- 學位論文
中子照相應用於小管徑矩形管之雙相流可視化研究
Application of neutron radiography to visualization of two phase flows in narrow rectangular ducts
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摘要
本研究的目的為使用中子照相方法進行雙相流測量。中子照相被用來作為垂直流與水平流的流譜觀測與空泡分率之觀察工具，小管徑矩形管厚度分別為1mm和5mm，寬皆為60 mm。流譜操作在氣泡流與彈狀流之流動狀態，在快門時間皆為0.2秒時進行在金屬通道之氣-水雙相流的可視化拍攝。光學拍照做為與中子照相方法之比較，使用中子照相方法觀察與測量之結果顯示，這些圖像的品質並不好需要靠影像處理改善，故建立處理雜訊與空泡分率計算之流程。 由於雙相流設備與測量中子之閃爍器兩者距離33cm，影像受到中子散射導致失真。使用Σ-scaling method計算雙相流之空泡分率，5mm矩管雙相流空泡計算結果得知，快門時間太長導致空泡分率計算結果高於理論值。1mm矩形管易受表面張力影響導致漂移速度幾乎為0且氣泡速度比5mm矩形管慢，實驗結果可知，隨著氣流量增加，空泡分率也隨之上升。為了了解Σ-scaling method之結果與實際值之誤差，使用1~4mm厚度不同之金屬分別放入5mm金屬矩形管。結果顯示4 mm水層厚度之測量誤差最小，5 mm受到中子通量不均勻而導致測量誤差上升。
並列摘要
The purpose of this study is to investigate measurement of two-phase flow by using a neutron radiography. This system is utilized to determine the flow regime and void fraction in narrow rectangular vertical and horizontal channels, gap widths 1mm and 5 mm with widths of 60 mm. The channel is operated as a bubbly and slug flow state. Visualization of air-water two-phase flow in a metal channel was performed at exposure time 0.2 second for neutron radiography. The optical system is used to compare with neutron radiography method. Results confirm the qualities of these images are not good to observe and measure two-phase flow in neutron radiography. A process was established in this study in order to reduce the noise and calculate void fraction. Separation distance between the test section and the scintillator is 33 cm, this results in image distortion due to neutron scattering. Σ-scaling method was used for void fraction of two-phase flow. Exposure time is so long that the void fraction calculation for gap widths 5 mm is larger than predicted value. For gap widths 1mm of two-phase flow, the result of void fraction is smaller than predicted value. Due to surface tension, the drift velocity is almost 0 and bubble velocity is not faster than gap widths 5 mm. Results confirm the void fraction tends to increase as the gas velocity increases. In order to understand the difference between the result by Σ-scaling method and true vlaue, metallic thickness from 1 to 4 mm are put into 5 mm metallic duct, respectively. It is found that the measured error for water layer thickness of 4mm is minimum. The measured error for full water is greater because neutron flux is not well-distributed.
參考文獻
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