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ISSN Online: 2377-424X

ISBN Print: 978-1-56700-474-8

ISBN Online: 978-1-56700-473-1

International Heat Transfer Conference 16
August, 10-15, 2018, Beijing, China

FLOW AND HEAT TRANSFER CHARACTERISTICS OF A THERMOSYPHON LOOP WITH HIGH FILLING RATIO

Get access (open in a dialog) DOI: 10.1615/IHTC16.her.022167
pages 4501-4508

Résumé

A wickless two-phase closed thermosyphon loop is designed and fabricated in this work to completely remove the capillary limit associated with the conventional loop heat pipes. High filling ratios of R134a working fluid, ranging from 80% to 93%, is chosen to avoid the possible evaporator dry-out under wobbling and variable tilt angle situations. An experimental facility is constructed to visually observe the fluid velocity and to simultaneously measure the pressure and temperature fluctuations. The effects of heat load and filling ratio on the working temperature, the pressure and the fluid velocity, and eventually on the thermal resistance, the temperature uniformity and the heat transfer capacity are investigated in detail. The instability phenomenon in the cold start-up and the chaotic behavior in the stable operation are analyzed using the Power Spectrum Density (PSD) method. The results show that the intense flow boiling occurring at the evaporator section produces powerful bubble pumping effect, driving the fluid circulation within the loop, and keeping the evaporator section at relatively uniform temperature. The average velocity in the intense boiling flow is twice as much as that of the single-phase flow. The velocity could increase by about 32% when the bubble size doubles. The merits of liquid thermal properties and extended surface can be exploited to enhance the heat transfer at the cooler section, which is similar to the forced convection. An optimal filling ratio of 87% is obtained for the minimum thermal resistance due to the competition between the evaporator and the condenser sections.