Dielectric liquids natural convection on small rough Cu surfaces at different orientations

This paper reports heat transfer correlations for predicting the rate of heat removal by natural convection of dielectric liquids from small uniformly heated surfaces, simulating a computer chip, while in the standby mode and in case the motherboard is mounted at different orientations dictated by packaging requirements. Experiments performed to investigate the effects of surface roughness, oxidation, and inclination angle, and liquid subcooling on natural convection cooling of small (10 × 10 mm) uniformly heated Copper (Cu) surfaces using degassed PF-5060 dielectric liquid. The rough Cu surfaces have been shown to markedly enhance nucleate boiling of dielectric liquids, which is the primary mode of cooling the submerged chips in the full power mode. This work employed three non-oxidized and two oxidized rough surfaces. The non-oxidized surfaces with average roughness, Ra, of 0.039 (smooth polished), 0.80 and 1.79 μm are investigated at inclination angles, θ, of 0° (upward facing), 60°, 90° (vertical), 120°, 150°, and 180° (downward facing) and liquid subcooling of 0 K (saturation), 10 K, 20 K, and 30 K. The oxidized Cu surfaces with Ra = 0.15 and 1.83 μm are investigated at saturation temperature in the upward facing orientation. Results show that surface roughness and oxidation and liquid sub-cooling insignificantly affect natural convection cooling, and the rate of heat removal increases with surface temperature to the 1.2 power. The cooling rate is highest in the upward facing orientation and decreases by ∼5%, ∼10% and ∼28% with increasing surface inclination angle to 60°, 120°, and 180°, respectively. The developed natural convection correlations, which agree with experimental data to within ±10%, are compared to those reported in the literature for PF-5060 and FC-72 liquids on porous graphite, microporous Cu, micro structured surfaces, and smooth Cu and silicon.