環形貧油火焰特性與注入空氣共伴流之影響

Abstract

本文探討不同當量比與出口流速下的環形貧油甲烷火焰特性，並提出注入空氣共伴流來改造火焰型態的設計理念，以大幅改善貧油燃燒之駐焰性能。研究分析上透過燃燒流場的火焰影像、流場可視化、高速PIV量測分析(1000, 2000 fps)與溫度分布來解析火焰型態與駐焰的機制。 結果顯示改變貧油燃氣的當量比(φmid=0.5–0.95)與出口流速(Umid=0.5–4.0 m/s)可歸納出環形貧油火焰的三種特性，第一特性是從燃燒狀況良好的錐焰(cone flame)到熄滅(extinguishment)之前會有半錐焰(half-cone flame)、飄焰(lift-off flame)的過渡區間，在操作燃燒器時，透過火焰外觀即能判斷燃燒反應的強弱。第二特性是貧油可燃極限會隨著燃氣出口速度的不同而改變，研究結果中最低的貧油可燃極限達0.52，發生在燃氣出口速度為1 m/s時，此時火焰面上的質量傳遞(往下游)與熱量傳遞(往上游)達到最佳平衡。第三特性是其燃燒流場包含三種緊密互動的週期性變化：火焰的內外擺盪、迴流區的大小變動、週期性的縮頸(necking)現象。注入空氣共伴流的概念是在環形貧油火焰瀕臨熄滅前，即火焰型態為半錐焰或飄焰的情形下，將空氣共伴流注入火焰的內側或外側，以流場來改變火焰型態。研究驗證顯示在火焰外側注入空氣共伴流且Uout /Umid 大於0.3–0.7 (依燃氣出口速度Umid而定)，因燃氣迴流至兩環噴流之間的低壓區形成共伴迴流結構，使燃氣出口附近區域的流場與溫度分布改變，進而促使火焰型態由半錐焰轉變為鬱金香焰(tulip flame)，顯著地強化燃燒反應與駐焰性能。如此僅利用空氣共伴流而不需添加額外的高成本物質或燃料即能改造火焰結構，使貧油燃燒反應更強烈且穩定。

The characteristics of annular lean methane flames with different equivalence ratios(φmid=0.5–0.95) and outlet flow velocities(Umid=0.5–4.0 m/s), and the effects of the injection of air co-flow are experimentally studied in a bluff-body multi-ring burner by flame image, flow path line, high speed PIV(1000, 2000 fps) and temperature mea-surement. Result shows that annular lean flames can be classified into four modes: cone flame, half-cone flame, lift-off flame, and extinguishment. Combustion is strong and efficient at cone flame mode, and then gets weaker through half-cone flame, lift-off flame and finally to extinguishment. In order to enhance chemical reaction in weak combustion mode such as half-cone flame mode, there is no need to add extra materials or fuel, just injecting air co-flow with velocity ratio Uout /Umid > 0.3-0.7(depending on reactant flow velocity Umid) around the outer side of annular lean flame can promote chemical reaction and make the flame transfer into a more stabilized mode – tulip flame.