Experiments and correlations of saturation boiling of hfe-7000 dielectric liquid on rough inclined copper surfaces

The saturation temperature of only 34 degrees C at atmospheric pressure, the high boiling figure-of-merit (BFOM), Critical Heat Flux (CHF) and maximum nucleate boiling heat transfer coefficient (h(MNB)) make HFE-7000 dielectric liquid a good choice for immersion cooling applications. Pool boiling experiments are performed to investigate the effects of inclination angle on saturation boiling of HFE-7000 dielectric liquid on 10 x 10 mm rough copper surfaces. Experiments varied surface average roughness, Ra, from 0.039 to 1.44 mu m and inclination angle of the uniformly heated Cu surfaces, theta, from 0 degrees (upward facing) to 180 degrees (downward facing). Captured images of nucleate boiling in various regions of the boiling curve and near CHF using high-speed camera at 1,000 fps help the interpretation of the experimental results. The values of CHF, h(MNB), and corresponding surface superheats as well as those of the surface temperature superheat prior to boiling incipience, Delta T-exc, are correlated and compared to reported values for other dielectric liquids. The h(MNB), which occurs at the end of the fully developed nucleate boiling region, is higher than the heat transfer coefficient at CHF and the corresponding surface superheat is lower. The CHF and h(MNB), increase with increasing Ra and/or decreasing theta, while those of the corresponding surface superheats decrease with increasing theta and/or Ra. In the upward facing orientation (theta = 0 degrees), increasing Ra from 0.039 mu m (smooth polished) to 1.44 um increased CHF by similar to 49%, from 20.7 to 30.85 W/cm(2), and decreased the corresponding surface superheats by similar to 38.7% from 23.5 to 14.4 K. In this orientation, h(MNB) increased 250% from 0.88 to 2.2 W/cm(2) K, with increased surface roughness from 0.039 to 1.44 mm and the corresponding surface superheat decreased similar to 41.4% form 22.7 K to 13.3K. The values of CHF and h(MNB) in the downward-facing orientation (theta = 180 degrees) are 27 - 33% and similar to 40%, respectively, of those in the upward facing orientation (theta = 0 degrees). In the discrete bubbles' region at low superheats, the nucleate boiling heat transfer coefficient increased, while that in the fully developed nucleate boiling region at high superheats, decreased with increased theta. Developed correlations of CHF, h(MNB )and the corresponding surface superheats as well as of Delta T-exc are in good agreement with present experimental results and those reported for other dielectric liquids. Results confirmed the promising potential of HFE-7000 dielectric liquid for immersion cooling applications. (C) 2020 Elsevier Ltd. All rights reserved.