利用微觀紋影技術觀察二元混合流體在微矩形管內沸騰時之Marangoni對流現象

Visualization of the Marangoni Convection During Boiling of Binary Mixtures in a Square Capillary Tube by Micro-Schlieren Technique

黃建元(Chien-Yuan Huang)

指導教授 ： 孫珍理

國立臺灣大學/工學院/機械工程學研究所/碩士(2013年)

https://doi.org/10.6342/NTU.2013.00045

摘要

本研究以控制熱通量的加熱條件，搭配微觀紋影技術，觀察耐熱玻璃製成之方型毛細管內的二元混合系統沸騰現象。分別探討方型毛細管管徑、工作流體濃度與不同二元混合系統對熱傳效果的影響及濃度所引起之Marangoni對流可視化之關係，最後利用紋影技術進行液氣介面附近濃度梯度之量化。本研究選用的方型毛細管管徑分別為200微米、500微米及900微米，工作流體為不同濃度的異丙醇/水 (2-propanol/water) 、仲丁醇/水 (2-butanol/water)、正丁醇/水 (1-butanol/water)、乙二醇/水 (ethylene glycol/water)、乙酸/水 (acetic acid/water)及丁酸/水 (butyric acid/water) 的混合溶液。二元混合系統依其表面張力與溫度之關係，可分為正混合系統與負混合系統兩種，其中正混合系統為異丙醇/水與仲丁醇/水兩種，負混合系統為正丁醇/水、乙二醇/水、乙酸/水及丁酸/水四種；而依有無共沸點可分為：具有共沸點的異丙醇/水、仲丁醇/水、正丁醇/水及丁酸/水四種，及不具共沸點的乙二醇/水及乙酸/水兩種。
實驗結果顯示，在方型毛細管管徑對熱傳效果影響方面，異丙醇/水及丁酸/水的混合系統，在不同濃度下，皆會受到管徑的影響且趨勢一致；正丁醇/水、乙二醇/水及乙酸/水的混合系統則是在高濃度之實驗條件下，較易受到管徑的影響；而在仲丁醇/水的混合系統中，熱傳係數之趨勢則是較不受到管徑改變之影響。在工作流體濃度對熱傳效果影響方面，異丙醇/水、仲丁醇/水、正丁醇/水、乙二醇/水及乙酸/水混合系統之熱傳係數皆會因莫耳濃度的不同而改變，而在丁酸/水的混合系統中，濃度的改變對於熱傳係數較無影響。在二元混合系統對熱傳效果影響方面，異丙醇/水及乙酸/水的混合系統受到Marangoni效應影響較為顯著，在低濃度時，異丙醇/水之混合系統受到Marangoni效應的影響較大，將遠離加熱面的冷流體拉引至靠近加熱面的區域，進而提升熱傳效果；而在乙酸/水的混合系統中，在低濃度時，乙酸/水之混合系統受到Marangoni效應的影響較大，將靠近加熱面的流體牽引至遠離加熱面的區域，進而降低熱傳效果。而在具有共沸點的異丙醇/水、仲丁醇/水及正丁醇/水混合系統中，實驗觀察到當管徑較小、濃度接近共沸點及工作流體之液態莫耳分率高於氣態莫耳分率時，成核氣泡會發生震盪現象。
在紋影觀察方面，當管徑越大時，越易觀察到紋影條紋，而在管徑為200微米時，皆無觀察到紋影條紋；經由計算並比較各實驗條件下溫度改變所造成之折射率變化及濃度改變所造成之折射率變化，可證實本研究所觀察到之折射率梯度主要係由濃度變化引起，因此利用濃度梯度與影像灰階值之關係式即可量化沸騰時液氣介面附近的濃度梯度。

關鍵字

微觀紋影技術 ； 二元混合系統 ； Marangoni對流 ； 折射率梯度 ； 濃度梯度之量化 ； 局限沸騰

並列摘要

In this study, we employ a micro-schlieren system to observe Marangoni convection induced by concentration gradient during boiling of binary mixtures in square capillary tubes of various sizes. The effects of channel size, concentration of working fluids and mixture characteristic on boiling heat transfer are investigated experimentally. From the relation between refractive index gradient and grayscale ratio, we are able to quantify the concentration gradient near the vapor-liquid interface.
The inner widths of square capillary tubes we use are 200 micrometer, 500 micrometer and 900 micrometer, and our working fluids are binary mixtures of 2-propanol/water, 2-butanol/water, 1-butanol/water, ethylene glycol/water, acetic acid/water and butyric acid/water. In general, binary mixtures can be classified into positive mixtures and negative mixtures. In this work, positive mixtures include 2-propanol/water and 2-butanol/water.
Our experimental results reveal that both size of the square capillary and concentration play a prominent role in the heat transfer coefficients. Yet, heat transfer of 2-butanol/water mixtures is independent of size of capillary and varying concentration has no impact on the heat transfer of butyric acid/water mixtures. Results of this work confirm that the concentration induced Marangoni convection is significant to the boiling of binary mixtures. For 2-propanol/water and acetic acid/water mixtures, we find out that the Marangoni effect is particularly vital at low concentration. Heat transfer of 2-propanol/water mixture is enhanced at low concentration because the surface tension gradient acts to induce liquid motion towards the heated surface. Nevertheless, opposite trend is found for acid/water mixture. An interesting phenomenon, bubble oscillation, is discovered in smaller square capillary tube when concentrations are close to the azeotropic point and the mole fraction of liquid phase is higher than that of vapor.
Usually, the schlieren patterns can be easily found in square capillary tubes of larger size. No schlieren phenomenon is observed in capillary of 200 micrometer in width. By comparing the refractive index derivatives with respect to temperature and concentration, we verify that the schlieren indeed corresponds to the concentration induced Marangoni convection. With the correlation of refractive index gradient and grayscale ratio, we are able to quantify the concentration gradients near the liquid-vapor interface during boiling of binary mixtures when the Marangoni convection is strong.

並列關鍵字

micro-schlieren technique ； binary mixtures ； Marangoni convection ； refractive index gradient ； quantification of concentration gradient ； confined boiling

參考文獻

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