雙組份生質柴油與醇類液滴於微重力下燃燒與微爆現象之研究

On the combustion and the microexplosion of biodiesel/alcohol droplet in microgravity

謝志偉(Chih-Wei Hsieh)

指導教授 ： 潘國隆

國立臺灣大學/工學院/機械工程學研究所/碩士(2015年)

https://doi.org/10.6342/NTU.2015.01281

摘要

本研究以生質柴油預混醇類(甲醇、乙醇、異丙醇)液滴利用落塔(drop tower)的自由落體方式達到微重力燃燒，在此環境下浮力的影響可被忽略，液滴燃燒將呈現球對稱火焰。不同直徑大小液滴(0.38 mm、0.5 mm)透過液滴產生器產生並靜置在兩條陶瓷纖維線交叉的中心點，再置入燃燒室內以電熱絲點火進行實驗。液滴之後將經歷0.68秒的微重力過程，並在落下期間利用攝影機觀察其燃燒現象。除了落塔的實驗外，本實驗也透過數值計算的方式模擬雙組份生質柴油與異丙醇液滴的燃燒，希望藉此驗證實驗的燃燒結果。由於生質柴油與醇類的混合液滴點燃後會發生機率性的微爆(microexplosion)，其燃燒趨勢將與不發生微爆的燃燒完全不同，而大部分的文獻都呈現微爆的結果。前人雖有完成不發生微爆的燃燒，但所呈現的燃燒趨勢並不明顯，且在混合液滴具有高重量百分比的醇類時(>45%)無法完成燃燒不具有微爆的結果，因此本文的重點為了解微爆現象發生的原因、解決微爆的問題以及研究非微爆的液滴燃燒。
實驗發現內部不均勻現象(nonhomogeneity)為導致微爆發生的原因，當其出現於液滴內部時，受熱後會被汽化膨脹，撐破液滴表面並直接導致微爆。內部不均勻物的發生與相對濕度並無明顯的關聯，而與醇類及生質柴油間互溶現象有關。微爆的發生為機率性，小尺寸液滴(0.38 mm)微爆機率較小，而大尺寸液滴(0.5 mm)發生微爆機率高，不過兩者皆在醇類與生質柴油組分比1:1的情況下機率最高。醇類加入生質柴油液滴有助於增加其燃燒速率，在不微爆時，混合液滴在醇類重量百分比25%時達到最高；在微爆狀況下，則在50%時因微爆最劇烈而使平均燃燒速率最高。整體而言，重量百分比25%的甲醇在所有的組份中具有最高的燃燒速率，但其燃燒速率在過了此組分之後會隨著甲醇組分比增加而劇烈下降，而異丙醇混合液滴燃燒速率下降的趨勢則相對最緩。

關鍵字

微重力 ； 液滴燃燒 ； 生質柴油 ； 醇類 ； 微爆

並列摘要

In the present work, combustion of a multi-component droplet, blended with biodiesel and alcohol (methanol, ethanol and 2-propanol, respectively), was studied in a gravity-reduced environment. A “drop tower” facility was utilized to achieve the microgravity condition, and the falling time was about 0.68 s. Droplets of different sizes, i.e., 0.5 mm and 0.38 mm in diameter, respectively, were initially produced by a droplet generator. They were suspended at the crossing point of two ultra-thin fibers and ignited by a pair of electrical hot wires. The droplets then underwent a convection-free combustion, leading to formation of spherically symmetric flame. It was found that the existence of internal inhomogeneity directly contributed to occurrence of microexplosion. Via observation by using a CCD camera, explosion was yielded when a bubble was heated during the burning period to sufficiently high temperature. Furthermore, influence of different relative humidity of the environment was tested. The results revealed its little relevance to the formation of inhomogeneity in the droplet that was suspected to be caused by absorption of water on the surface. The stochastic nature of microexplosion was then found to correlate with the size of droplets, with a lower possibility of explosion in smaller ones. The maximum frequency occurred at 50% mass fraction of alcohol in all sizes of droplets, resulting in the corresponding highest averaged burning rate in the same proportion. In the meantime, the addition of alcohol into biodiesel showed significant enhancement of burning rate in the cases without explosion, with largest increment created when the mixture included 25% alcohol.

並列關鍵字

microgravity ； droplet combustion ； biodiesel ； alcohol ； microexplosion

參考文獻

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