預混火焰與尾流交相干涉之實驗研究; Premixed flame interacting with a turbulent wake.

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题名:	預混火焰與尾流交相干涉之實驗研究;Premixed flame interacting with a turbulent wake.
作者:	黎文孝;Wen-Shiau Ree
贡献者:	機械工程研究所
关键词:	擾動尾流;雷射斷層攝影術;流體動力;turbulent wake;laser tomographic technique;hydrodynamic
日期:	2000-07-05
上传时间:	2009-09-21 11:34:51 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	本實驗使用一具新型直立式擾動尾流燃燒器研究預混紊流燃燒，它主要的特徵含(1) 燃燒器頂部有個大口徑釋壓閥能使預混火焰引燃的瞬間同時釋放膨脹的氣體，故壓力效應可忽略不計，並於定壓(1大氣壓)下由上往下傳遞，浮力效應也可忽略；(2) 燃燒器之電子控制器可精確地控制測試區抽板延遲作動的時間，使預混火焰與抽板所產生的擾動尾流充分地交相干涉，可作為研究預混紊流燃燒之基礎實驗。實驗研究的重點有觀測擾動尾流冷流場及預混火焰與擾動尾流交相干涉和定量量測與分析預混紊焰燃燒速度。我們固定抽板最大速度並於尾端個別增加各種厚度尺寸的擾流片，以產生遞增的尾流強度(u')，再應用雷射斷層攝影術，擷取甲烷-空氣及丙烷-空氣(前者當量比f=0.65及1.4；後者f=0.6)之預混火焰於測試區與擾動尾流間交相干涉之二維預混紊焰瞬時平面影像，結果發現火焰傳遞可視為為近似統計穩態，故可用於量測紊流燃燒速度。另外，由影像觀測出預混火焰面受低紊流流體動力不穩定性效應產生較不規則皺摺(cell)外觀類似自生擾動火焰(self-turbulizing flame)屬於皺摺火焰。除此之外，紊流強度增加，熱釋放明顯受到較大拉伸效應影響，因此焰前紊流是增大熱釋放主因之一。對於燃燒速度的量測採用非置入流場方式之攝影機法，優點是不會干擾流場結構，由攝影機量測結果得知，火焰波面受到紊流場不穩定影響產生拉伸使火焰接觸面積增加，但在碎薄焰區，火焰表面拉伸不僅會受到流體動力之影響，而且Lewis (Le)數效應的影響也必須要考慮，在Huygen's傳遞理論中，火焰面積增加造成預混紊流燃燒速度提升(ST~u')，在低紊流場，火焰波面受曲率影響控制，火焰拉伸有限，對較大紊流場而言，火焰波面的切線應變效應變成為主要掌控拉伸之因數，因此迅速遞增紊流燃燒速度直到過渡至碎薄焰區。至於，Le數效應由本實驗結果得知；(1) Le<1之預混火焰速度會隨著紊流場變化而增加移動速度(displacement speed)。(2) Le<1之局部焰前的切線應變率比Le>1強，使凸向(凹向)反應物(生成物)預混火焰面受熱擴散效應(thermodiffusive effect)增加反應率(reaction rate)。 This experiment investigates premixed turbulent combustion using a new vertical turbulent wake combustor. The combustor is a long square vessel with a pressure release plate on its top, which eliminates the pressure effect and provides a premixed-flame propagating downwardly at normal pressure. Moreover, we use an electronic controller panel that exactly controls the withdrawing motion of a sliding plate to generate the turbulent wake for the interaction with the downward propagating premixed flame. We focus on three points: the observation of turbulent wake flows, visualizations of flame-turbulent wake interactions, and measurements of turbulent burning velocities. The sliding plate is fixed at its maximum mean speed of 160 cm/s. Emphasis is on obtaining 2-D images of premixed flame-turbulent wake interactions via laser tomographic technique. We perform combustion experiments for methane-dry air mixtures at the equivalence ratio phi=0.65 and phi=1.4, and propane-dry air mixtures at phi=0.6. From these instantaneous sequent images of propagating flame fronts, we find that flames propagation is statistically stationary. This is essential for measurements of turbulent burning velocities. We use a high-speed video camera to measure burning velocities. From Huygen's propagation principal, the flame surface is influenced not only by stretching due to aerodynamic strain and the curvature of the front, but also the Lewis number (Le). Increasing the flame surface area increases turbulent burning velocity (ST~u'), where u' is the r.m.s. turbulent intensity and ST is the turbulent burning velocity. In weak turbulent flows, the flame front is controlled by curvature effects. For strong turbulence, the stretching effect is dominated by the strain of flame fronts. Concerning the Lewis number effects; tangential strain rate effects on local flame curvature are intensified for Le<1 and hampered for Le>1. The reaction rate may be increased by thermodiffusive effects due to flame front convex (concave) reactants (products).
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