層狀化燃燒流場之時空動態特性及穩焰機制研究

Abstract

本研究以層狀化燃燒器(stratified burner)為載具，建立包含化學螢光(chemiliminescence)和粒子影像測速儀(particle image velocimetry, PIV)之光學量測技術量測燃燒流場之火焰和流場暫態連續影像，並利用本征正交分解(proper orthogonal decomposition, POD)處理，進行燃燒流場結構重組和時空動態特性分析。主題依序分別為進流數效應和其穩焰操控方法，以及應用生質合成氣(氫氣及一氧化碳)於衝擊燃燒流場之燃燒特性，探討包含火焰型態、平均燃燒流場特性、同調結構(coherent structure)、模態頻譜分析，最終歸納出不同層狀燃燒流場之結構特性和提出穩焰機制及應用方法。 進流數效應依據進流數分為單股、雙股和三股預混甲烷火焰探討，由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。透過POD處理發現燃燒流場的第一模態均為主導模態，隨進流數增加低模態能量會分散至高模態的結構。在空間分佈方面，水平及垂直方向上擺動振盪隨增加進流數而加劇；在時間變化特性方面，進流數增加和細碎結構增加且生成、消散的轉變快速造成高階模態週期減小和反轉處不規則抖動的間歇特性。在進流數效應操控穩焰方法部分，首先是加入空氣共伴流(air co-flow)之單股貧油甲烷火焰，其燃燒流場的火焰型態可分為四類：錐焰、飄焰、近吹熄和熄滅。在飄焰內側加入空氣共伴流並無明顯的影響，然而當外側加入之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，其與POD分析所出現之分層結構相似，形成衝擊反轉流場結構減少垂直方向速度分量，並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，顯示共伴流效應強化火焰結構並使反應強度提升。第二，在三股預混丙烷燃氣進流具速度梯度差之層狀化燃燒時，由於迴流區改變尾流流場結構造成的混合及各股火焰間的預熱效果，壓縮渦對(compressive vortex pair)的形成可擴展操作區間至phi = 0.5。壓縮渦對形成一股加速射流加強熱量和質量傳遞效應，具有提升火焰強度，達到幫助燃燒穩焰的功能。第三，三股預混丙烷燃氣在不同當量比(phi = 0.6－1.6)可將火焰型態分為融合火焰、穩定中環火焰和焰尖開口火焰等三種類型。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，在第一模態中，phi = 0.6時由低頻大尺度結構主導，但在phi = 1.6則由出口處高頻小層流化結構主導。顯示在貧油和富油燃燒不同情況下，熱擴散不穩定性效應具有主導影響化學螢光強度和速度場分佈的特性。 V型燃燒器具兩股45°燃氣進流，在迴流區的衝擊流場中具有增強混合、預熱與蓄熱等特性，藉由強烈的火焰與流場交互作用顯著強化富油丙烷火焰操作區間及其穩定性。在含生質合成氣之貧油燃燒部分，固定丙烷流率的情況下添加不同比例之氫氣與一氧化碳之火焰可燃下限可分別拓展至0.38和0.50，具有M型及丘型兩種火焰型態。在當量比0.6時，M型的H2/C3H8/air火焰溫度(1435 ℃)約為1.37倍丘型的CO/C3H8/air火焰(1050 ℃)，而此時C3H8/air火焰已經熄滅，廢氣排放量(一氧化碳)也隨之改變。結果闡明在衝擊燃燒流場中預混火焰添加氫氣及一氧化碳下，火焰和流場交互作用機制，包含火焰結構轉變、迴流低速流場特性及化學動力學影響。本研究建立PIV和化學螢光法結合POD之光學量測分析技術，探討層狀化燃燒器之平均燃燒流場特性、重組及動態特徵，並歸納出操控方法及其交互作用機制。本研究核心及貢獻在於：發展有效實用之燃燒流場光學量測實驗及分析方法、研究層狀化燃燒之交互作用機制、建立燃燒流場重組及動態特徵分析方法、分析層狀燃氣進流數效應並提出共伴流、壓縮渦對及熱擴散不穩定性對火焰和流場交互作用之穩焰效應，以及添加生質合成氣之衝擊燃燒流場穩焰機制。

An experimental method of PIV and chemiluminescence coupled with POD was constructed to capture transient images for both flames and flows with procession of the reconstruction, and temporal/spatial dynamic characteristics on a stratified burner. The issues in this study include the effects of inflows on the combustion characteristics with its mechanisms of flame stabilization, and application of syngas combustion on V-shaped burner. The effects of inflows categorized with single, double and triple inflow mode were investigated for premixed methane flames, respectively. The increase of inflow number was found to enhance the peak value and broaden the higher level probability distribution function (PDF) for turbulence intensity, demonstrating that the presence of shear layer structure is the dominating factor. The mode 1 was found to be the dominant mode for all cases, but the energy-contained of low-rank mode was diverged to the high-rank mode. Both the horizontal and vertical oscillation was intensified with increase of inflow number; the presence of augmented oscillation and irregular vibration in turning point for high-rank mode was responsible for the fractalized structure. The effect of air co-flow on the single lean methane flame was investigated firstly. A variation of the position of co-flow injection shows that the inner one has no impact, whereas the outer one surpassing effective velocity ratios has a definite impact with flame configuration altered from a lift-off flame to a cone-like flame. This characteristic is similar with the presence of turning point the vertical oscillation because of a reversed flow with accumulated hot combustion products in the co-recirculation zone. It results in a lift-off flame propagating nearer the burner exit and demonstrats enhanced flame stabilization. Second, for stratified combustion of three premixed propane mixtures with velocity gradient, the operation region was expanded to phi = 0.5 with compressive vortex pair because of the enhanced preheating and mixing effects in the wake region. The compressive vortex pair structure effectively induces greater turbulent intensity to enhance the flame intensity, and thus achieves a salient performance of stabilization. Third, the flame intensity of the triple premixed propane flames with phi = 0.6－1.6 were found to correspond well with turbulence intensity in lean flames, but inversely in rich flames. With the mode 1 in POD analysis, the large scale vortex structures dominated in lean flames with low frequency, whereas the small stratified structures dominated in rich flames with high frequency. It indicates that the combustion characteristics influenced by the lean and rich flames were dominated by the change of diffusion-thermal instability. For the V-shaped burner the impinging region is capable of enhancing stabilization of rich propane flames due to benifits from the intense interaction between flame and recirculation. For the combustion characteristics with syngas addition the lean flammability of H2/C3H8/air is expanded to 0.38 and that of CO/C3H8/air is expanded to 0.50 with M type and hill type flame configurations. At phi = 0.6, the flame temperature of H2/C3H8/air with M-type flame is 1.37 times that of CO/C3H8/air with hill type flame, while the C3H8/air flame is extinguished; the CO emissions also change. The mechanisms of flame/flow interaction including alternation of flame structures, characteristics of recirculating flow, and chemical kinetics for impinging flames with H2 and CO addition were revealed.