當流體的壓力超過其臨界壓力且其溫度超過臨界溫度時，該流體即屬於超臨界狀態。超臨界二氧化碳作為工作流體已廣泛應用於動力工程、化學以及能源等諸多技術領域。超臨界二氧化碳具有相對良好的熱物理性質，如具有相對較低的臨界壓力(~73.8 bar)與接近室溫的臨界溫度(~304.25 K)。二氧化碳的熱物理性質與輸送現象在接近其假臨界區域時會歷經劇烈的變化，進而顯著地影響超臨界狀態下的熱傳特性。因此，對於超臨界二氧化碳在流道中局部的熱傳特性，有必要進行研究探討，以提升系統的設計、穩定性能與安全之運轉。
　　本研究選用較符合環保的二氧化碳作為實驗環路的工作流體，目的在於探討超臨界二氧化碳在內徑2.2 mm光滑圓管下的熱傳特性。論文內容包含:超臨界熱傳實驗系統的架設與標準操作程序的建立，超臨界流體的熱物理性質蒐集與分析，並且進行超臨界二氧化碳在不同實驗參數(如: 系統壓力、質量流率、流動方向)下的熱傳實驗。
　　實驗所量測的流體溫度其對應之熱物理性質均可由NIST REFPROP 數據庫查詢而得。本研究進行不同流動方向，包含水平流動、垂直向上與垂直向下，的超臨界二氧化碳熱傳實驗。實驗結果顯示當流體溫度超過臨界溫度時(即為超臨界狀態)，其熱傳能力有惡化的現象；當流體溫度遠離假臨界點後，熱傳能力再次增強。在不同流動方向下，系統壓力的增加對於熱傳能力的影響不盡相同。三種流態下，質量流率的增加能顯著強化流體的熱傳能力；水平流動的熱傳能力最佳，垂直向下流動次之，垂直向上流動的熱傳能力最差。

Supercritical fluid with the pressure and temperature above the pseudo-critical point has been widely adopted in power engineering, chemical process, energy system and other technical fields. Supercritical carbon dioxide relatively has a low critical pressure (~73.8 bar) and a low critical temperature (~304.25 K) close to room temperature at the critical point. The thermophysical properties and transport phenomena for supercritical carbon dioxide will experience dramatic changes near the pseudo-critical point to affect the heat transfer characteristics under supercritical conditions. Therefore, for the enhancement of the design and safe operation of such supercritical systems, it is necessary to carefully explore the local heat transfer characteristics of supercritical carbon dioxide in the flow channel.
Carbon dioxide, which is more environmentally-friendly, is adopted as the working fluid in this experimental loop. The objective of this study is to investigate the local heat transfer characteristics of supercritical carbon dioxide in the smooth circular miniature tube with an inner diameter of 2.2 mm. This study includes the establishment of the supercritical heat transfer experimental loop and experiment procedures. Meanwhile, the collection and analysis of thermophysical properties for supercritical carbon dioxide. The heat transfer experiments are conducted under various system parameters, i.e. operating pressures, mass flow rates and flow directions.
The local thermophysical properties of the carbon dioxide fluid corresponding to local measured temperatures can be acquired from the NIST REFPROP database. The heat transfer experiments of supercritical carbon dioxide are conducted under different flow directions, i.e. horizontal, vertical upward and vertical downward. The experimental results show that the heat transfer deterioration of the fluid would occur when the fluid temperature passes through the critical temperature point. However, the heat transfer will be enhanced again while the fluid temperature is far from the pseudo-critical point. The increase of the mass flow rate will enhance the heat transfer, while the effect of system pressure is different, among the cases in three flow directions. As the comparisons among three flow directions in this study, the horizontal flow has the best heat transfer performance and thus the vertical downward flow, whereas the vertical upward flow is the worst one.

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