| Summary: | 碩士 === 國立中央大學 === 機械工程學系 === 102 === 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.
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