預混紊流氨氣/空氣燃燒: 從引燃到火焰整體熄滅;Premixed Turbulent Ammonia/Air Combustion: From Ignition to Flame Global Quench

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Title:	預混紊流氨氣/空氣燃燒: 從引燃到火焰整體熄滅;Premixed Turbulent Ammonia/Air Combustion: From Ignition to Flame Global Quench
Authors:	Chen, Yi-rong;Chen
Contributors:	機械工程學系
Keywords:	預混氨氣/空氣火焰;層流和紊流最小引燃能量;火焰整體熄滅;向下傳遞火焰與中心引燃火焰;等向性紊流場;Premixed ammonia/air flames;Laminar and turbulent minimum ignition energy;Flame global quench;Downward propagation versus central ignition;Intense near-isotropic turbulence
Date:	2023-05-11
Issue Date:	2024-09-19 17:25:43 (UTC+8)
Publisher:	國立中央大學
Abstract:	本論文使用十字型風扇擾動紊流燃燒設備，搭配可精確控制放電能量的高壓電極火花引燃系統，量測預混氨氣/空氣燃料在層紊流下的最小引燃能量(Minimum Ignition Energy, MIE)與火焰整體熄滅極限。十字型燃燒設備在水平方向配有兩對卧且相反方向轉動的10馬力風扇與一對空孔板，可在設備中心處產生一等向性紊流場，且方均根紊流擾動速度(u′)可控制在0 ~ 7.2 m/s。在垂直方向有上下對稱兩圓管，引燃藉由一對電極探針在設備中心處或在垂直上下管頂端處引燃，可分別產生中心引燃、向上與向下傳遞型態之火焰。研究分為引燃及火焰熄滅兩大方面: (1)在引燃方面，我們比較預混氨氣/空氣燃料使用傳統電極放電(Conventional Spark Discharge, CS)與奈秒脈衝放電(Nanosecond repetitively pulsed discharge, NRPD)的MIE，也探討當氨氣/空氣燃料混氫時，對引燃的影響。(2)在火焰熄滅方面，使用不同引燃能量，比較向下傳遞與中心引燃兩種火焰型態的整體熄滅極限之異同，而向上與向下傳遞的火焰的整體熄滅結果雷同。結果顯示在當量比f = 0.75 ~ 1.2時，CS比NRPD的MIE來得低，這是因為NRPD火核會因連續放電的影響，多分裂成兩小火核使得熱損失較大。然而，我們發現NRPD可有效拓展貧富油可燃極限，從CS的f = 0.75 ~ 1.2到NRPD的f = 0.66 ~ 1.42，這是因為NRPD的長時間連續放電有助於活躍分子和自由基的累積，最終產生可自我傳遞的火焰。此外，當氨氣/空氣燃料混氫時，最小引燃能量也可大幅下降、貧富油可燃極限也被大幅拓展，即使僅添加2 vol.%的氫氣，貧富油可燃極限可被拓展從純氨/空氣燃料的f = 0.75 ~ 1.2到f = 0.5 ~ 1.5。另一方面，火焰熄滅的結果顯示，向下傳遞比起中心引燃更難以被紊流整體熄滅，但向下傳遞火焰僅能在f = 0.75 ~ 1.2時傳遞，這是因f < 0.75或f > 1.25時，層流火焰速度(SL)太低且火焰厚度(dF)太厚，故火焰無法向下傳遞。最後，我們發現向下傳遞火焰臨界熄滅所需之紊流方均根擾動速度臨界值u′c會隨值增加而明顯增加，從f = 0.75之u′ = 1.4 m/s到f = 1.2之u′c = 7 m/s；向下傳遞的富油火焰出乎意外地能抵抗強紊流，使整體熄滅變得更為困難。此現象我們透過量測燃燒後生成物的氫氣含量體積比[H2]來解釋，結果顯示，在f = 0.9時，[H2] = 1.4 vol.%，但是在f = 1.2時，[H2] = 6.4 vol.%，此結果與先前Kobayashi的模擬結果有相同趨勢。是故，在富氨/空氣火焰燃燒後之氫氣含量的大幅增加，使得富氨火焰更能抵抗強烈紊流。前述結果，對未來使用氨為燃料之燃氣渦輪機設計應有所助益。;This dissertation experimentally measures laminar and turbulent minimum ignition energies (MIE) and flame global quench (GQ) conditions of premixed ammonia/air mixtures using a fan-stirred cruciform burner and a well-controlled high-voltage pulse ignition device. The fan-stirred cruciform burner was constructed by a large horizontal cylindrical vessel and a pair of vertical tubes symmetrically welded on its central region. A pair of counter-rotating fans driven by two 10 HP motors with perforated plates were installed in the two ends of the horizontal vessel that can be used to generate a controllable near-isotropic turbulence having a controllable r.m.s. turbulence fluctuation velocity (u′ = 0 ~ 7.2 m/s) with negligible mean velocities in the central region of the cruciform burner. Three ignition locations using a pair of electrodes positioned at the center of the burner, the top of the upper vertical tube, and the bottom of the low vertical tube are selected to generate a central-ignited flame, a downwardly-propagating flame, and an upwardly-propagating flame, respectively. The experiments can be divided into two parts, i.e. (1) MIE measurements and (2) flame GQ studies. For MIE measurements, we compare the ignition similitudes and differences of premixed ammonia/air mixtures between two ignition systems, i.e. conventional spark discharge (CS) and nanosecond repetitively pulsed discharge (NRPD). Moreover, we also investigate the influence of hydrogen addition on MIE. For flame GQ studies, we compare flame GQ limits of central-ignited flame and downwardly-propagating flame using different ignition energies. Here we note that flame GQ limits between downward propagation flames and upward propagation flames are roughly the same. (1) The MIE measurement results show that the MIEs of CS are smaller than those of NRPD in the range of f = 0.75 ~ 1.2 due to the difference of the flame kernel propagation between two ignition systems. However, the flammability ranges of ammonia/air mixtures can be extended by using NRPD where f = 0.66 ~ 1.42 as compared with using CS where f = 0.75 ~ 1.2. This is because the continuously pulsed NRPD can accumulate the active species and radicals. For hydrogen addition ammonia/air flame, even just adding 2 vol.% H2 MIE is significantly reduced by an order of magnitude and the flammability limits can be also extended from pure ammonia/air mixture range of f = 0.75 ~ 1.2 to f = 0.5 ~ 1.5. (2) The flame GQ results show that the large downwardly-propagating flames initiated from the top vertical tube of the cruciform burner are much more difficult to be globally quenched by near-isotropic turbulence than the small central ignition kernels for spherical flames. For the downward propagation flames, flame-turbulence interactions in the central uniform region only occur within a range of equivalence ratio f = 0.75-1.2; flames cannot propagate downwardly at too lean (f = 0.7) and too rich (f = 1.25) conditions because of too small laminar burning velocity (SL) and large laminar flame thickness (dF). The critical values of u′ (u′c) for GQ of downwardly-propagating ammonia/air flames continue to increase with increasing f from u′c = 1.4 m/s at f = 0.75 to u′c = 7 m/s at f = 1.2. Such surprising resistance of rich downward propagation ammonia/air flames to GQ by intense turbulence is explained by measurements of the remaining H2 mole fraction percentage in the product that varies from 1.4% at f = 0.9 to 6.4% at f = 1.2 having the same trend as that predicted by a chemical kinetics model proposed by Kobayashi. These results may be useful to design a better combustion strategy for future ammonia gas turbines.
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