| 摘要: | 本論文定量量測氫氣/一氧化碳合成氣於高壓條件下即p = 0.1 ~ 1.0 MPa，之預混層焰和紊焰之燃燒速度(SL和ST)，利用風扇擾動式紊流場之大尺度高壓燃燒系統，執行一系列中心引燃往外傳遞之預混層焰與紊焰燃燒實驗。該高壓燃燒系統，為一雙腔體設計，由內爐與外爐所組成。內爐為參考本實驗室已發展多年之十字型燃燒器所設計而成，由水平與垂直鋼管所構成，透過水平腔體兩側之特製風扇與空孔板，可於十字型燃燒器中心處區域產生一近似等向性之紊流場，其均方根紊流擾動速度(u')最高可達8.4 m/s。在十字型內爐垂直腔體上下對稱位置，設有四個靈敏的釋壓閥，引燃爆炸後，內爐壓力若高於外爐約25 kPa，釋壓閥即會啟動，故火焰傳遞過程是在接近等壓條件下進行。外爐為一大型保護腔體，可於實驗中吸收由內爐所釋放之壓力，以確保實驗之安全性。利用高速高解析度攝影機，透過內爐與外爐之石英玻璃視窗，量測火焰之動態傳遞時序影像，經影像分析計算可獲得火焰燃燒速度。本研究針對挾帶流床氣化法所產出合成氣65%CO/35%H2之成份組合，及其貧油當量比在 當量比 0.5與0.7為主要研究對象，進行層流與紊流燃燒速度量測。另外，為了探討不同合成氣成份組合對於層焰與紊焰燃燒速度之影響，因此我們再選擇兩種不同合成氣成份組合，即50%CO/50%H2與95%CO/5%H2，在同樣u'= 0.7條件下，進行層流與紊流燃燒速度量測實驗。結果顯示，貧油合成氣之層紊火焰在高壓條件下呈現高度不穩定狀態，導致向外傳遞之層流球狀火焰表面和紊流擴張火焰均充滿蜂巢狀結構；並發現在不同當量比值和不同合成氣成份組合之層流燃燒速度，均會隨壓力增加呈現負指數下降之趨勢，可表示成SL ~ p-n，其中n值範圍為0.10 ~ 0.20。此下降趨勢與相同受高壓影響之貧油甲烷層焰相比(SL ~ p-0.41)來的緩和許多。相反地，當固定u=1.4 m/s之條件下，貧油合成氣之紊流燃燒速度則會隨著壓力增加而增加，可表示ST ~ pm，其中m值範圍為0.11 ~ 0.22。也可發現在任一相同壓力條件下(p = 0.1 ~ 1.0 MPa)，隨著合成氣中氫氣濃度或是當量比增加，皆會具有較高之SL和ST值。接著，為了更進一步探討紊流效應對於ST之影響，於不同初始壓力0.1 MPa與0.5 MPa，量測ST在不同紊流強度下之變化趨勢。結果顯示，增加紊流強度與增加壓力相比，能夠更有效地增加ST/SL值，但隨著紊流強度持續增加，ST/SL值增加幅度有越來越小之趨勢，產生所謂的彎折效應(bending effect)。此外，在高壓條件下此彎折效應仍會存在，且提升壓力似乎能夠使彎折效應延後產生。本研究結果對於合成氣於燃氣輪機與內燃機之直接應用，應有重要之幫助。 This study quantitatively measures the laminar and turbulent burning velocities (ST and SL) of premixed hydrogen/carbon monoxide syngas flames over an initial pressure range of p = 0.1 ~ 1.0 MPa. A series of centrally-ignited laminar and turbulent combustion experiment are performed, using a high-pressure, fan-stirred, large-scale, turbulent combustion system. This turbulent combustion system included an inner chamber and an outer chamber, as a double-chamber design. The inner chamber applies the same design of the cruciform burner previously used at National Central University led by Professor Shy, which is constructed by two mutually crossing cylindrical vessels, a horizontal vessel and vertical vessel, forming a cruciform shape. Using two identical frequency-controlled counter-rotating fans equipped at two ends of the horizontal vessel, a near-isotropic turbulent flow field can be generated in the central region of the inner chamber. In it the maximum value of turbulent fluctuating velocities u' can be up to 8.4 m/s. Furthermore, four sensitive pressure-releasing valves placed symmetrically around the vertical vessel of the inner chamber are applied. These valves are immediately activated when the pressure inside the inner chamber is 25 kPa greater than that of the outer chamber during explosion. Thus, outwardly propagating premixed flames interacting with isotropic turbulence can be obtained under nearly constant pressure condition. Besides, the outer chamber is large enough to absorb pressure releasing from the inner chamber, assuring the safety of experiments. Flame visualizations are carried out via optical-accessed quartz windows of both chambers using high-speed, high-resolution CMOS cameras. After the image processing, the flame burning velocities can thus be obtained. In this study, we select the syngas derived from entrained-flow gasifier with a fuel composition 65%CO/35%H2 at two different equivalence ratios of ? = 0.5 and 0.7 to measure laminar and turbulent velocity. Besides, in order to investigate effect of different syngas fuel composition for laminar and turbulent burning velocities, we also select two different syngas fuel composition 50%CO/50%H2 and 95%CO/5%H2, at the same ? = 0.7 to measure the laminar and turbulent burning velocities. Results show that lean syngas laminar and turbulent flames at elevated pressure are highly unstable resulting in cellular structures all over the expanding flame front surface. It is also found the laminar burning velocities all decrease with increasing pressure in a minus exponential manner for different equivalence ratios and syngas fuel composition, by which SL ~ p-n where the values of n range from 0.10 to 0.20. This trend is more modest than the same effect of pressure on lean methane laminar flame (SL ~ p-0.41). Contrarily, at a fixed u? ≈ 1.4 m/s, values of lean syngas ST increase with increasing pressure, by which ST ~ pm where the values of m range from 0.10 to 0.22. It is also found that, at any given pressure condition varying from p = 0.1 ~ 1.0 MPa, increasing hydrogen or equivalence ratios of syngas increases values of SL and ST. Moreover, in order to further investigate effect of turbulence on ST, we measures the changes of ST with different turbulent intensities at two different initial pressure 0.1 MPa and 0.5 MPa. Results show that increasing turbulent intensity is still a way much more effective in increasing value of ST/SL than increasing pressure, but the increments of ST/SL with u?/SL is getting smaller and smaller, revealing the bending effect. Moreover, this bending effect also exist at high pressure condition and pressure elevation seems to be able to delay bending effect. The present result should be of help in the direct application of syngas for gas turbines and internal combustion engines. |
