON. Then, after a critical value about RON = 90, the MIEL increases drastically showing a non-linear curve. (2) The MIE of PRF95/air mixture is measured under laminar (u′ = 0 m/s) and turbulent (u′ = 2.76 m/s) condition with different electrode gap distance (dgap) and trying to find the critical dgap that occurs turbulent facilitate ignition (TFI, namely MIET < MIEL). Results show that TFI is found only at dgap = 0.8 mm < 1 mm that MIET = 30.1 mJ < MIEL = 26.8 mJ, while MIET = 24.4 mJ ? MIEL = 24.9 mJ at dgap = 1 mm. The situation comes back to common notion that turbulence makes ignition more difficult that MIET > MIEL at dgap = 1.5 and 2 mm > 1 mm. The result suggests that the critical dgap might be dgap = 1 mm. Moreover, according to the correlation of MIE and dgap, MIEL is proportional to dgap-3, but MIET is proportional to dgap-0.3 showing that MIEL is relatively sensitive to dgap effect compare to MIET. (3) MIET is measured over a wide range of u′ with two different dgap which is 0.8 mm (TFI) and 2.0 mm (No TFI), respectively. At dgap = 0.8 mm, we discover the curve of MIET versus u′ is non-monotonic: TFI (MIET < MIEL) occurs when u′ < 2.76 m/s, where the lowest MIET takes place at u′ = 1.84 m/s. When u′ > 2.76 m/s, MIET increases drastically and the scenario returns back to the common notion that turbulence renders ignition more difficult. Furthermore, at dgap = 2 mm, a MIE transition is found, in which the value of MIET firstly increases linearly with the increase of u′ and then its increase becomes exponentially when u′ > 2.3 m/s. Moreover, the critical Karlovitz number (Kac) indicating the Ka at turning point of MIE transition is calculated. The result plotted with previous data: lean hydrogen ( = 0.18, Le = 0.3), rich hydrogen ( = 5.1, Le = 2.3), methane ( = 0.6 ~ 1.3, Le ? 1.0), and iso-octane ( = 0.8, Le = 2.98). It shows that Kac increases with the decrease of Le having a correlation of Kac ~ Le-2, which suggests that the criterion of Kac that occurs MIE transition in Borghi diagram should consider about the effect of Le.
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