併排之雙Tsuji燃燒器上逆流火焰間的交互作用

Abstract

本論文包括了兩個部分，其中第一部份是將陳俊勳教授及翁芳柏學長於1990年所發展的燃燒模式加以改良成具有四步化學反應機構的模式，然後在以此來研究Tsuji燃燒器上火焰穩定及吹離的現象。至於第二部分則是探討兩併排之雙Tsuji燃燒器間的火焰交互作用現象，然而在本部分中所採用的化學反應模式則是單步之反應機構，但在格點方面卻導入了多區塊格點系統以進行運算。另一方面，在單燃燒器的模擬中，吾人所探討的參數是空氣進氣速度（Uin）和燃燒器之噴油面積（S），而在進行正式模擬前吾人先將目前的計算結果和Tsuji於1982年所量測得到之火焰吹滅曲線相比較，結果發現吾人目前的模擬比之前陳俊勳教授和翁芳柏學長於1990年時所模擬出的結果更接近Tsuji的實驗量測值，因此可以確定目前之模擬是較準確的，另外，本研究的數值預測和Dreier等人1986年實驗量測數據之間的吻合度也非常高。再者，當進氣速度增加時，火焰會在完全熄滅前依序由包封擴散火焰轉變成尾流火焰、吹離火焰及尾流火焰，而前者之尾流火焰係由包封火焰轉變而來，但後者卻是由吹離火焰轉變而來，至於吹離火焰的存在則可自張哲誠2002年所做的相應實驗觀測中獲得確認。當進氣速度達到每秒1.05米時會造成1.7D的最大火焰吹離高度，且此高度可維持到進氣速度等於每秒1.09米時，在此之後隨著進氣速度的增加卻會導致火焰吹離高度之逐漸降低，而此現象可視為一種回火的過程，另外，在這些吹離火焰生成後，原本存在於圓柱形燃燒器後方的迴流區便會消失，然後當進氣速度上升到每秒1.16米時尾流火焰又會再度出現，而於此之前，即進氣速度介於每秒1.13至1.15米之間時則會生成一由吹離火焰轉變成尾流火焰的過渡時期，最後當進氣速度大於每秒2.12米時則火焰就會完全熄滅掉。 在第二部分中，首先為了確認此部分所用多區塊格點系統的正確性，因此吾人先進行流體流經雙圓柱上之冷流模擬，然後在正式進行參數研究前先將目前計算出的阻力係數曲線和Hori於1959年實驗所量得的曲線相比較，而其結果說明了目前的模擬是值得信賴的。另一方面，對於雙圓柱形燃燒器間之火焰干涉現象而言，吾人首先也是將目前的模擬結果和王景盈（1998）及張哲誠（2002）所量測到的火焰轉變曲線相比較，結果發現目前的模擬可以正確地計算出火焰轉變速度隨著圓柱間距之變化趨勢。在數值與實驗結果交互比對完之後吾人便開始進行參數研究，而所探討的參數分別是圓柱間距（L）和進氣速度（Uin），而在固定進氣速度下改變圓柱間距的例子中，當圓柱間距增大時，由包封火焰轉變成尾流火焰的火焰轉變速度卻會下降，但燃燒效率卻會隨著圓柱間距之增加而上升。此外，當圓柱間距小於等於1.5D時，則雙包封火焰就會合併成一個較大的包封火焰，至於在1.5D或2D圓柱間距的情況下，每個圓柱後方只有一個漩渦存在，然而當圓柱間距為1.2D時，圓柱後方的迴流區就會完全消失。另一方面，當圓柱間距大於等於3.5D時，兩火焰間就不會再有任何的交互作用，而造成此雙火焰間交互作用的控制機構則是由於雙火焰間之氧氣缺乏現象。接著吾人將探討固定圓柱間距為3D時改變進氣速度對雙火焰所造成的影響，此時當進氣速度增加到每秒0.79米時，則雙包封火焰就會轉變成雙尾流火焰，而當進氣速度進一步增加到每秒1.96米時，則雙尾流火焰就會熄滅，且由於火焰拉伸的作用，故進氣速度越大則火焰溫度越低。此外，在接近熄滅極限的情況下，每個圓柱後方均有三個漩渦之產生，而這與其他例子的情況是迥異的。一般來說，在固定圓柱間距的情況下，由於火焰間交互作用的影響，故雙火焰會有彼此互相吸引的傾向，然而當進氣速度增加到接近熄滅極限時卻會造成雙火焰的互相排斥。

