多U型地埋管熱交換器之流場最佳化設計

Abstract

在淺層溫能結合熱交換器中，最常見的型式為地埋管熱交換器，而地埋管熱交換器的換熱方式及殼內回填材料大都以土壤為主。本研究將針對多U型殼管式熱交換器埋入地表下深10公尺的地下水中，並且將熱交換器底部多處開孔，讓地下水滲入殼內，與傳熱管進行熱交換。本研究將利用CFD模擬軟體ANSYS FLUENT進行分析，針對多U型地埋管熱交換器的入水孔相對位置、入水孔高度、擋板間距、傳熱管排列角度及傳熱管間距差之參數的改變，將流場可視化，並算出其壓損及流場均勻度指標，用其數據來評判熱交換器的流場及熱傳效能，為達到熱交換器優化設計，利用實驗設計法中的田口法結合灰關聯來分析，求出多U型地埋管熱交換器的最佳水準組合，品質特性採用壓損、X方向均勻度及Z方向均勻度，最後結果顯示，其最佳化組合參數為：入水孔相對位置：down、入水孔高度295 mm、擋板間距59 mm、排列角度30ﾟ及傳熱管間距差5 mm。且其灰關聯度與預測值之相對誤差僅0.54%，因此證明其方法能準確及有效改善流場特性，最後利用巨觀的殼側熱傳係數分析方式，驗證了只利用流場分析的有效性，並能有效改善熱傳性能。

Among the various kinds of shallow geothermal energy combined with exchangers, the most common type is borehole exchanger (BHE). The heat transfer methods and the backfill material in the shell of BHE relies mainly on the soil. In this research, we sink the Multi-U type BHEs (MUBHEs) ten meters deep down to the groundwater. After that, the exchanger bored multiple holes will be placed at the bottom, and the tube transferred heat with water which flowed into the shell part. The analytic methods to carry out this study are the use of the software, CFD “ANSYS FLUENT”, the change of parameters, including the relative position of water hole, the height of water hole, the baffle spacing, the tube layout and the pitch difference to calculate the press drop and the flow field uniformity. To optimize the results of MUBHE, we use the Taguchi method and grey relational analysis to testify the result. The pressure drop and the flow field uniformity are used in the evaluation of MUBHE's quality characteristics. According to the numerical results, the result can be found by the optimal combination of parameters is the relative position of water hole: down, the water hole height of 295 mm, the baffle spacing of 59 mm, the tube layout of 30ﾟand the pitch difference of 5 mm. Thus, the relative error of the actual grey relation and the predictand is only 0.54%, by means, this method can accurately and effectively improve the characteristics of the flow field.