| 摘要: | 本論文定量量測高壓條件下(p = 0.1 ~ 1.0 MPa)之預混層焰和紊焰之燃燒速度，探討壓力效應如何影響預混火焰之流力不穩定性(hydrodynamic instability)與熱擴散不穩定性(thermodiffusive instability)，以及其與紊焰之交互作用關係。一系列中心引燃往外傳播之預混層焰與紊焰燃燒實驗，於高壓預混紊流十字型燃燒系統執行，該系統為一雙腔體設計，分內爐與外爐。內爐為本實驗室已發展多年之十字型燃燒器，由水平與垂直鋼管所構成，透過大水平管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，其方均根紊流擾動速度(u')最高可達8.5 m/s。在內爐垂直管對稱位置上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，釋壓閥即啟動，故火焰傳播過程是在接近等壓條件下進行。外爐為一大型保護外爐，具有甚大空間，於實驗中維持與內爐一樣之初始壓力，可確保實驗之安全。利用高速攝影機，透過內爐與外爐之石英玻璃視窗，觀測火焰之動態傳播，再經影像分析計算獲得火焰燃燒速度。本實驗共有四組不同燃料成份，其中兩組為採用無因次參數Lewis (Le)數相近之貧油和富油甲烷空氣預混燃氣，其當量比(equivalence ratio)為? = 0.8 (Le = 0.98)和? = 1.2 (Le = 1.02)，其中Le = ?/D，?與D分別為熱與質量擴散係數。另外兩組，採用在常壓下具有相同之未受拉伸層流燃燒速度(SL)之貧油(? = 0.76)和富油(? = 1.4)丙烷燃氣，相對應之Le值為1.61和 0.98。實驗結果顯示，所有不同燃氣成份之SL值均會隨壓力增加而下降，可以SL ~ p-m來表示，其中m值介於0.35 ~ 0.41。這是因為燃氣密度會隨壓力上升而增加，但預混層焰僅具有限之質量燃燒率，所以SL會隨壓力增加而下降。然而在固定u' = 1.4 m/s，所有紊流燃燒速度(ST)值，均會隨壓力增加而提升，呈現與層焰(SL)完全相反之趨勢。在任一固定壓力條件下(p = 0.1 ~ 1.0 MPa)，比較Le < 1紊焰與Le > 1紊焰之ST/SL值，可發現前者具有較大之ST/SL值，約是後者之1.5倍。這是由於Le < 1紊焰，其流力不穩定性與熱擴散不穩定性同時受高壓紊流之強化所導致；而Le > 1紊焰，僅具流力不穩定性影響，故其ST/SL值在同樣p值條件下，比Le < 1紊焰低，顯示熱擴散不穩定性對高壓預混紊流燃燒速度有重要之影響。本研究結果，應對瞭解內燃機之高壓燃燒機制有所助益。 This study quantitatively measures the laminar and turbulent burning velocities of premixed flames over an initial pressure range of p = 0.1 ~ 1.0 MPa, so that the pressure influence on hydrodynamic and thermodiffusive instabilities and its interaction with premixed turbulent flames can be investigated. A series of centrally-ignited laminar and turbulent combustion experiments are conducted in a high-pressure premixed turbulent combustion system consisting of an inner chamber and an outer chamber. 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 8.5 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 maintain the initial pressure assuring to the safety of experiments. Flame visualizations are carried out via optical-accessed quartz windows of both chambers using high-speed CMOS cameras. After the image processing, the flame burning velocities can thus be obtained. In this study, four combustible mixtures are used including two methane/air mixtures at different equivalence ratios of ? = 0.8 (Le = 0.98) and ? =1.2 (Le = 1.02) and two propane/air mixtures at ? = 0.76 (Le = 1.61) and ? =1.4 (Le = 0.98) having the same value of unstretched laminar burning velocity (SL), where Le ≡ ??D is the Lewis number and ??and?D?are the thermal and mass diffusivities of premixtures. Results show that values of SL for all mixtures decrease with increasing pressure, by which SL ~ p-m where the values of m range from 0.35 to 0.41. Such decrease in values of SL is because the density of the combustible mixture increases in proportion to the pressure, but laminar premixed flames have the limited mass burning capability. However, at a constant turbulent fluctuating velocity u' = 1.4 m/s, all values of turbulent burning velocities (ST) increase with increasing pressure, showing an opposite trend to that of SL. Moreover, at any given pressure conditions varying from 0.1 MPa to 1.0 MPa, values of ST/SL for Le < 1 flames are found to be about 1.5 times larger than those of Le > 1 flames for both methane and propane flames. This result suggests that both hydrodynamic and thermodiffusive instabilities can be effectively augmented by turbulence at high pressure conditions, so that Le < 1 flames with both instabilities have larger burning rates than Le > 1 flames. The latter has only the hydrodynamic instability. This suggests that the thermodiffusive instability is also important for the enhancement of ST. The present results should be useful to our understanding of high-pressure combustion mechanism in internal combustion engines. |