This dissertation consists of two parts. The first one is to study the flame stabilization and lift-off over a Tsuji burner using a four-step chemical kinetics in the combustion model developed by Chen and Weng (1990A). The second part is to investigate the flame interference/interaction phenomena between two cylindrical burners in a side-by-side arrangement. A multi-block grid system is introduced to implement the model, however, one-step overall chemical kinetics is used in this part. For the single burner case, the parameters of interest are the inflow air velocity (Uin) and fuel-ejection area (S) of the cylindrical burner. Comparing the blow-off curve of Tsuji (1982) with that of Chen and Weng (1990) reveals that this simulation yields a much better prediction than that in the latter reference. Also, the present predictions have an excellent agreement with the measured data of Dreier et al. (1986). As Uin increases, the envelope diffusion flame, wake flame, lift-off flame, and wake flame appear in order before complete extinction. The formal wake flame is transformed from envelope one and the latter is from the lift-off flame. The existence of a lift-off flame is verified by a corresponding experimental observation (Chang, 2002). The maximal lift-off height is 1.7D when Uin is 1.05 m/sec, and this height is retained up to Uin = 1.09 m/sec. Then, the height declines gradually as the inflow velocity increases, whose process can be regarded as flashback. No recirculation flow exists behind the cylindrical burner for these lift-off flames. A transition from lift-off to wake flame occurs between 1.13 to 1.15 m/sec. The wake flame reappears at Uin = 1.16 m/sec. Finally, the flame is extinguished completely when Uin > 2.12 m/sec. An explanation for flame’s lifting off and dropping back is given. In the second part, a preliminary study for a cold flow over the twin cylinders is given first. Comparing the drag coefficient curve of Hori (1959) with the predicted one by the present study reveals that this simulation yields a reliable prediction. After that, a parametric study is given as well. For flame interference between dual cylindrical burners, comparison of the flame transition curve between Wang (1998) and Chen’s (2003) experiments and the present simulation indicates that this simulation can correctly predict the trend for the variation of flame transition velocity with intercylinder spacing. For the parametric studies, the interested ones are the intercylinder spacing (L) and inflow velocity (Uin), respectively. For flame interference, in general, the wider the intercylinder spacing, the lower the flame transition velocity, which transforms the envelope flame into the wake flame. However, the combustion efficiency increases with L. The twin envelope diffusion flames merge into a larger envelope diffusion flame completely as L is equal to or less than 1.5D. There is only one vortex behind each burner as L = 1.5D or 2D. However, no vortex is found behind in each burner as L = 1.2D. As L is equal to or greater than 3.5D, there is no interference at all between the two flames. The controlling mechanism of mutual interaction for twin counterflow diffusion flames is the oxygen deficiency between the dual flames. In the case of varying Uin under fixed L = 3D, as Uin increases to 0.79 m/sec, the dual envelope diffusion flames transform into dual wake flames. When Uin further increases to 1.96 m/sec, the dual wake flames extinguish. Raising the inflow velocity can enhance the mutual interaction between dual envelope flames. The larger the inflow velocity, the lower the flame temperature due to the flame stretch effect. Besides, there exist three vortices behind each cylinder as flames are near extinction, and this is quite different from another case. For fixed intercylinder spacing, the dual flames have a tendency to attract each other normally due to the mutual interaction between flames. However, as the inflow velocity increases to near extinction limit, the dual flames repel each other.