| 摘要: | 本論文介紹一新型高壓預混紊流燃燒設備系統及相關高壓預混紊焰實驗研究。此高壓燃燒設備採內外雙腔體設計，含內部的風扇擾動式紊流燃燒爐與外部的壓力緩衝安全氣艙，其腔體四面均具備光學量測視窗。本論文採用中心引燃向外傳播之球狀火焰，分別於靜態層流與等向性均勻紊流場中，針對0.1 ~ 1.0 MPa等不同初始壓力(p)條件進行燃燒速度之量測。實驗結果顯示，不同於層流燃燒速度(laminar burning velocity, SL)所呈現之高壓弱化效應，提高壓力確實可提升紊流燃燒速度(turbulent burning velocity, ST)。我們推測高壓之所以能提升紊焰速度，主要是因為高壓下火焰厚度變薄，導致火焰面不穩定性被強化，進而提高火焰面皺摺程度的緣故。與先前的本生燈紊焰速度比較後發現，球狀紊焰之ST/SL數據較低，但其隨壓力變化之趨勢相同。依運動黏滯係數(kinematic viscosity,?)隨高壓遞減之比例，調整高壓燃燒設備之紊流場參數，我們也針對等雷諾數(Reynolds number, ReT = u?LI/?)條件進行高壓ST量測，其中u?與LI分別為均方根紊流擾動速度與紊流積分長度尺度。等ReT實驗結果指出，ST如同SL亦隨壓力增加呈負指數遞減，顯示燃燒速度隨壓力變化之一致性。整體而言，當p固定時，ST隨ReT增加而增加；但當ReT為定值時，ST/SL隨u?/SL變化之曲線均呈現極為顯著的彎折現象。實驗結果並顯示，在p = 0.1 ~ 1.0 MPa而ReT = 6,700 ~ 14,200的變化範圍內，ReT對於強化ST/SL所扮演的重要效應，即紊流強度與流體黏性之耦合流場效應，其強弱並非由ReT的數值來決定。這是因為ReT對於提升ST的實際作用主要還是取決於紊流強度而非高壓效應。此外，我們發現校正至平均傳遞變數 = 0.5之球狀紊焰ST值與先前本生燈紊焰之數據相當吻合。這意味著對於不同紊焰幾何之ST比較，以 = 0.5所對應之火焰面來定義ST是一較佳的選擇。最後，我們會針對ST/SL隨數種不同無因次參數之整體變化關係進行討論。 A new apparatus for studies of turbulent premixed flames at atmospheric and elevated pressures is presented. This apparatus is of dual-chamber optically-accessible design, combining an inner fan-stirred turbulent burner and an outer pressure-absorbing safety chamber. Burning velocity measurements for centrally-ignited, outwardly-propagating lean premixed spherical flames under both quiescent and turbulent conditions are conducted over an initial pressure range of p = 0.1 ~ 1.0 MPa. It is found that, contrary to the suppressing effect on laminar burning velocities (SL), pressure elevation significantly enhances turbulent burning velocities (ST). It is suggested that such flame speed magnifications are attributed primarily to boosted flame wrinkling induced by augmented flame front instabilities on thinning flame at elevated pressure. Our spherical flame data show similar variation trends but have lower values of ST/SL as compared with previous Bunsen flame data. By adjusting turbulence properties in proportion to the decreasing kinematic viscosity (?), measurements of ST at increasing p under constant Reynolds numbers (ReT ? u?LI/?) conditions are also performed, where u? and LI are respectively the r.m.s. turbulent fluctuation velocity and the integral length scale of turbulence. Results show that ST decreases similarly as SL with increasing p in minus exponential manners at constant ReT of 6,700 ~ 14,200, revealing the global response of burning velocities to pressure. In general, at fixed p, the higher ReT, the higher ST; however, the curves of ST/SL as a function of u?/SL are all found to exhibit very strong bending under constant ReT conditions. These results also clearly reveal that the important effect of ReT, comprising the joint fluid flow effect of turbulent intensity and fluid viscosity, on ST/SL enhancement depends implicitly on the non-equal contributions primarily from turbulence and secondarily from pressure, not explicit on the magnitudes of ReT. Moreover, we find that the modified values of ST at mean progress variable c ̅ ? 0.5 show good agreements between previous Bunsen-type and present spherical flames, suggesting that the turbulent burning velocity determined at flame surfaces with c ̅ = 0.5 may be a better representative of itself regardless of the flame geometries. Finally, various general correlations of turbulent burning velocities are discussed. |
