臺灣博碩士論文加值系統

控制閥在操作過程中容易產生空蝕作用, 空蝕作用即為當流體在局部壓力低於同溫度所對應之
飽和蒸汽壓力時, 會產生氣泡, 而當其流至壓力較高區域, 如下游流道有所壓力回復時, 氣泡會
因壓力變化而破裂, 此時管路壁面受到極大之壓力波衝擊而造成損害, 同時伴隨著噪音、震動及
效率降低。因此, 在控制閥的設計上如何避免或減少空蝕成為重要的課題。本研究將現實中生產
的球型閥與內部套筒重建成三維的流道模型, 利用數值的空蝕模型去模擬在球型閥內所產生的
空蝕情況與變化, 所使用的內部套筒分為單層多孔式套筒與單層階梯式套筒,而為了找出套筒的
存在對空蝕造成的差異, 額外考慮一個沒加套筒的單純球型閥。
根據數值模擬的結果, 從壓力等位面和流線圖可以得知流體在沒加套筒的球型閥會直接撞
擊閥體下緣, 造成一個高壓區域, 而渦漩會發生在其左側和下游管路, 而加裝了套筒後的球型閥
壓力分佈較小, 從壓力分佈曲線能得到在流體通過套筒時,比起沒加套筒的球型閥會產生一個強
烈的壓降, 但過了閥體靠近下游管路的部份時, 壓力變化則較平穩, 最後從空蝕現象的預測中,
我們可清楚看到空蝕發生在沒加套筒的球型閥的閥內和下游管路,當球型閥加上套筒後, 空蝕區
域則會被套筒所侷限於套筒的流道附近, 由此可知空蝕產生的範圍會因為套筒的有無而產生變
化, 故加上套筒後能進一步避免閥體和下游管路的損害。

Cavitation in a valve leads to many troubles and inconvenience for factories. It ruins valves in a piping system so that replacing those valves every several months is required. In order to reduce the cost caused by cavitation in a valve, a cage is utilized to make cavitation occur only in the region adjacent to the cage itself. Therefore, it only requires to replace the cage rather than the valve. To validate the design of a cage, simulation of the turbulent flow field inside globe valve and the occurrence of cavitation are necessary for a valve designer. To reach this purpose, prediction of the cavitation inside the globe valve with and without a cage is undertaken in this study. A cavitation model is established in this study. The percentage of vapors in each computational cell is calculated using the proposed cavitation model. Two various cages, the one-stage perforated cage and the one-stage step cage, are considered in this study.
Pressure contours and streamtraces show that the main flow passes through the plug and directly impinges the bottom of a globe valve without a cage in the present numerical results. Consequently, a high pressure region happens at the bottom of the valve. Vortices occur inside the valve body and at the downstream region of the globe valve without a cage while the vortex inside the globe valve with those two cages shrinks. The high pressure region at the bottom of the valve is reduced as well. It is observed that a steep pressure occurs when fluids flow through those cages, but the pressure variation at the
downstream region becomes milder in comparison to the result of the globe valve without a cage. It is found that vapor due to cavitation appears in the vortices existing inside the valve and at the downstream region of the globe valve without a cage. Nevertheless, vapor does not occur in those regions in the globe valve with those two cages. Vapors mainly appear at the exits of the flow passages of those cages. In other words, cavitation inside the globe valve with those two cages mostly occurs in the vicinity of the cages. It prevents that cavitation ruins the valve body and downstream region as those two cages
are installed in the globe valve. In addition to the globe valve, the proposed cavitation model can be applied to prediction of cavitation in other control valve as well.

Chinese Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v
Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
Nomenclatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x
List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii
List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xv
1 INTRODUCTION 1
2 MATHEMATICAL FORMULAE AND NUMERICAL MODEL 5
2.1 Numerical Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1.1 Governing Equations . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1.2 Cavitation Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1.3 Numerical methods and parameters . . . . . . . . . . . . . . . . . . 8
3 RESULTS AND DISCUSSION 11
3.1 Analysis of flow field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.1.1 Flow patterns and pressure contours . . . . . . . . . . . . . . . . . 11
3.1.2 Turbulence kinetic energy . . . . . . . . . . . . . . . . . . . . . . . 12
3.1.3 Prediction of cavitation . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.2 Analysis of pressure drop along a streamtrace . . . . . . . . . . . . . . . . 14
3.3 Calculation of total amount of vapor . . . . . . . . . . . . . . . . . . . . . 15
3.4 Characteristic coefficients . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4 CONCLUSIONS AND FUTURE WORK 17
4.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.2 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
CURRICULUM VITAE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

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