7-1 結論
本研究以層狀化燃燒器為載具，透過自行建立之光學診斷結合POD 之量測技 術，探討進流數效應對暫態火焰和流場的影響，包括火焰型態、平均燃燒流場特性、
紊流強度和螢光強度交互作用特性及PDF 分佈等，並透過 POD 處理進行燃燒流場 結構重組、模態能量特性分析、時空動態分佈特性以及模態頻率響應特性。基於 POD 解析的層狀燃燒流場特性歸納三種操控穩焰方法，包含加入空氣共伴流穩焰、
壓縮渦對穩焰以及利用熱擴散不穩定效應增幅或抑制燃燒流場特性等。另一方面，
應用兩股45°添加氫氣及一氧化碳之預混進流形成之衝擊燃燒流場，具有低速高溫 迴流區可大幅提升燃燒穩定性、提高火焰溫度並降低一氧化碳排放量。
在不同進流數影響下，火焰長度和分佈範圍隨進流數增加而擴大，此外火焰位 置集中於進流間的區域，顯示對於飄盪貧油火焰而言剪流層對交互作用具有關鍵 影響。由 PIV 和化學螢光所獲得之平均燃燒流場分佈發現隨進流數增加會增強燃 燒流場中紊流強度峰值，同時也會增加高紊流強度的分佈機率，證明剪流層區主導 改變進流數時之燃燒流場結構；隨進流增加火焰高度雖然變高且分佈變廣，但仍集 中於剪流層區，藉由指標平均機率強度則顯示火焰的交互作用影響增幅變大，並透 過高速影像解析證實此時的貧油不穩定燃燒流場由剪流層區火焰主導。POD 處理 後顯示第一模態均為主導模態，隨進流數增加主結構能量會分散但仍具有明顯的 能量比例差異，因此對於燃燒流場重建而言，隨進流數增加則需越多模態相加來趨 近原始燃燒流場。隨進流數增加，水平及垂直方向上擺動振盪加劇和高模態時出現 細碎化結構。由於燃燒場在進流數增加後主流方向開始轉變，透過交互作用明顯使 火焰在完整厚實的剪流層區域成為主導結構，造成這個以較微小的結構為主的局 部區域振幅變小。此外流場部分由於出現高階模態週期減小和反轉處不規則抖動 的間歇特性，證明進流數增加和細碎結構增加且生成、消散的轉變快速相關。模態
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峰值頻率與原始燃燒流場特徵頻率相近，且隨進流數增加，次要峰值明顯增加，而 累加一二模態幾與原始頻率分佈相同，因此驗證燃燒流場重組的可依賴性。
藉由PIV 和化學螢光光學量測結合 POD 分析，本研究歸納三種層狀化燃燒之 穩焰操控方法，分別探討加入空氣共伴流之影響、壓縮渦對之穩焰機制以及熱擴散 不穩定性對火焰和流場交互作用之穩焰效應。在加入空氣共伴流的部分，單股貧油 甲烷火焰之燃燒流場的火焰型態可分為四類：錐焰、飄焰、近吹熄和熄滅，藉由中 央鈍體之迴流結構可拓展操作區間至當量比 0.56。空氣共伴流在飄焰內側加入空 氣共伴流並無明顯的影響，然而當加入外側之空氣共伴流超過特定流速閾值時，則 會改變火焰結構至較穩定之類錐焰型態，透過高速 PIV 發現共伴迴流區形成衝擊 反轉流場結構，使高溫燃氣蓄集在共伴迴流區當中，因此火焰能向上游傳播而形成 類錐焰結構，強化火焰穩定性。在壓縮渦對之穩焰部分，具速度梯度差之層狀化燃 燒所產生的壓縮渦對使穩定操作區間在開放空間下擴展至
= 0.50，由於尾流區中 型成之壓縮渦對促使流體在兩渦漩中的通道被擠壓加速，加上受到浮力效應所捲 入的外側氣流捲曲向下游拉伸，形成一股加速射流加強尾流區的熱量和質量傳遞 效應。以作為火焰強度指標的 Uvar/SL峰值和紊流強度峰值兩者比較，其位置特性 與流場互相吻合，顯示壓縮渦對可提升火焰強度及穩焰。受熱擴散不穩定性影響 下，不同丙烷預混火焰在當量比(
= 0.6-1.6)時，富油火焰受到火焰前緣不穩定性 主導，火焰尖端產生破裂並於焰尖開口兩側出現螢光強度峰值。由統計所得之紊流 強度分析燃燒流場中火焰和流場的交互作用特性，發現在貧油燃燒時火焰強度與 紊流強度呈現正相關之趨勢；反之，在富油燃燒時則呈現負相關之趨勢。透過POD 處理後，富油端第一模態能量含量約為貧油端之兩倍，代表貧、富油兩端具不同程 度之紊流化；而在燃燒流場空間分佈中具主導性的第一模態的特性方面，
= 0.6 時 由低頻大尺度結構主導，但在
= 1.6 則由出口處高頻小層流化結構主導，因此可 藉由熱擴散不穩定性具有主導影響火焰和流場之特性進行增幅和抑制。添加生質合成氣於衝擊燃燒流場的部分，首先探討富油丙烷火焰利用相互衝
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擊的流場，兩股相近的火焰之相互預熱並在 V 型溝處因停滯面產生低流速區蓄積 熱能相對減少逸散熱損失，因此強化火焰穩定性。比較 C3H8/air、H2/C3H8/air 和 CO/C3H8/air 火焰在不同當量比和不同氫氣或一氧化碳添加比例下的燃燒特性，添 加氫氣或一氧化碳可分別擴展操作區間至0.38 和 0.5，火焰長度隨添加氫氣或一氧 化碳比例增加而縮短。由於添加氫氣和一氧化碳始化學動力學特性改變，由 H2 + OH = H2O + H 和 CO + OH = CO2 + H 兩反應式主導火焰色澤的變化，使 CH*和 OH*呈現相反分布趨勢，此外火焰溫度因為碳氫燃料比例下降、衝擊低速回流區以 及氫和一氧化碳反應特性而提升，並大幅降低一氧化碳排放量，顯示利用衝擊燃燒 器可有效提升燃燒問焰特性。
7-2 自評與未來展望
本研究建立PIV 和化學螢光法光學診斷結合 POD 之量測分析技術，探討層狀 化燃燒流場特性，藉由平均分佈、紊流強度和火焰螢光強度、POD 分解、重組及 時空動態特徵，歸納空氣共伴流、壓縮渦對效應以及熱擴散不穩定操控穩焰方法，
並應用添加氫氣及一氧化碳之衝擊燃燒流場擴展燃燒穩定性、提升火焰溫度及降 低一氧化碳排放量。核心及貢獻在於：發展有效實用之燃燒流場光學診斷結合POD 之實驗分析方法、層狀化燃燒之火焰和流場交互作用機制、燃燒流場重組及時空動 態特徵分析方法、歸納共伴流、壓縮渦對及熱擴散不穩定效應之穩焰機制、衝擊燃 燒流場之混燒特性分析。研究核心概念與貢獻架構如圖7-1 所示。
火焰穩定性與流場結構的交互作用特性或許是最美且最引人入勝的燃燒現 象，人們若能對火焰有深入的了解，將使科學和工程更相緊密的結合。本文的量測 結合分析技術之開發及研究發現不僅在學術研究上有所貢獻，也期望能在未來應 用於民生工業、航空和國防等用途，以增進國家社會福祉，並保障人類生活健康和 永續生存的生態環境。
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圖7-5 本研究之核心概念與貢獻
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