再生代能源不僅可減少石化能源消耗，進而使造成全球暖化的溫室氣體排放減少，亦可減少對石化能源的依賴，因此再生能源已成為使環境永續發展不可或缺的角色。再生能源除了常見的風能和太陽能外，亦有海洋能源發電，雖然其中的洋流發電具有很大的能源潛力，但其效益卻仍未顯現出來。洋流發電有一項新的技術，是在傳統的水平軸渦輪機加上擴散導流罩來增加性能，此導流罩使洋流渦輪機明顯增加動力輸出。本研究的主要目的，是在導流罩外附加一個噴嘴裝置，來探討進一步提高擴散導流罩式渦輪機的流體動力學性能的可能性及物理現象，研究的途徑是利用流體力學計算分析軟體來做流場的模擬分析，並與未裝載具有噴嘴裝置之渦輪機進行性能比較。噴嘴的設計係採流道收縮式設計，此噴嘴裝置在擴散導流罩的外圍近出口處， 目的在於加速出口流體，因此增加導流罩入口與出口之間的壓力差，以及相對應的導流罩內的質流量，而跨過渦輪葉片表面的壓力差也變大了，進而產生了較大的扭力與功率輸出。各種設計配置在透過參數分析後，可確定哪些幾何參數是如何影響系統性能。實驗結果證實，擴散導流罩式渦輪機加入了噴嘴裝置之後的整體流體動力效率有進一步顯著的提高，流體動力效率最高可提升28.2％之多。

Nowadays, renewable energy plays a crucial role in building a more sustainable life for the world by cutting back the energy consumption from traditional fossil fuel sources-the main cause of global warming. Beside the conventional kinds of renewable energy such as wind and solar, marine current energy has yet been significantly deployed although it has the greatest potential of energy. One of the recent technology for marine tidal current energy conversion is the utilization and adaptation of the conventional horizontal-axis turbine with the implementation of diffuser-augmented duct. This duct enables the turbine performance be raised a significant percentage. The main purpose of this research is to study the possibility and physics that can further enhance the hydrodynamic performance of the diffuser-augmented tidal turbine with a nozzle add-on device. Hydrodynamic performance assessment and model design is based on computational fluid dynamics (CFD) simulations. A comparison of the potential performance of the original ducted turbine and the modified models with nozzle add-on was also carried out.
The add-on nozzle is designed to create a converged channel surrounding the duct outlet in order to accelerate the outbound flow and create a pressure drop surrounding the duct outlet. Hence, it creates a larger pressure difference between the inlet and outlet flow of the system. Wider pressure difference will result in a greater mass flow entering the system leading to the improvement in power output. Moreover, the pressure difference also is bigger across the blade surface and generates bigger torque and power output. Parametric study of the design in various configurations, mainly the nozzle length and the nozzle inlet/outlet area ratios, have been conducted to understand the influence of the geometric parameters on the turbine performance. The results confirmed that the overall hydrodynamic efficiency of the diffuser-augmented turbine has been significantly improved. The maximum hydrodynamic efficiency improvement was found to be equal to 28.2% with the nozzle add-on device implemented.

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