Application of neutron radiography to visualization of two phase flows in narrow rectangular ducts

• Main Authors: Hsiao, Yu-Hsun, 蕭鈺勳
• Other Authors: Chen, Shao-Wen
• Format: Others
• Language: zh-TW
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/38839748865645671562

碩士 === 國立清華大學 === 核子工程與科學研究所 === 103 === 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.