| 摘要: | 本論文針對預混紊流燃燒領域提出一實驗研究，含三個重點：預混紊焰之碎形分析、輻射熱損失效應對預混紊焰之影響及預混紊焰整體熄滅(global quenching)機制之探討。實驗用燃氣以研究級甲烷和丙烷為主，其與空氣混合後之當量比(?)，從貧油到富油甲烷燃氣為? = 0.6 ~ 1.45 而丙烷燃氣為? = 0.6 ~ 1.95，含蓋甚廣之燃氣當量比範圍。燃燒實驗於一十字型燃燒器進行，十字型燃燒器乃由兩個相互垂直圓柱鋼管所組成，在大的水平圓柱管兩底端各裝置了一組反向旋轉風扇及空孔板，可於兩圓柱中心測試區內產生一強烈近似等向性紊流場。我們使用高速雷射斷層攝影術，由高速CCD攝影機擷取記錄預混紊焰前緣隨時間變化之動態影像，並用co-dimension和stepping-caliper碎形分析方法，獲取預混紊焰隨紊流強度變化之碎形維度和內外截止長度等之物理量。有關輻射熱損失效應之研究，藉著加入不同比例之稀釋氣體(含氮氣和二氧化碳)，來改變燃氣之輻射熱損失程度，並以自製離子探針及光電倍增管等設備來量測輻射熱損失對紊流燃燒速度的影響。有關預混紊焰整體熄滅之研究，我們於測試區前視窗及下視窗各裝置一組高速攝影機作同步拍攝，來決定預混紊焰向下傳播之動態過程。若預混紊焰被具足夠大強度之紊流所熄滅，無法通過強烈近似等向紊流中心區，即為整體熄滅，反之則為非整體熄滅。我們並以氣體分析儀量測實驗後之殘餘氣體濃度，比較未發生和發生預混紊焰整體熄滅之燃料殘留濃度，以確定整體熄滅發生之紊流拉伸條件，即以Bradley所定義之紊流Karlovitz 數，K = 0.157(u?/SL)2ReT-0.5=0.157Ka來判斷，其中u?為能量平均紊流強度，SL為層流燃燒速度，ReT = u?LI/v為紊流雷諾數， LI為紊流積分長度和v為反應物之運動粘滯係數，而Ka則為一般所定義之Karlovitz數。 本研究使所量得之碎形維度遠低於先前大部分相關之實驗結果，平均僅約2.18，且其值於1 < u?/SL < 10 範圍內變化不大；先前大部分實驗，應用不同反應紊流場如Bunsen flames, V-shape flame 及 IC engine flame 等，得到當u?/SL > 3 時，碎形維度會趨近於2.33。我們找到內外截止長度均接近一定值，大體上不隨u?/SL和?值而變，外截止長度略小於未反應之紊流積分長度且比內截止長度大一個級數。我們以K和?值來界定預混紊焰整體熄滅發生和未發生之邊界，將此邊界的臨界值表示為Kc。在固定反應燃氣SL ? 10 cm/s的條件下，比較加入氮氣(具低輻射熱損失)和加入二氧化碳(具高輻射熱損失)的甲烷-空氣預混焰，發現在貧油條件? = 0.7時，加入氮氣的預混紊焰若要整體熄滅Kc值需達約為4.8，而加入二氧化碳之預混紊焰Kc值僅需約3.7。而在富油條件? = 1.4時，加入氮氣和加入二氧化碳的甲烷-空氣火焰，Kc值均約為1.3。由此可知輻射熱損失對預混紊焰之影響，在貧油燃燒時特別顯著，但於富油燃燒時幾無效應。本研究有助於建立一以物理參數為基準之評估機制，可用於瞭解預混紊焰傳播和整體熄滅，對內燃機、新一代往復式引擎和大氣氣爆有所助益。 This thesis is related to the study of premixed turbulent combustion using a large cruciform burner that has previously established in our combustion laboratory at NCU. The cruciform burner consisted of a long vertical vessel and a horizontal vessel. The former can be used to provide a downwardly propagating premixed flame with pressure release valves via an ignition modulus on the top of the vertical vessel. The latter was equipped with a pair of counter-rotating fans and perforated plates at each end to generate intense near-isotropic turbulence between the two perforated plates. Thus, flame-turbulence interactions without the influence of ignition can be achieved. Turbulence statistics and visualizations of turbulent flame fronts were obtained using laser Doppler velocimetry and a high-speed laser tomography technique. Three focuses are respectively fractal characteristics of these wrinkled turbulent flames, effect of radiative heat losses on turbulent burning velocities, and global quenching conditions of premixed turbulent flames. Concerning effect of radiative heat loss on turbulent premixed flames, we employ several diluted CH4/air flames with different degrees of radiative heat loss, from small (N2-diluted) to large (CO2-diluted), in the cruciform burner, over a wide range of turbulent intensities (u?/SL) where SL is the laminar burning velocity. Using both the co-dimension and the stepping-caliper methods to analyze these laser tomography wrinkled turbulent flame images, we found that the mean fractal dimension d3 increases slowly from about 2.1 when u?/SL ? 1 to only 2.18 when 5 < u?/SL < 10, nearly independent of u?/SL. This is finding is in support of a recent Bunsen-flame result found by Gülder and his co-workers (2000), but conflicting with several previous results in which the fractal dimension was found to approach a value of 2.33 when u?/SL > 3. The inner and outer cutoffs, ?i and ?o, respectively, are found to be nearly constant for all flames studied. ?o is found to be slightly smaller than the integral length scale of unreacted turbulence and is an order of magnitude greater than ?i. It is found that the present fractal data cannot predict ST/SL correctly when the available fractal closure model such as the model of Gouldin (1987) was used, indicating a limit of the fractal model. By comparing N2- and CO2-diluted CH4/air flames of the same SL, effect of radiative heat loss plays an important role on lean CH4/air flames, while it has little influence on rich CH4/air flames. For lean mixtures at the same SL and u?/SL, values of ST/SL and/or Kc for CH4/CO2/air flames (large heat loss) are found to be considerable smaller than that of CH4/N2/air flames (small heat loss), revealing that the radiative heat loss may inhibit turbulent premixed flame propagation. These results may be relevant to the development of internal combustion engines, new-generation reciprocating engines, and atmospheric explosions. |
