| 摘要: | 本論文量測高溫高壓條件下，異辛烷之預混層流與紊流火焰之燃燒速度。主要探討熱擴散不穩定性(thermodiffusive instability)對紊流燃燒速度ST之影響，並分別探討在固定流場紊流雷諾數ReT,flow = u′LI/v和固定紊流擾動速度u′條件下，壓力效應對ST之影響，其中LI為紊流積分長度尺度，v為運動黏滯係數。實驗於一高壓高溫雙腔體十字型風扇擾動預混紊流燃燒設備進行，它可產生一近似等向性紊流場，可在固定溫度(T)、壓力(p)、u′和ReT,flow條件下進行實驗。我們使用液態燃料異辛烷作為燃料，先將異辛烷注入已抽真空之另一加熱腔體，使其完全氣化後再以分壓方式將適量的異辛烷注入十字型風扇擾動爐體內，並在實驗前與空氣預混。實驗開始於中央引燃，以高速紋影法紀錄往外傳播球狀火焰半徑之時序圖(t)。我們量測三種不同當量比(phi = 0.9, 1.0, 1.25)之異辛烷/空氣燃氣，分別對應之 Lewis數為Le ≈ 2.94、Le ≈ 1.43和Le ≈ 0.93。比較當量比phi = 0.9(Le > 1)與1.25(Le < 1)之燃氣，兩者均使用相同實驗範圍進行實驗，即在T = 358K、p = 1 ~ 5atm和u′ = 1.4m/s或ReT,flow = 6700。而有關phi = 1.0之燃氣，也是在T = 358K和p = 1 ~ 5atm範圍，但更擴大紊流實驗範圍，即在u′ = 1.0, 1.4, 2.8 m/s和ReT,flow = 6700, 9100, 11600，以深入探討ReT,flow效應。結果顯示，在固定u′時，ST值會隨p增加而增加，這主要是因為ReT,flow會隨壓力增加而增加之故。反之，在固定ReT,flow時，ST則會隨p增加而下降，其趨勢與層流燃燒速度(SL)相似，即ST和SL對壓力效應均呈指數下降之關係，這顯示ReT,flow對ST之重要性。在固定壓力下，ST值會隨ReT,flow增加而上升。另外，Le數對紊流燃燒速度也有重要之影響，我們發現Le < 1紊焰(phi = 1.25, Le ≈ 0.93)其ST值明顯高於Le > 1紊焰(phi = 0.9, Le ≈ 2.94)，請注意這是在兩者之SL和u′ = 1.4m/s為近乎相等的情況下所得的結果，其原因為Le < 1紊焰會額外受到熱擴散不穩定性之影響。
本論文也以實驗室常用之Damköhler number正規化關係式，即ST,c=0.5/u′ = A1(Da)B1，以及由Kobayashi et al.與Chaudhuri et al.所提出之正規化關係式，分別為ST,c=0.5/SL = A2[(u′/SL)(p/p0)]B2與[(1/SLb)(d/dt)] = A3(ReT,flame)B3，來分析三種不同Le數之ST資料，其中下標c ̅為平均傳遞變數，Damköhler number Da = (LI/u′)(SL/L)，L為層流火焰厚度，p0為一大氣壓，A1 ~ A3和B1 ~ B3為實驗常數，SLb為生成物之層流燃燒速度，火焰紊流雷諾數ReT,flame = (u′/SL)(/L)。我們發現原相當分散之正規化ST資料，可經由加入Le數之考量修正後合併成單一曲線，分別為ST,c=0.5/u′ = 0.087(DaLe-1)0.5、ST,c=0.5/SL = 3.5[(u′/SL)(p/p0)Le-1]0.24與[(1/SLb)(d/dt)] = 0.35(ReT,flameLe-1)0.33，顯示不同u′、p和phi值下之紊流燃燒速度經Le-1修正後具有相似性。很可惜的事，冪次常數B2 = 0.24和B3 =
0.33小於先前氣態甲烷燃料之值(B2 = 0.38; B3 = 0.53)，僅B1 =
0.5與先前氣態甲烷燃料(B1 = 0.53)接近。我們將先前文獻上僅有的異辛烷ST資料(由Leeds Prof. Lawes團隊所量得)，用ST,c = 0.5/u′ vs. (DaLe-1)0.5關係與目前ST資料作比較，得到相當吻合的結果，顯示ST,c = 0.5/u′ = (DaLe-1)0.5之關係為一較優的一般通式。此研究結果，對高溫高壓預混紊流燃燒相關之應用，如車輛及航空引擎之燃燒研究有所助益。
;This thesis measures quantitatively burning velocities of high-temperature, high-pressure premixed laminar and turbulent flames of iso-octane/air. We mainly explore the effects of thermodiffusive instability and pressure at constant turbulent Reynolds number (ReT,flow = u′LI/v) and constant r.m.s. turbulent fluctuation velocity (u′) on turbulent burning velocities ST, where LI and v are the integral length scale of turbulence and the kinematic viscosity of reactants respectively. Experiments were conducted in a high-pressure, high-pressure, double-chamber, cruciform fan-stirred premixed turbulent explosion facility, capable of generating a near-isotropic turbulence for conducting combustion experiments at fixed temperature (T), pressure (p), and u′ or ReT,flow conditions. We apply the liquid fuel (iso-octane) as a fuel which is first injected into a pre-vacuumed heating cylinder to make sure it is fully vaporized. Then we inject the pre-vaporized iso-octane into the cruciform fan-stirred burner by using the partial pressure method. The pre-vaporized iso-octane and are at T = 358K are well-mixed before a run. A run begins by