| 摘要: | 本論文針對貧油預混甲烷/空氣之弱漩渦噴流火焰，執行一系列高紊流雷諾數(遠大於先前相關研究)燃燒實驗，利用二維雷射斷層攝影術(laser tomography，LT)及質點影像測速技術(particle image velocimetry，PIV)，定量量測紊焰之碎形特性與紊流燃燒速度。所採用之弱漩渦噴流燃燒器(weak swirl jet burner，WSJB)，類似先前研究Bédat & Cheng (1995)所設計，本研究共有兩組不同噴嘴直徑之WSJB，一為25 mm另一為50 mm，它可提供穩定駐留於燃燒器噴嘴出口之火焰。弱漩渦噴流火焰動態時序影像，係以高速高解析之CMOS攝影機(5000張/秒，512 × 512 pixels)所攝取，將影像二值化後，利用stepping-caliper碎形分析方法獲取火焰面之碎形參數，包括碎形維度(D3)和內外截止長度(?i；?o)等物理量。本實驗無因次紊流強度的範圍u?/SL ≈ 2.6 ~ 20.4，其中u?為均方根紊流擾動速度而SL為層流燃燒速度，而相對應之紊流雷諾數(ReT = u?LI/ν) 之範圍則從400至7,000，其中LI為流場之積分長度尺度而ν為空氣之運動黏滯係數。當u?/SL > 3，D3值僅為2.22且與u?/SL之大小無關，此結果不同於先前大部分學者之研究結果(如I.C. engines、Bunsen flames及V-shape flames等研究結果，找到D3 ≈ 2.33)。但本結果與Gülder團隊(2000)在本生燈型態火焰(ReT < 500)所獲之結果相符。而?i和?o值僅會隨著u?/SL的增加而略微下降，其減少變化範圍：當u?/SL從2.6增加到20.4，?i和?o值會從1.5 mm和12 mm略微減少到1.0 mm和10 mm。將前述實驗所得之碎形參數代入Gouldin (1987)所提出之紊流燃燒速度(ST)碎形理論模型中計算，並與本實驗PIV量測值做一比較，我們發現模型計算之ST值無法預測實際量測值，顯示出目前碎形數學模型有需要再做進一步的修正。 This thesis measures fractal properties and turbulent burning velocities (ST/SL) of lean premixed methane/air weak swirl jet flames using laser tomography and particle image velocimetry (PIV). Two weak swirl jet burners (WSJB) of 25 mm and 50 mm diameters are applied, which are similar to the previous design by Bédat & Cheng (1995) for providing stabilized flames above. The instantaneous images of weak swirl jet flames were recorded by a high-speed, high-resolution CMOS camera (5,000 frames/s, 512 ? 512 pixels). After binarization, the flame front images were analyzed using the stepping-caliper method to obtain the fractal dimensions (D3) and inner and outer cutoff length scales (?I and ?o). In this study, the dimensionless turbulent intensities (u′/SL) of turbulence can be controlled from u′/SL = 2.6 to u′/SL = 20.4 with corresponding turbulent Reynolds number (ReT = u?LI/ν) ranging from 400 to 7000 which are much larger than previous studies, where u′, SL, LI and ? are the r.m.s. turbulent intensity, laminar burning velocity, integral length scale of turbulence, and kinematic viscosity of reactants, respectively. It is found that values of D3 are only 2.22 independent of u′/SL. This result differs drastically with most of previous studies, such as I.C. engines, Bunsen flames, and V-shape flames, that reported values of D3 ≈ 2.33 when u′/SL > 3. However, D3 ≈ 2.22 is in support of a previous study using Bunsen-type flames at smaller values of ReT (<500) by Gülder et al. (2000). As values of u′/SL increase from 2.6 to 20.4, values of ?i and ?o decrease from 1.5 mm and 12 mm to 1.0 mm and 10 mm. Finally, these fractal parameters obtained at high ReT cannot predict ST/SL correctly using available fractal area closure model, indicating a need for further improvement of existing fractal models. |
