| 摘要: | 球閥應用相當廣泛，除了當做流體的開關，還可調節流體的流量和壓力。它未來的發展趨勢，能夠用在大型化，高溫和高壓，能承受外界較惡劣的環境，管壁能耐腐蝕，又能用在長輸管線。其中高溫和高壓會衍生出空蝕問題。空蝕會把管壁材料受損，而影響整個球閥的性能。空蝕嚴重時會有哽塞流現象，使壓力差增加時，流量不會再增加，會失去閥的流量調節性。
本文用ANSYS FLUENT套裝軟體來進行數值模擬，研究重點是空蝕的預測。空蝕現象會有相變化，模擬空蝕需用兩相流模型。結果可以得氣態體積比例圖，來呈現球閥所含的蒸氣體積分量，這和空蝕現象描述相近，它是預測空蝕較準確的方式。本研究用不同壓力差和球閥轉角，來討論空蝕對不同壓力差和球閥轉角的影響。再用模擬結果算出球閥性能，討論性能對不同壓力差和球閥轉角的影響，還有對空蝕的相關性，並和氣態體積比例圖的結果比較，來驗證空蝕預測的準確性。
模擬結果得知，預測空蝕最準的方式是氣態體積比例圖，其次是空蝕係數和流動係數，最不準的為壓力圖。用壓力圖判斷空蝕比氣態體積比例圖會高估許多，所以壓力低於蒸汽壓空蝕不一定會發生。其空蝕的影響因素，球閥轉角的影響性會比壓力差的影響大很多，代表流場的幾何形狀是空蝕影響的主要因素。損失係數和流動係數主要由球閥轉角來改變，而對壓力差影響較小，而空蝕係數的改變，球閥轉角和壓力差都有影響。球閥適合操作的轉角和壓力分別為30度以內， 2 bar以下，若球閥完全沒有空蝕發生，轉角要調到20度以內。
;Ball valve has more applications, in addition to the switch as a fluid, it can regulate fluid flow and pressure. These are trend of development in the future, can be used in large-scale, high temperature and high pressure, can withstand harsh environments compared to outside, The wall can resistant corrosive, and can be used in long-distance pipeline. High temperature and pressure will rise to cavitation problems. Cavitation will damage the wall material, which will affect the performance of the whole ball valve. Severe cavitation will occur choking flow, so that the pressure difference increases, the flow rate does not increase, it will lose the flow adjustability .
In this thesis, uses ANSYS FLUENT software package for numerical simulation research which focused on cavitation prediction. Cavitation will change phases, so it required two-phase flow simulation model. The results can be obtained volume fraction. It can show the fluid which contains vapor volume fraction in the ball valve. It is similar to describe cavitation phenomenon, and is more accurate to predict cavitation method. In this study, using differential pressure and the ball with different angle, discuss cavitation the effects of pressure difference and ball valve angle. Simulation results calculate ball valve performances, discuss performances the effect of pressure difference and ball valve angle, as well as the relevance of cavitation. These results compare to volume fraction to verify the accuracy of cavitation prediction.
From simulation results, the most accurate way for cavitation prediction is volume fraction, secondly is cavitation index and flow coefficient, the most inaccuracy is pressure contour. Using pressure contour would overestimate a lot with volume fraction, so the pressure is lower than the vapor pressure of cavitation may not occur. Cavitation effects on factors, ball valve angle will be much effective than the pressure difference, the geometry of the flow field is a major factor in cavitation effects. Loss coefficient and flow coefficient is mainly to change ball valve angle, but has little effect on the pressure difference. For change cavitation index, ball valve angle and pressure difference will be affected. ball valve is operated suitable angle and pressure are within 30 degrees, 2 bar or less, if the ball valve is no cavitation occurs within 20 degrees angle to be adjusted. |
