| 摘要: | 本論文探討輻射熱損失對預混層焰拉伸率之影響，利用本實驗室已研發的紊流尾流直立式燃燒器(高度為1.5 m、截面積為15 × 15 cm2)及數位質點影像測速儀(digital particle image velocimetry, DPIV)，定量量測反應流場之空氣動力應變率、曲率、拉伸率與膨脹率。實驗一開始先將燃燒器抽真空，之後將已預混之燃氣灌入，使爐內壓力增至常壓，於燃燒器上方引燃產生由上往下的自由傳播預混層焰，同時燃燒器頂端原密封的頂板會打開，使得引燃時所產生的膨脹氣體可排出，避免壓力效應之影響。當往下自由傳播預混層焰到達燃燒器中間附近之水平置放抽板上方2公分時，抽板開始往左迅速移動產生一紊流尾流。我們使用高速雷射斷層攝影術和DPIV技術，來擷取紊流尾流與自由傳播預混層焰交相干涉之二維影像和獲得熱流場速度場，經統計分析後，可得到反應流場之空氣動力應變率、曲率、拉伸率與膨脹率等等參數之資訊。 為了研究輻射熱損失對於預混甲烷-空氣層焰拉伸率的影響，我們以(1)不加任何稀釋氣體、(2)加氮氣與(3)加二氧化碳於預混燃氣內來改變輻射熱損失程度的大小，上述三種情況都可找到當流場之負應變率增加，其正膨脹率會增加的結果，而當正應變率提高，則負膨脹率會增加，上述結果不受輻射熱損失效應的影響。由機率密度函數(probability density function, PDF)分布圖中觀察到，在富油甲烷-空氣的實驗中，其空氣應變率值在+200 s-1及-100 s-1區間中變化，而在富油甲烷-空氣-氮氣的實驗和富油甲烷-空氣-二氧化碳的實驗中，其空氣應變率值在 50 s-1區間中變化，所以空氣應變率會受到輻射熱損失之增加而降低。拉伸率為曲率項和空氣應變率項所相加而成，而輻射熱損失效應會明顯降低空氣應變率值，但輻射熱損失效應對曲率值的影響不大，再則因拉伸率受曲率項之影響較應變率項為大，故整體而言，輻射熱損失對富油甲烷火焰之拉伸率的影響甚小，幾乎可忽略不計。比較富油甲烷-空氣-氮氣和富油甲烷-空氣-二氧化碳之膨脹率，發現增加輻射熱損失的強度會降低反應流場的膨脹率（即火焰之燃燒強度）。由小波分析(wavelet analysis)方法，發現富油甲烷火焰受壓縮應變渦對和擴張應變渦對的影響，流場積分尺度會隨著時間增加而減少，顯示非定常(unsteady)效應對預混紊焰有影響，將於本論文中探討分析。 This thesis investigates experimentally effect of radiative heat losses on the stretch rate of a premixed flame interacting with a decaying turbulent wake. The decaying turbulent wake was generated by quickly withdrawing a horizontal plate in a long vertical burner with 1.5m height and with a cross-sectional area of 15×15 cm2. Before a run, the burner is evacuated and then filled up methane-air mixtures with or without diluents to 1 atm, including N2 and CO2, at a given equivalence ratio ( ). A run begins by ignition at the top of the burner where the top plate with 15×15 cm2 cross-sectional area is simultaneously opened in order to release the gas from thermal expansion and generate a downward-propagating premixed flame at 1 atm. When the flame propagates 1m downwardly, very near to the top of the horizontal plate, the horizontal plate is quickly withdrawn to create a turbulent wake for flame-wake interactions. We use high-speed digital particle imaging velocimetry to measure flame-wake interactions, so that the corresponding dilatation rate, aerodynamic strain rate, curvature, and thus the stretching rate can be obtained. Three cases of rich methane/air premixed flames at 1.4 or 1.45 are studied, including the pure methane/air case ( 1.45; negligible radiative heat loss), the N2-dilulent case ( 1.4; small radiative heat loss), and the CO2-diluent case ( 1.4; large radiative heat loss). It is found that the negative strain rate increases the positive dilatation rate, while the positive strain rate increases the negative dilatation rate. This correlation is the same among these three cases, regardless different degrees of radiative heat loss. The effect of radiative heat loss reduces the magnitude of the strain rate, but it has little influence on the curvature. It is found that the curvature term plays a more dominate role on the stretch rate than the strain rate term. When the dilatation rate of the CH4/air/N2 case is compared with that of the CH4/air/CO2 case, it is found that increasing the degree of radiative heat loss corresponds to a decrease of the dilatation rate, indicating the dilatation rate may be used as an indication of flame burning strength, as suggested by Driscoll and his co-workers (1996).Using the wavelet analysis, the integral length scale of flame-wake interactions among these three cases are obtained. Finally, the unsteady effect of flame-wake interactions will be also discussed. |
