質子交換膜燃料電池中流道幾何形狀對其性能影響之數值探討

Abstract

本論文係以數值模擬方法來探討不同幾何形狀的蛇形流場板對質子交換膜燃料電池的性能影響，並分析此項研究參數所得出的不同變數之分佈變化。本模擬使用商用套裝軟體CFD-ACE+來建構一個穩態、三維、雙相流、多物種並包含電化學反應的數值模型。論文內容可分為三部分。第一部分主要是分析流場、溫度場及其他電化學變數在質子交換膜燃料電池內的基本現象。從模擬結果得知，電流密度、溫度以及水含量彼此的分佈情形呈現緊密的正相關性。而其中存在於邊緣勒條的些許差異來自於等溫的邊界條例。另外，這三項變數從陽極流道入口逐漸向陽極流道出口遞減，說明其分佈情形主要受制於氫氣濃度的影響。除此之外，當操作溫度超過348K時，液態水的生成變得相當微弱並未在電池內產生水氾濫的情形。第二部分主要是研究在兩個不同操作溫度(323K、353K)下不同彎道角度(45°、60°、90°、120°與135°)的流場板對質子交換膜燃料電池的性能影響。數值計算結果顯示，因著質傳擴散速率的提升，結合60°與120°的流場板可以得到最好的性能尤其在低操作電壓下(0.4–0.6 V)。然而，不同角度的流場板的性能差異隨著操作溫度的降低而增加，說明彎道角度對性能的影響跟操作溫度呈負相關。另一方面，從薄膜溫度分佈圖中得知不同角度流場板的溫度分佈是相似的，說明改變流道角度並未能降低薄膜裡的溫度變化。第三部分接著探討不同彎道寬度的流場板在相同於第二部分的操作條件下對質子交換膜燃料電池的性能影響。從模擬結果得知，因著物質擴散能力的提升，擁有較寬彎道的流場板可以得到最好的性能。然而，從電流密度及溫度的分佈圖中得知，該兩項變數在較寬彎道流場的薄膜中分佈地相當不平均並且伴隨著熱點存在於彎道裡，而此現象對薄膜的使用壽命有相當程度的傷害。

This study numerically investigated how the geometry of serpentine flow pattern influences performance of proton exchange membrane fuel cell (PEMFC), and analyzed how these parameters lead to different distributions of model variables. Three-dimensional simulations were carried out with a steady, two-phase, multi-component and electrochemical model, using CFD-ACE+, the commercial CFD code. This thesis consists of three parts. In first part, the fundamental behaviors of the flow field, temperature and the electrochemical variables inside a PEMFC are analyzed. From the numerical results, it shows a close and positive correlation between the distributions of current density, temperature and water content with only a slight discrepancy existing at the marginal rib due to isothermal conditions. Also, these three variables decrease gradually from anodic inlet toward anodic outlet, indicating that their distributions are principally dictated by the hydrogen concentration. Additionally, with the cell temperature increased beyond 348K, liquid water formation doesn’t appear to be considerable nor result in flooding effect inside the cell. In the second part, the effects of bend angle on the PEMFC performance is studied with various angles (45°, 60°, 90°, 120° and 135°) with cell temperature of 323K and 353K. The numerical results indicate that the combination of 60° and 120° enables flow pattern to achieve the highest performance, especially at low operating voltages, due to the increase mass diffusion rate. Also, the differences in performance for different angles become more noticeable with decreasing cell temperature, implying that the influence of bend angle on the performance is inversely proportional to the operating temperature. On the other hand, the temperature distributions of flow patterns in the membrane with different angles are more or less similar indicating the variation of temperature in the membrane is not reduced from the change of bend angles. In the third part, the effects of bend width on the PEMFC performance are subsequently studied under the same operating conditions applied in the second part. Simulation results show that flow pattern with wider bend width achieves the highest performance compared to patterns with narrower width results from the enhanced mass diffusion. However, the distributions of current density and temperature in the membrane with wider bend show a high non-uniformity with the existence of hot spot at bending areas that is fatal to membrane lifetime.