預混焰與紊流尾流交互作用時非定常應變率、曲率和膨脹率之定量量測; Quantitative Measurements of Unsteady Strain Rate, Curvature, and Dilatation Rate in a Premixed Flame Interacting with a Turbulent Wake

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题名:	預混焰與紊流尾流交互作用時非定常應變率、曲率和膨脹率之定量量測;Quantitative Measurements of Unsteady Strain Rate, Curvature, and Dilatation Rate in a Premixed Flame Interacting with a Turbulent Wake
作者:	李志杰;Chih-Chieh Lee
贡献者:	機械工程研究所
关键词:	曲率;應變率;拉伸率;紊流尾流;預混焰;膨脹率;premixed flame;turbulent wake;stretching rate;strain rate;curvature;dilatation rate
日期:	2002-07-08
上传时间:	2009-09-21 11:39:59 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	摘要本論文探討預混層焰與紊流尾流交互作用之動態變化，並定量量測其間之火焰前緣應變率(strain rate)、曲率(curvature)和膨脹率(dilatation rate)隨時間變化之情形，應用高速質點影像測速技術(high-speed particle image velocimetry)和雷射斷層攝影術(laser tomography)。此為非定常拉伸率(stretch rate)之量測，挑戰性高，故已有之文獻資料不多，且本研究所採用之紊流尾流為一多渦結構，比先前單渦研究較為複雜，為本研究之主要特色。紊流尾流燃燒器的高度為1.5 m，截面積為15 × 15 cm2。實驗前，先將燃燒爐抽真空，再將選定當量比(equivalence ratio)之預混甲烷/空氣燃氣注入燃燒爐中，使爐內壓力達1 atm。於燃燒爐上方引燃產生由上往下傳播之預混層焰，在引燃的同時，燃燒爐頂端原密封的整塊平板立即打開，可釋出燃燒後產生的膨脹氣體，使預混層焰可在一大氣壓下傳播。紊流尾流產生器為距燃燒爐頂端1.0 m處水平放置之平板(厚0.50 cm)，當預混焰傳播至平板上方約2.0 cm時，自動控制之氣缸閥迅速抽開平板產生紊流尾流，使其與預混層焰進行交互作用。此紊流尾流由一連串位置交錯的壓縮/擴張應變渦對所組成，壓縮應變渦對頂點之垂直法線向量與預混層焰傳播方向接近同向，而擴張應變渦對則相反。實驗結果發現，預混層焰與壓縮應變渦對的交互作用遠比與擴張應變渦對激烈，這是因為壓縮應變渦對能將預混焰捲入其順時鐘旋轉方向的渦心之中，作激烈燃燒。當尾流紊流強度較大時，拉伸率乃由應變率和曲率所共同主導，而當尾流強度衰減後，曲率對拉伸率的影響會遠比應變率來得大。此外，膨脹率受火焰面熱釋放效應的影響，其峰值會出現在相當靠近火焰面的位置，因此局部膨脹率的峰值可視為局部火焰強度的指標。吾人發現預混富油甲烷/空氣火焰的膨脹率與應變率有密切的關係(曲率較不重要)，負應變率可提高其膨脹率，增強火焰強度，而正應變率會降低其膨脹率，減弱火焰強度。而預混貧油甲烷/空氣火焰的膨脹率則與曲率息息相關(與應變率相關性較弱)，正/負曲率分別會提高/降低火焰強度。由拉伸率的機率密度函數變化可知流場及火焰特性會隨時間變化而改變，因此非定常效應對拉伸率之影響是不能忽略不計的。 This study investigates experimentally a downward-propagating premixed flame interacting with a decaying turbulent wake for measurements of stretch rates in a 1.5 m long vertical burner with a cross-sectional area of 15×15 cm2. Before a run, the burner was evacuated and then filled with methane-air mixtures at a given equivalence ratio at 1 atm. A run began by ignition at the top of the burner where the top plate was simultaneously opened in order to generate a downward-propagating premixed flame at 1 atm. When the downward-propagating flame was approaching very near a horizontal plate of 0.50 cm thickness that was initially positioned at 1.0 m below the top of the burner, the electrically-controlled horizontal plate was quickly withdrawn to create a turbulent wake for flame-wake interactions. The associated wrinkled flame front and its velocity field were obtained using high-speed laser tomography and particle image velocimetry, so that the corresponding strain rate, curvature, and dilatation rate along the flame front at different times may be determined. The present turbulent wake consists of a parallel row of staggered vortex pairs, similar to the Kármán vortex sheet behind a circular cylinder. If the tip of the vortex pair is pointing toward/away from the downward-propagating flame, the corresponding vortex pair will experience an extensive/compressive strain. Flame-wake interactions are much more intense in vortices whose strain are compressive than those with extensive strain. This is because the compressive vortex pair can engulf the flame deeply into its clockwise-rotating vortex core. Combustion starts from the core with a burning rate much faster than the other half (counterclockwise-rotating) vortex. When the fully-developed turbulent wake starts to decay, the curvature becomes more and more important on the influence of the stretch rate than the strain rate does in the present flame-wake interactions. Furthermore, dilatation rates reveal peak values very near the flame front, so that the local peak dilatation rate may be used as an indicator of the local flame strength, similar to that found by Driscoll and his co-workers. We also found that the stretching rate in rich methane/air flame correlates strongly with the strain rate but much less with the curvature. This correlation alters when lean CH4/air flames are considered, in which the curvature is the dominate one in the stretching rate. For rich methane/air flame, the flame strength increases when the negative strain is applied, but the flame strength increases with the positive curvature for lean methane/air flame. Finally, variations of the probability density function of the stretch rates with time indicate that the unsteady effect cannot be neglected.
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