Experimental and analytical investigation of a 0.3-mm-thick loop heat pipe for 10 W-class heat dissipation

Abstract

In this study, a 0.3-mm-thick ultra-thin sheet-type loop heat pipe (LHP) for high heat flux (>5 W/cm2) for cooling high-performance mobile devices was designed, fabricated, and its heat transfer characteristics were experimentally and analytically evaluated. The LHP was designed after a steady-state numerical model specialized for ultra-thin copper LHPs was developed. The heat source size was 10 × 10 mm, and the overall size of the LHP was designed to be mounted on smartphones. Two copper sheets were half-etched and soldered together to form a prototype model of the LHP during the fabrication process. To evaluate the LHP's heat transport performance, we measured the temperature of each part of the LHP in response to heat input. Furthermore, the same test was conducted in five orientations (Horizontal, Roll ± 90°, Pitch ± 90°) to verify the effect of operating orientation on performance, assuming that the LHP is mounted on a mobile device. In the horizontal operation, the fabricated LHP exhibited a maximum heat transport performance of 10 W (10 W/cm2) and a minimum thermal resistance and effective thermal conductivity of 2.51 K/W and 2438 W/mK, respectively. The orientation change test resulted in a change in the thermal resistance of the LHP under thermal loads greater than 6 W. At a 10 W heat load, the Roll −90° orientation obtained a minimum thermal resistance of 2.41 K/W and an effective thermal conductivity of 2538 W/mK. On the other hand, the LHP achieved a thermal resistance of 3.54 K/W and an effective thermal conductivity of 1728 W/mK at the Pitch +90° orientation under the same heat load. On the other hand, the effect on the evaporator temperature and the heat source temperature is small, and the proposed LHP can be expected to be implemented in mobile devices that take any orientation. The proposed steady-state numerical model agreed well with the experimental results, and the estimated heat loss and heat dissipation due to LHP operation showed that 71.3% of the maximum heat load was dissipated in the condenser in the horizontal orientation, and the evaporation efficiency changed with the change of the orientation due to the ease of liquid reflux.