Two-phase secondary flow characteristics and heat transfer mechanism during boiling in a vertical helically coiled tube

Abstract

A VOF model was used to study the two-phase secondary flow characteristics and the heat transfer mechanism in a helically coiled tube. According to the vapor-phase volume fraction, the helically coiled tube was divided into four sections for analysis, considering the different heat transfer characteristics in various boiling states. The heat transfer coefficient, velocity vector, and circumferential temperature distribution of the tube cross-sections were studied during boiling. The numerical results reveal that the secondary flow plays different roles in heat transfer of the four boiling states. When water boils in a helically coiled tube, the secondary flow enhances heat transfer in the subcooled and convective boiling sections, and alleviates heat transfer deterioration in the dry-out section. However, due to the secondary flow, the heat transfer coefficient drops rapidly by about 36% in the nucleate boiling section.