Browsing by Author "Xu, Zhi"

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Heat performances of a thermosyphon as affected by evaporator wettability and filling ratio
(2018-01-25)
Xu, Zhi
;
Zhang, Yaning
;
Li, Bingxi
;
Wang, Chi-Chuan
;
Ma, Qin
A thermosyphon is considered an efficient heat dissipation device in engineering fields due to its low thermal resistance. The heat transfer mechanisms for thermosyphons at different evaporator wettability and filling ratios are not well detailed. A model considering evaporator wettability in terms of a contact angle is developed to detail the phase change process to explore the heat transfer mechanism for a thermosyphon in this study. The effects of evaporator wettability and filling ratio on the heat performances of a thermosyphon charged with water are investigated. It is observed that the simulated absolute temperatures with a contact angle are in better agreement with the experimental results with an average relative error of 0.15% than the simulation results without a contact angle (0.28%). The results show that a hydrophilic surface causes bubbles to easily depart the evaporator wall, thereby increasing the heat performance, whereas a hydrophobic surface causes bubbles to adhere to the evaporator wall, decreasing the heat performance. Further study shows that a low filling ratio of 12% will result in drying out, but a high filling ratio of 40% will prevent large bubbles from reaching the liquid surface, thereby decreasing the heat performance. The heat performance is best at filling ratios of 20-30% for an evaporator with a hydrophilic surface. (C) 2017 Elsevier Ltd. All rights reserved.
The influences of the inclination angle and evaporator wettability on the heat performance of a thermosyphon by simulation and experiment
(2018-01-01)
Xu, Zhi
;
Zhang, Yaning
;
Li, Bingxi
;
Wang, Chi-Chuan
;
Li, Yongji
A model considering evaporator wettability in terms of the contact angle is developed in this study. In addition, experiments are conducted using a copper thermosyphon with a length of 240 mm and internal and external diameters of 22.2 mm and 25.4 mm. The influences of the inclination angle and evaporator wettability on the heat performance of a thermosyphon charged with water are investigated. It is observed that simulated temperatures agree well with experimental data with a relative error of 0.12% for a modified thermosyphon. The results show that bubbles attaching to the wall of the evaporator decrease as the inclination angle increases from 15 degrees to 90 degrees, which decreases thermal resistance by 59.5%. As the heating power increases from 10 W to 14 W, thermal resistance reduces more significantly (44.1% decrease) for an evaporator with a hydrophilic surface than for an evaporator with a hydrophobic surface (20.6% decrease) because the bubble emission frequency of a hydrophilic surface rises more sharply (265% increase) than that of a hydrophobic surface (100% increase) at an inclination angle of 90 degrees. (C) 2017 Elsevier Ltd. All rights reserved.