Two-Phase Flow Visualization and Heat Transfer Characteristics Analysis in Ultra-Long Gravity Heat Pipe

The ultra-long gravity heat pipe has a long heat transfer distance and narrow working fluid flow channel within its tube. Due to these unique design features, the vapor–liquid counter-flow and heat transfer characteristics of these heat pipes are more complex than those found in conventional-size heat pipes. This paper innovatively proposes the design of a segmented visualization window structure of an ultra-long gravity heat pipe, which successfully overcomes the challenge of visualizing the internal flow during operations. A visualization experimental platform, measuring 40 m in height with an inner diameter of 7 mm and the aspect ratio up to 5714, was built to investigate the evolving characteristics of two-phase flows with an increasing heat input and the impact of the phase change flow characteristics on the thermal performance of ultra-long gravity heat pipes. The results obtained can provide guidance for the development of the internal structure of ultra-long gravity heat pipes that are being applied in exploiting geothermal energy. The results show that, at low heat input (200 W, 250 W), there are separate flow paths between the condensate return and the steam, but the high hydrostatic pressure due to the height of the liquid injection results in the presence of an unsaturated working fluid with a higher temperature in the liquid pool area, which has a lower evaporation rate, limiting the heat transfer through the heat pipe. It is found that if increasing the heat input up to 300 W, the evaporative phase change in the heating section becomes intense and stable. At the same time, despite the intermittent formation of liquid columns in the adiabatic section due to the vapor–liquid rolls, which increases the resistance to the vapor–liquid counter-flow, the liquid columns are blocked for a short period of time, and the path of steam rises and the condensate return is smooth, which does not seriously affect the steam condensation and liquid return evaporation. At this point, the overall temperature of the heat pipe is evenly distributed along the tube and the heat transfer performance is optimal. When the heat input further increases (350 W, 400 W), a large amount of condensate is trapped in the upper part of the adiabatic section and the condensing section by long liquid plugs for a long time. At this point, the condensate flow back to the heating section is significantly reduced, and the steam is seriously prevented from entering the condensation sect