水冷引擎的主要散熱方式和水套的幾何設計有直接的影響，因此評估水套的散熱能力對引擎的設計非常重要。本文建立400cc四行程單缸水冷引擎之水套流場分析模型、引擎純熱傳導模型（pure conduction model）及引擎共軛熱傳模型（conjugated heat transfer model）三種模型，透過計算流體力學（Computational Fluid Dynamics, CFD）的方法並搭配相關商用套裝軟體ANSYS Fluent模擬水套內之熱液動性質，並改變不同轉數下之入口流速，觀察流場分佈的改變以及散熱效率的提升，藉此分析水套對引擎溫度分佈的影響。而引擎熱傳模型則能夠觀察水套對引擎整體溫度分佈的影響，在設計初期就可以評估水套對於引擎重要部位如氣閥座與火星塞座之散熱效果及熱分佈情形，並分析冷卻系統能將引擎本體溫度均勻化的程度。
由數值結果可知，當水套入口流速由0.58m/s提升至2.40m/s時，壓差呈現類似拋物線之關係式，由716Pa上升至11663Pa；而平均熱對流係數則呈現線性關係，由2275 W/m2/K增加為6929 W/m2/K。模擬的結果能看出水套內流動緩慢處，如兩墊片開孔中間、環繞汽缸壁處等，藉由流場的分佈可以預估其產生高溫之區域。整體引擎熱傳模型與實驗比較的誤差中，汽缸頭最大誤差為15%、汽缸最大誤差在20%內且其餘量測點則小於10%，而純熱傳導模型與共軛熱傳模型之誤差則在5%以內。最後對排氣閥座進行溫度均勻性的分析，探討不同水套流速對排氣閥座的影響。

By using the computational fluid dynamics (CFD) commercial software ANSYS Fluent, different numerical models are established for a 400c.c four strokes single cylinder water-cooled motorcycle engine. They are jacket flow field model, engine pure conduction model and engine conjugated heat transfer model respectively. With these models, the flow field, thermal hydraulic characteristics and temperature distribution with jacket inlet velocity (or engine speed) can be observed and analyzed. These results could reduce the time and cost to develop a new engine. The numerical results show that when the jacket inlet velocity is increased from 0.58m/s to 2.40m/s, the pressure drop is raised from 716Pa to 11663Pa and seems a parabolic curve with inlet velocity; the average heat transfer coefficient is enhanced from 2275W/m2/K to 6929 W/m2/K and being linear with inlet velocity. Comparing with experiments, the average errors are under 10%.

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