Modeling of Vapor Mass Formation under Forced Convective Boiling Condition

Abstract

The development of a precise and reliable prediction method for a departure from nucleate boiling (DNB) is urgently needed to design and develop new-generation reactors that enable us to establish a carbonneutral society. Currently, the DNB has been predicted by the DNB correlation and a subchannel analysis code for the fuel assemblies in pressurized water reactors. It took a huge cost because the DNB correlation had to be obtained by the mock-up test every time the new type of fuel assembly was considered. Therefore, the prediction method of the DNB based on the mechanism has been required for a long time. In this study, we consider that the formation of a large vapor mass on the heating surface is the primary trigger of the DNB. Therefore, we embark on the development of a new model for vapor mass formation under forced convective boiling. We assume that the primary bubbles coalesced with each other, which are generated from the nucleation sites, and the coalesced bubble is formed. The nucleation sites are assumed to be distributed on the basis of the Poisson distribution. The coalesced bubbles grow on the basis of the 1-D bubble growth and motion equation, and the large vapor mass is assumed to be formed by the merging of coalesced bubbles when the diameter of the coalesced bubble satisfies the criteria of the slug formation by Mishima. The model reproduced the dependency of vapor mass formation heat flux on the bulk velocity and subcooling.