氧氣及氮氣於T型微混合器之混合效應研究

Mixing Effects of Oxygen and Nitrogen Gases in T-type Micromixers

萬紹安(Shaw-An Wan)

指導教授 ： 黃智永

國立清華大學/工學院/動力機械工程學系/碩士(2015年)

摘要

本研究以數值模擬與實驗量測探討氮氣與氧氣於T型微混合器中之混合效應。先以ANSYS CFX對T型微流道進行數值模擬，改變微流道深寬比及流體的雷諾數，以量化的方式探討其流場現象及混合效率，並做為實驗配置時的參考依據。實驗量測方面，以壓克力片與PET基材雙面膠帶製作了主流道與入口流道幾何尺寸相同的T型微混合器，一種長為10 mm、寬為550 um、深為125 um，另一種流道長度及深度不變，寬度增加為720 um。實驗利用PSP (Pressure-Sensitive Paints)螢光壓力感測技術，藉由螢光分子的光致發光及氧氣淬滅機制，結合螢光顯微鏡及2倍物鏡以非侵入的方式量測T型微混合器內全域且連續性之二維氧氣濃度分佈，並佐以即地校正方式及逐點影像校正法提高實驗量測準確度。
　　於低雷諾數範圍內，當微流道寬度為550 um、雷諾數為13.9時，微混合器出口的混合效率為95.92%，此時兩氣體間的混合由分子擴散機制主導。若是隨著入口氣體流量提升，軸向速度的增加使得未充分混合的流體不斷被帶往下游，這時氣體分子缺乏足夠的時間往橫向擴散。當雷諾數為50.9時，混合長度增長，且出口混合效率降低至64.64%。但是當微流道改變成720 um，此時雷諾數為14.0，氣體分子之間的擴散距離增長，且流道深寬比降低會使得上下壁面剪應力對混合氣體的影響提升，導致此時出口效率比550 um降低了8.7%。於高雷諾數範圍內，當流道寬度為550 um、入口氣體雷諾數為596.1時，實驗觀察到流體產生了翻轉現象，此時進入捲入流範疇，兩氣體間的接觸面積增加且分子擴散距離縮短使得混合效率提升。但是在相同入口流量的情況下，使用720 um寬微混合器中則未觀察到捲入流現象。其原因是流道深寬比較低，不利於渦漩產生，且上下壁面剪應力相對較強而抑制流體翻轉。另一方面，藉由單獨增加氧氣入口流量，非對稱之入口條件也能觀察到流體翻轉的現象，且隨著兩端雷諾數差異愈大，此現象將會愈明顯。
　　本文利用PSP螢光壓力感測技術，成功量測得T型微混合器中之二維氧氣濃度分布，同時探討改變入口流量及流道寬度對於氧氣及氮氣混合效應及流場特性的影響，並輔以數值模擬的結果進行比對，完成了微尺度下氣體混合效應之研究。

關鍵字

微混合器 ； 氣體混合 ； 螢光壓力感測技術 ； 微流體

並列摘要

This study aims to investigate the mixing effects of oxygen and nitrogen gases in T-type micromixers. Commercial CFD software ANSYS CFX is used to simulate the flow fields inside the T-type micromixers with different aspect ratios of microchannels and Reynolds number effect, and the physic phenomena in flow field and mixing efficiency are analyzed quantitatively. For experimental approaches, PMMA sheets and double-sided tape made by PET film are used to fabricate T-type micromixers, which have rectangular cross-section and inlet and main (mixing) channels are in the same size. The microchannel is 10 mm long, 125 um deep and 550 um / 720 um wide. A non-intrusive experimental technique, pressure-sensitive paints (PSP), is applied to T-type micromixers for acquiring the global flow field with detailed oxygen concentration during the measurements. The spatial resolution and accuracy in the acquired data has been further improved by integrating a luminescence microscope with 2X objective lens and applying in-situ and pixel-by-pixel calibration during data processing.
　　From the experimental results, it can be clear seen that molecular diffusion is dominant during gaseous mixing at low Re numbers from 13.9 to 50.9. The mixing efficiency at micromixer outlet can reach to 95.92% while the width of microchannel is 550 um and Re is 13.9. If axial flow speed in the main (mixing) channel increases and inlet gas flow rate increases (Re number of 50.9), the flow quickly brings fluid to downstream and only partly fluid are mixed; therefore, the mixing efficiency decreases to 64.64%. Furthermore, the molecular path increases and shear stress on the upper/lower wall become relatively large if using microchannels with width of 720 um since aspect ratio decreases. The mixing efficiency at the outlet decrease to 8.7% compared to the microchannel with 550 um width.
　　If the inlet flow rate further increases to 149.7 ccm in the 550 um wide microchannel (Re=596.1), the symmetry in the flow field is no longer exist and engulfment flow regime is identified. In this regime, the interfacial area between nitrogen and oxygen gases increases due to the engulfment flow which shortens diffusion path and improve mixing performance. The asymmetrical flow rates at the inlets of T-type micromixers can also lead to the improvement of mixing. Engulfment flow cannot be seen in 720 um wide micromixer with the same flow rate due to the lower aspect ratio.
　　To sum up, the feasibility of PSP sensor (Ru(dpp) with silicone rubber) in T-type micromixers has been demonstrated to provide global oxygen concentration information which has good agreements with numerical results.

並列關鍵字

Micromixer ； Gas mixing ； Pressure-sensitive paints (PSP) ； Microfludics

參考文獻

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