centrally-igniting the combustible mixtures. The timing diagram of spherical expanding flames radii are recorded by the high-speed Schilieren imaging technique. Three equivalence ratios (phi = 0.9, 1.0, 1.25) with different Lewis numbers (Le ≈ 2.94, 1.43, and 0.93). Comparing the flames of phi = 0.9(Le > 1) and 1.25(Le < 1) are measured under the same ranges of experimental conditions i.e. T = 358K, p = 1 ~ 5atm, and u′ = 1.4 ~ 2.8m/s or ReT,flow = 6700, then the phi = 1.0 is measured more ranges of turbulent conditions i.e. u′ = 1.0, 1.4, 2.8m/s and ReT,flow = 6700, 9100, 11600 in order to further investigate the ReT,flow effect. Results show that when u′ is fixed, values of ST increase with increasing p, which is mainly due to increase of ReT,flow with increasing p. On the contrary, when ReT,flow is fixed, we find that values ST actually decrease with increasing p. Such decreasing trend of ST ~ p-0.36 is similar to laminar burning velocities (SL), showing a global response of burning velocities to the increase of pressure. When p is fixed, values of ST increase with increasing ReT,flow. In addition, it is found that the effect of Le plays an important role on ST. Le < 1 flame (phi = 1.25, Le ≈ 0.93) clearly have higher values of ST than that of Le > 1 flames (phi = 0.9, Le ≈ 2.94), both with the same SL ≈ 0.4m/s and u′ = 1.4 ~ 2.8m/s. This is because Le < 1 flames experience additional thermodiffusive instablility.
This thesis also apply that commonly used correlation of Damköhler number in the laboratory, ST,c = 0.5/u′ = A1(Da)B1, and two correlations proposed by Kobayashi et al. and Chaudhuri et al. respectively, ST,c=0.5/SL = A2(u′/SL)(p/p0)B2 and [(1/SLb)(d/dt)] = A3(ReT,flame)B3 to analyze present data with three different Le, where the subscript c is the mean progress variable, Damköhler number Da = (LI/u′)(SL/L), L is laminar flame thickness, p0 = 1atm, A1 ~ A3 and B1 ~ B3 are experimental coefficients, SLb is laminar burning velocity of product, and turbulent flame Reynolds number ReT,flame = (u′/SL)(/L). We discover that when using the Le-1 correction, all scattering ST data with different values of u′, p and phi can be collapsed onto a single curve, i.e. ST,c = 0.5/u′ = 0.087(DaLe-1)0.5, ST,c=0.5/SL = 3.5[(u′/SL)(p/p0)Le-1]0.24 and [(1/SLb)(d/dt)] = 0.35(ReT,flameLe-1)0.33. Unfortunately, the power constants of B2 = 0.24 and B3 = 0.33 are less than values (B2 = 0.38 and B3 = 0.5) of previous methane fuel and only B1 = 0.5 is close to the previous methane fuel (B1 = 0.53). We compare the only iso-octane ST data in previous literature (measured by the Leeds Prof. Lawes team) by the relationship of ST,c = 0.5/u′ vs. (DaLe-1)0.5 and get quite consistent results showing the relationship of ST,c = 0.5/u′ ~ (DaLe-1)0.5 is a better general correlation. These results are useful for the application of high-pressure, high-temperature premixed turbulent combustion such as in auto and aviation engines. |
