The mechanism of bubbly to slug flow regime transition in air-water two phase flow: A new transition criterion

Air-water two-phase upward flow experiments are conducted in a tubular test section with the inner diameter of 25.4 mm in order to investigate the transition mechanism of bubbly flow to slug flow. Flow regime identification is carried out by using ReliefF-FCM clustering algorithm, i.e., a new objective flow regime identification method. It is found that the velocity ratio decreases with the increase of superficial gas velocity in bubbly flow at the constant liquid superficial velocity. And the velocity ratio always reaches its minimum during the flow regime transition. The present research finds that the changes of bubble size and shape may result in the decrease of velocity ratio in bubbly flow. From this point of view, a new mechanism for bubbly to slug flow regime transition has been proposed. A transition criterion based on the mechanism is also built. The comparison of the transition criterion with the experimental results at different flow conditions is carried out. Although the transition criterion is empirically modified based on the present experimental data, it shows the reasonable agreements.