Ukraine, Kyiv, National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute”.

The paper presents research on the visualization of boiling processes in a flat gravitational heat pipe within a range of thermal fluxes from 5 to 55 W. The main objective of the study is to identify visual patterns during boiling and correlate them with thermal characteristics obtained during research conducted with visual observations. Due to the high-speed nature of the processes, observations were made using a high-speed camera. Temperature values on the heat pipe's surface were also recorded using thermocouples and measurement systems. To obtain a comprehensive picture of the study, experiments were conducted at various inclination angles of the experimental heat pipe sample to the horizon, ranging from 0° to 90°. Visual schemes of boiling and evaporation were obtained and explained simultaneously with thermal performance of experimental sample. The study allowed discovering that, in terms of thermal resistance, 60° is the optimal inclination angle. On the other hand, the lowest evaporator temperature was obtained for 15° and 30° angles. Additionally, it was observed that the experimental sample is able to operate when positioned horizontally. Maximal transferred heat fluxes were extremely low compared to other angles. Nevertheless, even at horizontal orientation, thermal resistance was lower than for vertical position.

Keywords: heat transfer, heat pipe, boiling, vapor generation, thermal resistance.

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