Theoretical modeling of the boiling annular-mist flow in vertical channels

Abstract

Accurate prediction of the boiling annular-mist flow as well as the occurrence of the dryout is of particular importance for the safe operation of heat systems. Therefore, this paper focuses on the analysis of the boiling annular-mist flow and aims to propose a mechanistic model to describe this boiling process. Firstly, the physical model is established which divides the flow field into the bubble layer region and the core region in the radial direction. The bubble layer serves as a pseudo boundary for generating bubbles, whose thickness is adopted as the bubble departure diameter. In addition, the annular-mist flow is divided into the core region evaporation flow section and the bubble layer evaporation flow section in the axial direction, based on the relationship between the thickness of the bubble layer and the film thickness. Then, a set of two-dimensional steady-state conservation equations is proposed, which considers the energy, mass and momentum exchanges in both the bubble layer region and the core region. The accuracy of this model is verified with the experimental data and simulation results of void fraction and CHF. Based on this model, the variations of two-phase flow parameters in the bubble layer region and the core region can be obtained separately, such as the velocity, void fraction and pressure. The dryout point can be predicted based on the variation of the film thickness. These findings promote the fundamental understanding of the thermal–hydraulic characteristics of the annular-mist flow.