Visual experimental studies were conducted on the condensation process of a gravity-assisted heat pipe with electrical capacitance tomography (ECT) technology. The heat pipe was made of silica glass

and alcohol was used as working fluid. The heating temperature of its evaporation segment was controlled by a heater with different power outputs. The alcohol vapor in the condensation segment was cooled to condense through heat convection with cooling water. The condensation process of alcohol vapor was monitored by an ECT measurement system to observe the distribution characteristics of liquid and its vapor

and the formation and development of liquid film on the wall of the heat pipe. Considering the access limitations of the structure of the condensation segment

the traditional structure of the ECT transducer was changed. The shielding case was abandoned

and the measuring electrodes were sealed with insulating hydrophobic layer. This method made it possible to permit the transducer to be used in a liquid environment

and effectively expand the application fields of ECT technology. Alcohol vapor condensed in strips and unevenly on the wall surface when heating temperature of the evaporation segment was low. With increasing heating temperature

the condensing liquid would change to annular flow around the whole circumference. When heating temperature exceeded a specified limit

a kind of cyclical phenomenon happened. The liquid film gradually became thicker and even occupied the whole cross section of the heat pipe

and then fell off. This phenomenon occurred repeatedly

and its frequency increased with increasing heating temperature. When the gravity heat pipe was inclined 30° from the vertical direction

the same phenomenon appeared when heating temperature was 110℃.