空氣共伴流對甲烷反擴散火焰結構之影響

Abstract

本研究使用具有鈍體結構之三環燃燒器為載具，探討甲烷反擴散火焰添加空氣共伴流之燃燒特性，期望提升擴散火焰之燃燒效能與降低其汙染物排放。實驗分析方法採用攝影技術觀察添加空氣共伴流對甲烷反擴散火焰結構與穩定性之影響，並透過粒子影像測速法，分析不同出口流速時火焰與流場的交互作用、燃料反應機制及溫度場分佈，以發展低碳煙排放、高效能且安全的燃燒。 本文之火焰基本型態分為雙層反擴散火焰(double-layer inverse diffusion flame)、過渡型反擴散火焰(transitional inverse diffusion flame)和單層反擴散火焰(single-layer inverse diffusion flame)三種，隨著空氣共伴流流速增加，外層火焰面會發生部分跳脫、完全跳脫之現象。粒子影像測速法之平均速度場與渦度場結果顯示，添加外環空氣共伴流之甲烷反擴散火焰，會在內、中兩環之間之鈍體結構後方形成一反向渦漩對，其渦漩結構的產生系由於外環空氣共伴流與內環空氣的流速影響燃燒器後方渦漩結構的主導地位，而藉此能增進甲烷與空氣之混合。OH*自由基化學螢光法實驗亦證實，當此渦漩結構出現之條件下，火焰之化學螢光強度出現最大值，顯示此時反應最為強烈。經過溫度量測之結果，亦指出此時火焰之熱釋放表現優於未添加空氣共流或提供過高流速之空氣共伴流之情況。透過本研究之火焰型態觀察、PIV實驗、火焰化學螢光與溫度場分布，進一步了解空氣共伴流對甲烷反擴散火焰之流場與燃氣混合機制，與其對反應強度影響與溫度熱釋放之分析。本文之成果歸納空氣共伴流甲烷反擴散火焰之燃燒性能之增益，期望能對反擴散火焰燃燒技術做出改善，並兼顧汙染控制，對環境環保與能源永續之議題做出一份貢獻。

In this study, air co-flow is applied to surround methane inverse diffusion flame on a stratified burner. To discuss the effects of the exit velocity of co-flow on the characteristics of a methane inverse diffusion flame, the structure of combustion flow field and distributions of free radicals were investigated by the methods of Particle Image Velocimetry (PIV) and chemiluminescence. The flame temperature was also measured. The results reveal that the stability of the inverse diffusion flame becomes less stable while accelerating the air co-flow speed, but air co-flow enhances the mixture of fuel and oxidizer, hence improves the efficiency of combustion. The basic flame pattern was classified into three various types, including double-layer inverse diffusion flame, transitional inverse diffusion flame, and single-layer inverse diffusion flame. While applying co-flow, the outer layer of inverse diffusion flame started to lift off. Through PIV, it was found that air co-flow could avoid fuel flow moving downstream due to the appearance of reverse vortex structure, which may increase the mixture between methane and air. The distributions of free radicals show that the intensity of OH* radicals with a suitable air co-flow velocity is significantly higher than those with excessive air co-flow or without air co-flow. This result implies that there is more fuel involved in the reaction. Though air co-flow would decrease the flame stability, an appropriate manipulated velocity could make the reaction of fuel mixture more complete. Also, the temperature corresponding to the OH* intensity of the outer flame rose with an increasing air co-flow velocity, meaning that the unburned gas reacted with co-flow more completely at the outer flame and improved the overall combustion efficiency. The results of this study are to expand the application of methane inverse diffusion flame, to enhance the combustion efficiency and to achieve low-level soot pollution by adjusting the exit velocity of air co-flow.