- 學位論文
富氧共伴流增進甲烷預混火焰穩定性與反應強度研究
Enhancing Flame Stability of Lean Combustion and Reaction of Rich Combustion with Oxy-Coflow on Stratified Burner
摘要
本研究使用具鈍體結構之層狀化燃燒器為載具，以外環富氧共伴流包覆中環預混甲烷火焰，利用粒子影像測速法、化學螢光法觀察燃燒流場結構與反應自由基強度分布，並量測火焰溫度及廢氣排放，探討共伴流氧氣濃度及其出口流速對不同當量比之預混甲烷火焰的影響。研究結果揭示富氧共伴流較空氣共伴流更有助於貧油甲烷火焰之穩定，且降低27 % -99 %之一氧化碳排放量，而應用在富油燃燒時可提升其燃燒效能。 本文之火焰基本型態可分為錐焰、半錐焰與飄焰三種，加入共伴流後，在特定參數下能使不穩定之貧油半錐焰轉為形似錐焰的穩定鬱金香焰。研究顯示在貧油燃燒時，隨共伴流氧氣濃度增加，半錐焰外側飄起之火焰高度隨之降低，火焰趨向穩定。在當量比ϕ = 0.78 - 0.87時，觀察不同流速之純氧與空氣共伴流的火焰穩定性，發現純氧共伴流可在更低的流速下使半錐焰轉為穩定的鬱金香焰。燃燒流場結構暫態影像及速度場分布顯示，純氧共伴流在較低流速下即產生雙渦旋結構可使火焰進入型態轉換區間。半錐焰型態之OH*自由基分布顯示，純氧共伴流之半錐焰焰腳下方的OH*強度明顯高於空氣共伴流，因此具有更多的激發態氧分子可促使甲基(CH3)氧化鏈鎖反應進行，因此純氧共伴流較空氣共伴流更有助於火焰穩定，同時也使預混燃氣反應更加完全，降低CO排放。富油燃燒時火焰溫度及化學螢光強度分布顯示，外層火焰面之溫度與OH*強度隨共伴流氧氣濃度增加而提升，內層預混火焰與外層火焰可達1900 K，意即通過內層錐焰後未燃之燃氣與中間產物如CO、H2可與富氧共伴流在外層火焰面處充分反應進而提高整體燃燒效能。本文之研究成果可擴大富氧燃燒技術之應用，藉由改變外環氧氣濃度與共伴流流速調控火焰穩定性及燃燒效能，達到高效能、安全且低汙染之燃燒。
並列摘要
In this study, the oxy-coflow is applied to surround premixed methane flame on a stratified burner. To discuss the effects of oxygen concentration and exit velocity of co-flow on the characteristics of premixed methane flame, the structure of combustion flow field and distributions of free radicals were investigated through the methods of Particle Image Velocimetry(PIV) and chemiluminescence. The flame temperature and the emissions of exhaust gas were also measured. The results reveal that the stability of the fuel-lean premixed flame with oxy-coflow becomes more stable than that with air co-flow, further, the emissions of carbon monoxide decreased 27 % - 99 %. On the other hand, the oxy-coflow enhances the combustion efficiency of fuel-rich combustion. The basic flame pattern was classified into three various types, including lift off, half cone and cone flames. While adding co-flow, the unstable half cone flame transferred to the stable tulip flame. The flame stability of oxygen and air co-flow was observed under the equivalent ratio ϕ = 0.78 – 0.87 with different exit velocity. It was found that the oxygen co-flow could covert the half-cone flame to the tulip flame at a lower exit velocity due to earlier appearance of double vortex structure, which causes the flame pattern to transfer to the transition zone. The distributions of free radicals show that the intensity of OH* radicals with oxygen co-flow outside half-cone flame is significantly higher than that with air co-flow. This result implies there are more excited oxygen molecules promoting the chain reactions of methyl(CH3) oxidation. Therefore, the oxygen co-flow is not only more conductive to flame stability, but makes a more complete reaction of the premixed fuel mixture, which contributes to the reduction of CO emissions. For the fuel-rich combustion, the temperature and OH* intensity of the outer flame increased with an increasing concentration of oxy-coflow, It means the unburned gas and intermediate products such as CO, H2 reacted with the oxy-coflow more completely at outer flame, which improved the overall combustion efficiency. The results of this study expand the application of oxy-fuel combustion. By adjusting the oxygen concentration and exit velocity of co-flow, the flame stability enhances and a low pollution with a high efficiency combustion can be achieved.
參考文獻
延伸閱讀
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