This thesis investigates experimentally bio-fuel droplet combustion in a quiescent atmosphere using two different kinds of bio-fuels, namely biodiesel and sunflower oil, together with their blends with diesel oil over a wide range of blended volumetric percentages varying from 5% to 75% for each of the two bio-fuels. Symmetrically spherical droplets with small diameters ranging from 0.45 mm to 0.60 mm are generated by a home-built piezoelectrically-driven droplet generator, where the small upwardly-injected droplet is suspended at the intersection of two very fine horizontally-positioned, perpendicularly-aligned ceramic fibers of 20 μm in diameter. A pair of Khantal wires of diameter 0.2 mm having a half-oval shape is horizontally placed on both sides of the suspended droplet for ignition. As soon as the ignition of flame that envelopes the droplet occurs, the two heating wires are electrically removed at which the initial droplet diameter (d0) is determined by a high-speed camcorder. These sequentially recording images are analyzed to obtain the burning droplet diameter (d) as a function of time (t) that is used to determinate the droplet burning rate constant (K) based on the best fitting slope of (d/d0)2 versus t data (the well-known d2-law). Results show that values of K increase with decreasing values of d0 for any fuels studied here. Pure biodiesel has a higher burning rate constant than that of diesel. The latter is free from microexplosion, while the former has weak microexplosion that occurs only at the very late burning stage just before the completion of droplet combustion. This may be due to a small quantity of methyl linoleic acid (4.92 wt. %) existed in biodiesel. Furthermore, complicated but very interesting microexplosion phenomena are observed for pure sunflower oil and its blends with diesel oil, in which two different values of K are found during the process of droplet combustion. When more than 50% of sunflower oil is within the blended fuel, the blended droplet undergoes a transition from regular burning in the first period (K1) to strong swelling, puffing, and microexplosion during the later stage of droplet combustion (K2), where K1 < K2. But such transition reverses when less sunflower oil (< 50%) is presented in the blended fuel. In other words, strong microexplosion occurs almost immediately in the beginning of droplet combustion, where K1 > K2. This reversing process of microexplosion may be attributed to different fatty acids with different volatile components in sunflower oil. It is found that the diffusion process plays an important role on droplet combustion, because various (d/d0)2 vs. t data sets with different curve can be collapsed into one single curve when t is normalized by a diffusivity time scale, tdiff = d02/α, where α is the thermal diffusivity of the droplet fuel. These results should be useful for our understanding of bio-fuel combustion that is important to automobile and aviation industries using bio-fuels to substitute fossil liquid fuels in the future.

論文探討兩種不同生質燃料(生質柴油和葵花油)，以及它們分別與柴油混合燃料(生質燃料體積濃度比從5%到75%)，在常壓靜止條件下之液滴燃燒實驗。對稱球形微小液滴可由自製的壓電驅動液滴產生器所產生，液滴直徑(d)範圍從0.45 mm到0.60 mm，將小液滴懸浮在兩條非常細水平定位且相互垂直呈十字狀直徑為20 μm之陶瓷纖維上。加熱器為一對直徑為0.2 mm 之Khantal 導線，呈半橢圓形狀並將其水平地置放於懸浮小液滴的兩側邊，用來點燃液滴。一旦點燃後當火焰包圍液滴時，加熱器即斷電移開，此時液滴的直徑訂為初始直徑(d0)，由高速攝影機所決定。由液滴燃燒過程所攝之時序圖，我們以(d/d0)2對時間t的最佳擬合斜率(即d2-law)，來決定液滴的燃燒率常數(K)。研究結果顯示，當 d0值下降時，K值會上升，對所有本研究所使用之燃料均如是。100%生質柴油比一般商用柴油有較高的K值。後者沒有微爆現象，而前者在燃燒後期即將完成液滴燃燒前，有弱微爆現象，這可能是由於少量的甲基亞油酸(4.92wt%)存在於生質柴油中所導致。此外，複雜但有趣的微爆現象，可以在純葵花油和其與柴油之混合生質燃料中被觀察到，我們找到兩個不同K值於液滴燃燒的過程中。當葵花油在混合燃料中體積比超過50%時，混合液滴會經歷一轉變，從第一階段的規律燃燒(K1)到第二階段液滴燃燒後期(K2)的強膨脹和微爆現象，其中K1 < K2。但是當混合燃料中含有較少量葵花油(< 50%)時，前述轉變順序正好相反，換句話說，在液滴燃燒開始不久的第一階段，強微爆即發生，其中K1 > K2。這種微爆的反向過程，可能可以歸因於在葵花油中，具有不同揮發成分的脂肪酸所致。我們發現在液滴燃燒中，擴散過程為主導因素，因為具有不同(d/d0)2 vs. t曲線可以被擬合成單一的曲線，當時間(t)以無因次tdiff = d02/α，其中α為液滴燃料的熱擴散係數。前述結果，有助我們對生質燃料燃燒之了解，其對未來汽車和航空工業上使用生質燃料替代化石液體燃料，將有所助益。

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