醇類添加奈米顆粒之破碎與微爆機制研究

On the Disruption and Microexplosion Mechanisms of Burning Alcohol Droplets with Addition of Nanoparticles

賴彥偉(Yen-Wei Lai)

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(2020年)

https://doi.org/10.6342/NTU202002927

摘要

由於微爆現象的發生可以增加燃燒速率，因此在燃燒相關領域被廣泛的研究，而本篇主要探討在沒有顆粒燃燒及界面活性劑的影響下，奈米顆粒添加對於燃燒過程的影響，以及顆粒彼此間的性質差異所產生的不同燃燒情況，並期望燃燒過程中會有微爆及破碎現象的發生。本篇論文將三種陶瓷奈米顆粒(氧化鋁、氧化矽、氧化鈦)，添加進入三種醇類中進行分析(乙醇、異丙醇、丁醇)，實驗中採用懸掛液滴法在無對流與自然對流及自由液滴法在強制對流的環境下分別進行實驗，發現對流強度的增加及不同種顆粒的添加，可以使得原本會熄滅的液滴，熄滅時間推遲甚至燒完，也可以使液滴從燒完變成有破碎甚至有微爆現象的發生，而在燃燒過程中，微爆的發生可以使得其燃燒速率比純油料提高46%，大幅提高燃燒速率。
在燃燒過程中，添加氧化鋁及氧化矽，發現在液滴外層會有膠狀物質形成，其可以限制氣泡向外跑出，而這是造成微爆的其中一個條件，然而，醇類添加氧化鋁及氧化矽並不是在每一種情況下都會微爆，還需要在燃燒過程中，吸收到足夠的熱，因此也會受到油料、對流場強度的影響，例如在同一油料且同一對流場的情況下，顆粒紅外線吸收率不同也會導致吸收到的熱量不同，進而影響微爆的發生與否，如丁醇添加奈米顆粒在自然對流情況下，微爆只發生在添加擁有較高熱輻射吸收率之氧化矽，而添加氧化鋁則不會。總結來說，將奈米顆粒添加於油料中，微爆要發生的條件有兩者，一是利用顆粒的特性在液滴表面形成可以將氣泡限制在內部的膠狀物質，二是在燃燒過程，液滴內部油料可以吸收到足夠的熱量，使液滴內部產生成核現象，同時具備兩者則可以在醇類油料中造成微爆。
最後本篇論文也探討液滴初始大小，對於燃燒過程的影響，隨著液滴初始尺寸的增加，液滴在燃燒過程中產生的自然對流強度會上升，因此液滴的燃燒速率也會跟著上升，而在純乙醇的實驗當中，還發現隨著液滴初始尺寸的增加，液滴熄滅尺寸會跟著增加，原因是當液滴越大，其在燃燒過程中，可以吸收水分的表面積也會越大，因此在燃燒過程中，較大的液滴會吸收到更多的水分，也代表著隨著初始液滴直徑越大，熄滅尺寸也越大。

關鍵字

液滴燃燒 ； 醇類 ； 非均勻成核 ； 奈米流體 ； 微爆

並列摘要

Microexplosion is extensively studied in the field of combustion since the occurrence of microexplosion can increase the combustion rate. The objective of this study is to control the occurrence of microexplosion by using nanoparticles as additives without the effects of surfactants and ignitable particles. We added three kinds of ceramic nanoparticles (Al2O3, SiO2, TiO2) into three kinds of alcohols, including ethanol, isopropanol and butanol under different convection conditions. The suspended droplet method was performed in the convection-free or natural convection environment and free-falling droplet method was applied in the forced convection environment.
The combustion characteristics of droplets changed from extinction to burnout or from burnout to disruption or microexplosion because of increasing intensity of convection and the addition of different kinds of particles. During combustion, droplets would deform owing to the presence of gelatinous substance in the outer layer if alumina and silica were added. This is one of the reasons that causes microexplosion. Another is sufficient heat absorption during combustion. For example, when adding alumina and silica into butanol under natural convection, which both resulted in droplet deformation, microexplosion happened only in the case of SiO2/butanol. By comparing infrared absorption ability, it could be known that their ability to absorb infrared radiation was different. This heat absorption difference determined whether microexplosion happened or not. To summarize, there are two conditions for the microexplosion when nanoparticles are added into the fuel. One is the formation of gelatinous substance on the droplet surface, which can trap bubbles in the droplets. The other is enough heat absorption during the combustion. The occurrence of microexplosion could make burning rate increase by 46% in comparison with pure fuel.
Finally, this paper also discussed the effects of initial droplet size on the combustion. As the initial size of the droplet increased, the intensity of natural convection increased. Therefore, the burning rate of the droplet would increase. In the ethanol experiment, it was also found that the extinction size of the droplet increased as the initial size of the droplet increased because more water were absorbed by the larger droplets (larger sphere area).

並列關鍵字

Droplet combustion ； Alcohol ； Heterogeneous nucleation ； Nanofuel ； Microexplosion

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