貧油正丁烷與富氫燃料之最小引燃能量量測: 電極熱損失與紊流效應;Minimum ignition energy measurements of lean n-butane and very rich hydrogen fuels: Effects of electrode heat losses and turbulence

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Title:	貧油正丁烷與富氫燃料之最小引燃能量量測: 電極熱損失與紊流效應;Minimum ignition energy measurements of lean n-butane and very rich hydrogen fuels: Effects of electrode heat losses and turbulence
Authors:	黃世曜;Shih-Yao, Huang
Contributors:	機械工程學系
Keywords:	層流和紊流最小引燃能量;紊流促進引燃;差別擴散效應;電極熱損失效應;紊流消散效應;Laminar and turbulent minimum ignition energies;turbulent facilitated ignition;differential diffusion effects;electrode heat losses effects;turbulent dissipation effects
Date:	2018-11-08
Issue Date:	2019-04-02 15:11:20 (UTC+8)
Publisher:	國立中央大學
Abstract:	本研究使用高溫高壓雙腔體十字型風扇擾動紊流預混燃燒設備，結合火花引燃及量測系統，針對富油氫氣當量比 = 5.1 (有效Lewis數Le = 2.3 >> 1)及貧油正丁烷 = 0.7 (Le = 2.1 >> 1)燃料，量測在層流和紊流情況下的最小引燃能量(Laminar and Turbulent Minimum Ignition Energy, MIEL and MIET)，並探討新近發現之電極火花紊流促進引燃(Turbulent Facilitated Ignition, TFI)現象。TFI現象為紊流在特定限制條件下，不但不會增加MIET，反而是讓MIET << MIEL。Wu et al. (2014)是第一個發現TFI現象的研究團隊，而Shy et al. (2017)進一步定量量測MIEL和MIET值，並發現TFI僅存在於Le >> 1及很小電極間距(dgap < 0.8 mm)的條件下。主要影響MIE變化與TFI現象的效應主要有三: (1)差別擴散(Differential diffusion)；(2)電極熱損失(Electrode heat losses)；(3)紊流消散(Turbulent dissipation)。我們針對小電極間距下所增加的電極熱損失與紊流消散效應進行研究。為了研究電極熱損失對MIEL和MIET影響，我們使用不同的電極直徑dth和dgap與特製的陶瓷管包覆的鎢電極來控制電極熱損失；在紊流消散效應部分，藉由改變紊流擾動速度(u′ = 0 m/s ~ 8.3 m/s)觀察MIE的變化。實驗結果顯示，MIEL值會隨電極熱損失上升(dgap下降與dth增加)而上升，在小間距dgap = 0.58 mm及直徑dth = 1 mm時尤其明顯，甚至引燃能量Eig = 300 mJ仍無法引燃富氫燃氣。當使用較少熱損失之陶瓷包覆電極時，無論在層、紊流及不同電極間距下，皆能降低最小引燃能量，尤其在小間距dgap = 0.58 mm及層流時，下降特別明顯。但就算使用陶瓷包覆電極，在dgap < 1 mm時，TFI現象仍然發生，這表示TFI現象並非由電極熱損失所主導。接下來，我們針對紊流消散效應進行探討(TFI僅在dgap < 1mm時發生)，在兩種燃氣中皆觀察到類似的非單調曲線，MIET首先隨u′上升而下降，但當u′大於某臨界值，MIET會隨u′上升而上升。我們使用Karlovitz數(Ka)進行分析，發現在兩種燃料的轉折皆發生在Ka = 0.8。若使用上述的三種效應來檢視此非單調的曲線，在層流時，因為火核有很大的正拉伸率(火核曲率半徑很小)，差別擴散效應會降低Le >> 1火核之反應率，進而使引燃在層流時變得非常困難。在弱紊流時(Ka < 0.8)，紊流可能會使初始火核稍微移動離開電極中心且有可能使火核具有負拉伸率，如此會提升Le >> 1火核之反應率，使MIET隨u′增加而下降。在強紊流時(Ka > 0.8)，根據預混紊流火燄區域圖(Regime diagram of premix turbulent flames)，Ka > 0.8時火燄可能進入薄反應區區間(Thin reaction zone regime)，因為火燄變厚使差別擴散效應改變，同時紊流消散效應開始主導MIET值變化，使MIET隨u′增加而上升。;This thesis measures laminar and turbulent minimum ignition energies (MIEL and MIET) of rich hydrogen/air mixture at equivalence ratio  = 5.1 (effective Lewis number, Le = 2.3 >> 1) and lean n-butane/air mixture at  = 0.7 (Le = 2.1 >> 1) in high-temperature, high-pressure, fan-stirred premixed turbulent combustion facility combining with electric spark ignition and its measurement system. We also attempts to investigate the recently-discovered turbulent facilitated ignition (TFI) phenomenon. Wu et al. (2014) were the first research group to discover TFI suggesting MIEL >> MIET. Shy et al. (2017) measured MIEL and MIET qualitatively. They discovered that TFI only occurs in Le >> 1 mixtures and at sufficiently small spark gap distance (dgap < 0.8 mm). There are mainly three effects influencing MIE behavior and TFI phenomenon: (1) differential diffusion, (2) electrode heat losses and (3) turbulent dissipation. Therefore, we try to investigate electrode heat losses effects induced from small dgap and turbulent dissipation effects. In order to understand electrode heat losses effects on MIEL and MIET, electrodes heat losses are controlled by changing electrode thickness (dth), dgap and using a pair of special made ceramic sheathed (CS) electrodes. To investigate turbulent dissipation effects, we observe MIE behavior with large range of turbulent fluctuation velocity (u′ = 0 m/s ~ 8.3 m/s). Results show that MIEL will increase with electrode heat losses (decreasing dgap and increasing dth). Large ignition energy (Eig = 300 mJ) cannot even ignite rich hydrogen mixture in dgap = 0.58 mm and dth = 1 mm. When using CS electrodes with smaller electrode heat losses, both MIEL and MIET are smaller than that of the normal electrodes. However, even using the CS electrodes, MIEL is still larger than MIET when dgap < 1 mm suggesting that the occurrence of TFI is not dominated by electrodes heat losses. Next, we focus on turbulent dissipation effects (TFI only exists in dgap < 1mm). A similar non-monotonic trend can be observed in both fuels. MIET first decrease with u′ and then increase with u′ after a certain degree of u′. Karlovitz number (Ka) is used to analyze the aforesaid phenomenon and the transition of MIE decrement to increment happens at Ka = 0.8 for both fuels. We again used aforementioned three effects to examine this non-monotonic curve. In laminar conditions, the initial spark kernel is near spherical with large positive stretch (positive curvature owing to small flame kernel radius), which in turn reduces reaction rate of the Le >> 1 kernel due to differential diffusion effects. Thus, ignition becomes extremely difficult in quiescence. In lower turbulence (Ka < 0.8), turbulence could move the initial spark kernel slightly away from the center of electrodes and could also result in negative stretch spots that increase reaction rate of the Le >> 1 kernel. MIET decreases with increasing u′. In higher turbulence (Ka > 0.8), flame kernel might enter thin reaction zone regime basing on regime diagram of premix turbulent flames. Flame kernel thickness might change altering differential diffusion effects and turbulent dissipation effects start to dominate. MIET increases with increasing u′.
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