| 摘要: | 本論文量測高溫高壓層、紊流條件下，中心引燃之球狀火焰的燃燒速度(SL和ST)及其氮氧化物NOx和一氧化碳CO的排放量，以研究甲苯汽油替代燃料 (Toluene Reference Fuel; TRF85)加入15％和45％乙醇混合物所造成的影響。TRF85由77.4％異辛烷(i-C8H18)、17.6％正庚烷(n-C7H16)和5％甲苯(C7H8)所組成，其中i-C8H18、n-C7H16和C7H8的研究辛烷值(Research Octane Number, RON)分別為0、100和121。本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的紊流場。實驗溫度保持在373 K，其中方均根紊流擾動速度 (u′ = 0-4.3 m/s) 和初始壓力(p = 1-5 atm)。本研究有三個重點。(1)乙醇混合物在不同u′、p下對SL和ST的影響，結果顯示15％的乙醇對SL僅有小增幅，然而對ST的影響較大，增幅會隨著u′和p的增加而放大，這是因為紊流導致火焰表面積增加使ST與放熱率增加而顯著上升。此外，乙醇主要中間產物乙醛（C2H4O）可以促進燃料氧化過程。再者，紊流雷諾數ReT, flow = u′LI/v隨p的增加而增加，因為v?ρ-1?p-1，其中LI是紊流積分長度尺度，而v和ρ是運動黏滯系數與反應物密度。(2)針對ST一般通式在考慮Le數(Lewis number)下，對當前TRF85、添加15% 或45%酒精之TRF85及之前異辛烷和PRF95的 ST數據作相關分析，當前數據在這些一般通式中有良好的吻合。(3)在高壓和高紊流條件下添加45％乙醇對TRF85的CO和NOx排放的影響。因為乙醇潛熱較高，使火焰溫度降低，添加乙醇在3 atm和1 atm下分別減少約23％和11％的NOx排放量。然而主要中間產物乙醛的分子間鍵能較低，較容易分解產生CO，故在3 atm和1 atm下分別增加約5％和29％的CO排放量。NOx、CO排放量均隨p增加而上升，NOx的增加是由於氧和氮原子的增加，而CO的增加則是因為燃氣與氧氣反應的時間有限。而NOx和CO排放也會隨著u′的增加而增加。NOx的增加歸因於燃燒溫度的提高，而CO的增加則是因為時間太短，使燃料無法完全的氧化。最後，這些結果有助於我們對火花點火引擎火焰傳播之了解。
關鍵詞：甲苯參考燃料、乙醇混合物、紊流燃燒速度、高溫高壓燃燒、紊流速度一般通式、NOx和CO排放量
;In this thesis we investigate experimentally the influences of 15% and 45% ethanol blends in a stoichiometric gasoline surrogate (Toluene Reference Fuel, TRF85) via measurements of laminar and turbulent burning velocities (SL and ST) of spherically expanding flames and their associated NOx and CO emissions. TRF85 consists of 77.4 % iso-octane (i-C8H18), 17.6% n-heptane (n-C7H16), and 5% toluene (C7H8), where i-C8H18, n-C7H16, and C7H8 have 0, 100, and 121 research octane number, respectively. Experiments are conducted in a double-chamber, fan-stirred constant-temperature pressure cruciform burner capable of generating near-isotropic turbulence with negligible mean velocities. The initial temperature of the experimentation domain is kept constant at T = 373 K, where the r.m.s turbulent fluctuating velocity (u′) is varied from 0-4.3 m/s and the initial pressure (p) is varied from 1 atm to 5 atm. There are three main points found in this study. First is the effect of ethanol blends on SL and ST as a function of p at various u′ conditions. Results show that 15% ethanol blend only has a minor enhancing influence on SL, but it has a more significant influence on ST especially with increasing u′ and p due to the fact that turbulence can increase ST through the increase of flame surface area, thus resulting in an increase of heat release rate. Further, the presence of ethanol major intermediate species, the acetaldehyde (C2H4O), can promote the oxidation process. Moreover, the flow turbulent Reynolds number ReT,flow = u′L/v increases with p because v ~ ρ-1 ~ p-1, where LI is the integral length scale of turbulence and v and ρ are the kinematic viscosity and the density of the reactants. The second point concerns general correlations of the present ST data of TRF85, TRF85+15E, TRF85+45E together with previous ST data of iso-octane and PRF95 with the consideration of Le. Results show that the present ST data can be collapsed well on the general correlations proposed. The third point relates to the influence of 45% ethanol blends on CO and NOx emissions of TRF85 under high pressure and high turbulence condition. It is found that the ethanol diluents reduce NOx emission for about 23% and 11% at p = 3 atm and p = 1 atm due to the high latent heat of ethanol, which lowers the flame temperature. However, CO emission increases with the addition of ethanol diluents for about 5% and 29% in p = 3 atm and p = 1 atm due to the rise of acetaldehyde as an indication of low bond dissociated energy. Both NOx and CO emissions increase with p. The increase of NOx is due to the enhancement of oxygen and nitrogen atoms. CO increase is because the limited time to react with oxygen. Similarly, both NOx and CO emissions also increase with increasing u′. NOx increase is owing to the enhancement of combustion temperature. CO increase is because the time is too short to complete the fuel/air oxidation. Finally, these results should be useful for our understanding of flame propagation in spark ignition engines. |
