密閉式冷卻水塔為工業界常用之熱交換器，其具有冷卻能力較一般乾式熱交換強之優點。密閉式冷卻水塔有三個工作流體：管内流體、淋水與濕空氣，管內流體可以是水或壓縮空氣等流體，管内流體經由管壁將熱量傳遞給淋水，而淋水則透過顯熱熱傳與潛熱熱傳將熱量傳遞至濕空氣，由於熱量傳遞的過程中有潛熱熱傳的存在，故管內流體理論上可以達到的最低溫度為空氣進口濕球溫度，低於乾式熱交換器所能達到的最低溫度。
本論文首先會依據能量與質量守恆推導逆流式密閉式冷卻水塔的一維數值解數學模型，並透過適當的假設將一維數值解模型簡化為一維近似解析解模型。藉由一維數值解與一維近似解析解可以初步了解密閉式冷卻水塔中各工作流體之間熱量傳遞的過程，以及各工作流體的溫度與空氣的濕度分佈。
接著本論文會使用計算流體力學軟體搭配使用者定義函數(user define function, UDF)進行數值模擬，由於密閉式冷卻水塔的物理現象較為複雜，難以透過數值方法進行完整的模擬，故本論文會針對淋水與濕空氣在管群中的熱質傳遞過程進行數值模擬。本研究的模擬模型為四根交錯排列的圓管所組成之管群，圓管管壁為等溫壁面，淋水由圓管上方滴淋至管壁上並形成水膜；空氣則由流道下方進入，與淋水呈逆向流動。本論文針對不同的濕空氣進口速度、相對濕度以及淋水密度進行模擬，並討論各參數的改變對熱質傳過程的影響。
模擬結果顯示，濕空氣進口流速的改變對熱傳量具有明顯影響，當濕空氣進口流速由2m/s增加至3.5m/s時，總熱傳量增加29.33%，其中熱傳量的增加是水膜表面的顯熱熱傳係數與質傳係數增加所造成。濕空氣進口相對濕度愈高則總熱傳量愈低，而總熱傳量的減少主要是潛熱熱傳量減少所致，進口相對濕度的改變對總熱傳量的影響顯著，當進口相對濕度由40%增加至70%，總熱傳量減少了14.86%。淋水密度增加會使總熱傳量減少，但熱傳量的減少幅度相較於淋水量的增加幅度而言十分小，當淋水密度增加至原本的4倍，總熱傳量僅減少19.22%。
本論文的最後會透過Visual Basic程式開發密閉式冷卻水塔電腦輔助設計軟體。本軟體有兩個主要功能：性能評估與尺寸設計，性能評估功能可以根據使用者設計之工作流體進口條件與冷卻水塔尺寸評估該冷卻水塔的性能，即計算該冷卻水塔的工作流體出口狀態、總熱傳量以及壓降等性能參數；尺寸設計功能則是在已知工作流體進口狀態的條件下，根據使用者所需要達到的總熱傳量推算所需的工作流體質量流率與密閉式冷卻水塔的尺寸。

The purpose of this study is to analyze the heat and mass transfer process of the closed-type cooling tower. There are three parts in this study. First, this study derived the 1-D numerical model of a closed-type cooling tower, then simplified it to 1-D approximate analytic model. The difference of the total heat transfer rate between the two mathematical model is 1.79%.
In the second part, this study used computational fluid dynamic software with the assist of user define function (UDF) to simulate the heat and mass transfer between the falling film and humid air in tube bundle, and discussed the effect of different air inlet velocity, relative humidity, and film density. The result of the simulation shows that the increase of air inlet velocity affects the heat transfer rate significantly. As the air inlet velocity increases from 2m/s to 3.5m/s, the total heat transfer rate increases 29.33%. Air inlet relative humidity also has big effect on heat transfer rate. As the air inlet relative humidity increase from 40% to 70%, the total heat transfer rate decreases 14.86%. However, the increase of film density has no large effect on heat transfer rate. The total heat transfer rate decreases 19.22%, as the film density increase from 0.05kg/m·s to 0.2kg/m·s.
In the third part, this study developed a CAD software for the closed-type cooling tower. There are two main functions in the CAD software: rating and sizing. The rating function calculates the performance of a closed-type cooling tower as the inlet conditions of working fluids and the dimensions of the cooling tower are known. The sizing function assists users with designing the dimensions of cooling tower and deciding the flow rate of each working fluid in order to achieve the required performance.

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