Visualization of vaporization events in two-phase thermosyphons operating in different orientations

Currently, thermosyphons are used to cool such devices as power amplifiers of radio frequency systems, data center hardware, LED light sources, etc. One of the important factors affecting the efficiency of such cooling systems is the orientation of the thermosyphons in space. This paper is dedicated to research and visualization of vaporization events in two-phase thermosyphons, primarily focusing on investigating and visualizing the influence of orientation in space on vaporization. The studies were performed for100% fill ratio. Vaporization was video recorded at 240 frames per second, whereupon the obtained footage was converted into image sequence. The analysis of the obtained materials has shown that at intensive boiling for tilt angle range of 5—45º, a part of the working fluid is always outside the evaporator. When the angle is decreased, the portion of the working fluid outside the evaporator increases. Moreover, for the 5° tilt angle, the evaporator can be completely drained at certain time intervals. It is proposed that the thermal resistance at low heat flux values for smaller tilt angles can be lower than for vertical orientation because of thin liquid films. Evaporation is more effective in thin liquid films than in large volumes. This prompts the conclusion that smaller angles will allow obtaining lower thermal resistance for the same filling ratio. On the other hand, maximum heat transfer ability decreases dramatically for the angle range of 0—10º. The obtained results can be used in further studies to analyze and explane the aspects of heat transfer in two-phase thermosyphons with a short evaporator.