Estimation of average void fraction in vertical two-phase flow channel under low liquid velocity

During the reflood phase in a postulated loss-of-coolant accident of a pressurized water reactor, the driving force to feed the emergency core coolant into the core is the effective water head of a two-phase flow in the downcomer surrounding the core in the reactor vessel. The effective water head in the downcomer is dominated by the characteristics of the average void fraction in a vertical two-phase flow channel. This paper makes clear the effects of the physical properties of fluids, the channel diameter and superficial velocities of two phases on the void fraction in a vertical flow system by both carrying out the experiment of an air-water system under 1 atm and investigating the previous experimental results of steam-water and air-water systems. As the results, the presentation of the average void fraction could be obtained within errors of +- 15% by dimensional analyses in cases of both steam-water and air-water systems in the range of low superficial water velocities ( lt 30 cm/s) including the stagnant water condition. (author)