於強制脈衝流下對具發泡金屬塊之新穎平板式太陽能集熱器的熱傳性能研究

Thermal Performance Investigation of a Novel Flat-plate Solar Thermal Collector Using Metal-foam Blocks under Forced Pulsating Flow

陳志誠(Chih-Cheng Chen)

指導教授 ： 黃博全

國立臺北科技大學/機電學院/機電科技研究所/博士(2012年)

https://doi.org/10.6841/NTUT.2012.00663

摘要

由於傳統石化原料的日趨匱乏，且會衍生相關的環保問題，因此替代能源中之太陽能近年來受到重視。在太陽熱能系統中之主要元件為作為熱能轉換之集熱器，其效率的提昇可縮小設備尺寸、獲得更高溫熱源以作更廣泛的應用。本文以數值模擬方法探討在強制脈衝流下，對裝置有發泡金屬塊之新穎平板式太陽能集熱器的熱傳增強特性。文中統御方程式於純流體區流場遵守非穩態Unsteady Navier-Stoke方程式；於多孔材質區流場則以暫態Transient Forchheimer-Brinkman-Darcy模式來模擬多孔質熱沉內部之流動狀態；而能量方程式則依非局部熱平衡(LTNE)假設，使用固流兩相能量方程式模式，上述統御方程式並導入發泡金屬重要性質之實驗迴歸公式。接著以流線函數-渦度轉換公式及有限體積法(Finite volume method)來求解上述複合層熱流場之聯立方程組。在求解時藉由變化各項參數，來探討熱流場的變化及其對熱傳增強的效益。經由計算結果顯示，脈衝流週期性地改變由發泡金屬塊所引起的迴流結構，會影響到流場及熱場特性。此種結合發泡金屬熱沉與強制脈衝流的方式能有效增加熱傳率，藉此可提高平板式太陽能集熱器的集熱性能。此外，達西數為影響局部熱平衡（LTE）有效性之最主要參數。整體而言，在固相傳導為主導的熱傳輸模式下，隨固液兩相間之界面熱傳效果的增加，會導致局部熱平衡現象變得更加明顯，且太陽能集熱器的傳熱性能亦同時增強。最後，本文使用統計學上的迴歸分析理論，利用數值計算所獲得之數據，推導歸納出有用的迴歸關係方程式來估算平均紐賽數，可作為實際工程設計上之應用。

關鍵字

太陽能集熱器 ； 脈衝流 ； 發泡金屬 ； 非局部熱平衡 ； 多孔材質

並列摘要

The solar energy, which is one of alternative renewable energy, becomes more important recently due to the gradual scarcity and derivative environment problems of traditional fossil fuels. The major component of any solar thermal system is the solar thermal collector for thermal energy convert. The efficiency improvement for flat-plate solar collector can reduce its size and obtain higher temperature fluid for wider application. A numerical simulation was carried out to investigate the characteristics of heat transfer enhancement for a novel flat-plate solar thermal collector using metal-foam porous blocks as heat sinks under forced pulsating flow. The analysis is based on the use of unsteady Navier-Stokes equation in the fluid region, the transient Darcy-Brinkman-Forchheimer flow model in the porous region, and the two-phase energy models employing local thermal non-equilibrium (LTNE) assumption on the thermal field. The above governing equations are incorporated with empirical equations of metal-foam. A finite-volume integration method is employed to solve the dimensionless coupled governing equations for this porous/fluid composite system through the use of a stream function-vorticity transformation. This study details the effects of variations in the major control parameters to illustrate important fundamental and practical results. The results show that the periodic alteration in the structure of recirculation flows, caused by metal-foam blocks and flow pulsating, has a direct impact on the flow and thermal characteristics. The synthetic method indeed can be considered as an effective method to augment heat transfer and improve the efficiency of solar thermal collector. Besides, the Darcy number is the most influential parameter in determining the validity of local thermal equilibrium (LTE). In general, when solid conduction is the dominant heat transfer mode, the more efficient the interfacial heat transfer between solid and fluid phases is, the more obvious the local thermal equilibrium becomes. Further, the useful correlated equations to predict Num are proposed by regression analysis here for the application of realistic engineering design.

並列關鍵字

Solar collector ； Pulsating flow ； Metal foam ； Local thermal non-equilibrium ； Porous media

參考文獻

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