Study on transition criterion from bubble flow to slug flow in rectangular channels

Using air and deionized water as working fluids, flow pattern characteristics of two-phase flow was investigated experimentally for two vertical rectangular channels with the cross sections of 1.41 × 40 mm² and 10 × 40 mm². Experiment was conducted under atmospheric condition, and the superficial velocities of air and water were in the ranges of 0.015 ∼ 0.59 m/s and 0.025 ∼ 3.74 m/s. Four classical transition criteria from bubbly flow to slug flow are evaluated against present experimental data and that from open literatures. The result shows that the four transition criteria are limited to some extent. Through the analysis of the dataset, it is found that aspect ratio is a key parameter to determine the critical transition void fractions for which two correlations are given according to the threshold of 10 mm of the hydraulic diameter. As a result, the modified criterion is obtained and is satisfactory agreement with the present data and the existing experimental data, and it is better than four classical transition criteria. (authors)