Effect of heater orientation on pool boiling heat transfer from sintered copper microporous coating in saturated water

In this study, we investigate the heater orientation effects at inclination angles from 0 (horizontal upward) to 180 degrees (horizontal downward) on pool boiling heat transfer of saturated water using a durable high-temperature thermally-conductive microporous coating (HTCMC) created by sintering copper powder of an average particle size of 67 pm onto a 1 cm x 1 cm plain copper surface with 300 mu m thickness. The HTCMC surface showed a critical heat flux (CHF) of similar to 2 MW/m(2) and a maximum nucleate boiling heat transfer (NBHT) coefficient of similar to 400 kW/m(2) K, which are 2 and 8 times higher than those of a plain copper surface, respectively, at the upward horizontal inclination angle. The current experimental results showed that the CHF values of the HTCMC were maintained as similar to 2 MW/m(2) at upward inclination angles from 0 degrees to 90 degrees, whereas the CHF values decreased as the inclination angle changed from 90 degrees to 180 degrees, at which point the values were similar to 1.4 MW/m(2). The reduction at 180 degrees is due to individual bubbles merging and forming larger bubbles by blockage from the surface. This increases the vapor residence time on the surface and prevents liquid access to the heated surface at downward inclination angles. However, it is noted that the CHF value of the HTCMC at 180 degrees is similar to 4.5 times higher than that of plain copper surface (0.3 MW/m(2) at 180 degrees ). HTCMC also shows a noticeably higher CHF value at 180 degrees compared to nanocoating or other enhanced surfaces. This significant CHF enhancement is believed to be due to a large number of small feed jet bubbles created from porous structures, postponing the dryout caused by forming large bubbles on the heated surface. (C) 2016 Elsevier Ltd. All rights reserved.