Visualized experiments on phase-change heat transfer of a metal “endoscopic” two-phase closed thermosyphon

Abstract A novel metal “endoscopic” two-phase closed thermosyphon (METPCT) which could be used to intuitively investigate the phase-change heat transfer and two-phase flow characteristics were developed. The visualized experimental set-up was established by using a rigid borescope and a high-speed camera. The phase-change heat transfer characteristics of the internal working fluid inside the METPCT under various cooling water temperatures (10, 15, 20 °C) and input power (100 ~ 900 W) have been investigated. Different heat and mass transfer regimes, e.g. start-up, geyser boiling and steady-state, were analyzed. During the start-up, the heat transfer in the evaporator is mainly heat conduction, natural convection and surface evaporation. Meanwhile, the start-up time decreases as the increases with cooling water temperature. The geyser boiling includes the incubation and expulsion stage, and the wall temperature response shows slight fluctuation and sharp fluctuation during the expulsion stage. In the steady-state, from bottom to top of the liquid pool in the evaporator, the two-phase flow pattern evolves from bubbly flow to slug flow, churn flow and then dispersed flow, and its transition position is closer to the bottom with the increase of input power. In addition, the heat transfer modes at the condenser are the combination of droplet condensation, discontinuous film condensation, and uniform film condensation. This work provides a new visualized experimental method to investigate the phase-change and two-phase flow properties in a TPCT, which helps to understand the heat and mass transfer mechanism of the TPCT.