門關閉引致空化的流場計算分析

A Numerical Analysis of Cavitating Flow Due to a Closing Valve

在管流中，閥門的關閉作動往往對流體的運動帶來非常明顯的干擾，進而引發許多負面的效應，譬如在許多應用中，往往造成空化的出現，導致管材的腐蝕、噪音的產生、以及效率的降低、甚至引發振動。本論文運用計算流體力學的方式，配合動網格的技巧，以及「完全空化模型」（full cavitation model），分析流場可能產生的現象。計算結果顯示：在閥門行將關閉的前後剎那，先後有兩個空化氣泡出現在閥門長邊末端的導緣與尾緣，前者最後會脫離閥門表面，進入下游流場，但後者則始終附著在閥門表面，直到遭到壓破為止。在本論文中，我們也觀察空化前的流場現象、空化所持續的時間長度，計算結果與文獻中的實驗數據兩相比對之下，發現彼此相當吻合，顯示本計算模式可應用性。 In many engineering applications, cavitation may occur when a pipe flow or channel flow passes a closing valve Since cavitation may cause severe pitting due to erosion, create vibration and noise, and result in significant reduction of efficiency, it is vital to understand the cavitation phenomenon in order to avoid undesirable effects in pipe systems In the present study, we studied the physical phenomenon computationally. The full cavitation model was employed and the strategy of moving grid was developed for the investigation. The results show that two cavitating bubbles form subsequently when the valve is almost fully closed. One of them appears first at the leading edge of the end of the valve, grows, and finally moves into the downstream flow field: the other one appears somewhat later and attaches to the trailing edge till it collapses. Furthermore, the duration of cavitation and maximum speed al various time obtained in the present study agree quite well with those available in the literature.