Experimental Investigation of Rising Gas Bubble Characteristics from a Vertical Tube under CCFL Condition

This paper describes an experimental study of gas/liquid countercurrent flow in a vertical circular tube. CCFL experiments were carried out with three different water levels in the upper plenum, two different tube diameters. Measurements were made for liquid and gas flow rates, time variations of pressure at locations of the upper entry of the tube and lower plenum. Visual observations were also conducted to investigate the relationship between rising gas bubble characteristics and time variation of gas pressure at the upper entry of the tube. The results indicate that one bubble formation cycle (e.g., bubble growth, expansion, and detachment into the water pool) corresponds to one pressure fluctuation cycle. For the 20？mm diameter tube, it was confirmed that there was a characteristic waiting time between bubble cycles in which no bubble was formed at the upper entry of the tube. The waiting time is a favorable time for a liquid introduction into the tube from the upper plenum. The bubble volumes are compared with existing bubble formation correlations. 1. Introduction Countercurrent gas/liquid flow is generally characterized by an interaction between a gas flowing upwards inside a conduit and a liquid falling counter-currently along its wall. It has been an important design criterion in a variety of industrial equipments, such as chemical reaction in a chemical reactor, evaporators, reflux condensers, and two phase heat exchangers. The departure from stable countercurrent gas/liquid flow is of major importance in the operation of Emergency Core Cooling System (ECCS) during a postulated loss of coolant accident in light water nuclear reactor. There is a possibility that the downward flow of emergency coolant may be limited by uprising steam generated from coolant evaporation, therefore, accurate prediction of CCFL is a significant aspect for evaluating the performance of core cooling devices. Countercurrent flow has been studied both experimentally and theoretically for a long time by many researchers. Although many studies have been made on the fundamental processes of the countercurrent flow, there is still some uncertainty about the precise mechanism in countercurrent flow transition. The countercurrent flow transition can be classified into two regimes. One is generally called flooding which means the flow transition from the annular flow to the mixing flow occurred through an entire length of a flow channel and initiated by a large liquid entrainment due to choking of the gas flow path in the channel on increasing gas flow rate and/or liquid flow