Visualized-experimental assessment on a two-phase thermosyphon loop downcomer state with wide range of filling ratios

•A visual two-phase thermosyphon loop test is established.•Two-phase flow in the downcomer at high filling ratio is discovered.•Liquid column height oscillate in a partially liquid-filled downcomer.•Ideal state entails the downcomer being a high void fraction two-phase liquid. The two-phase thermosyphon loop (TPTL), which has an excellent heat transfer performance with simple, compact structure and pump-free advantages, can effectively solve the heat transfer problem under high heat flux. It is necessary to optimize the TPTL to operate in good conditions, especially the state of the downcomer in which the liquid head is one of the critical parameters to determine the driving force of the circulation. However, in the existing studies, the flow features under high filling ratio and high heat flux were ignored in the downcomer. Since a high filling ratio has been proved to have good performance at high heat flux. Herein, the flow characteristics of the TPTL downcomer were seen and quantified using an experimental visual setup under different filling ratios in this study. Two flow features under high heat flux, for the first time, are reported: 1. two-phase flow in the downcomer at high filling ratios. 2. liquid column height oscillation under a partially liquid-filled state. According to the results, the filling ratio significantly impacts the flow characteristics of the downcomer. The TPTL downcomer state, contrary to the traditional understanding, can be partially filled with liquid but not totally filled with fluid all the time, implying that the driving force may be less than the typical assumption. TPTL with a high filling ratio can also provide a maximum gravity pressure head driving force due to the full downcomer. In addition, the bubble pump, which influences by the heat flux, also accelerates the mass flow in the TPTL. Consequently, TPTLs with a high filling ratio showed a significant thermal performance. A partially filled with the liquid phenomenon is demonstrated in moderate filling ratios, and the height of the liquid column will change as the heat flux increases, which indicates the self-regulation of TPTL. When the filling ratio is low, the liquid column height is greatly influenced by the filling ratio. The high liquid column will also give the TPTL excellent thermal performance. The ideal state of a TPTL downcomer is the downcomer filled with two-phase liquid under a high void fraction, which is realized by a high filling ratio TPTL. This pa