Effects of surface inclination and type of surface roughness on the nucleate boiling heat transfer performance of HFE-7200 dielectric fluid

In the present work, the effects of surface roughness (including roughness from machined roughness and sandblasted roughness), surface inclinations, and heat fluxes on the nucleate pool boiling heat transfer of HFE-7200 dielectric liquid at a working pressure of 1 atm. are examined in detail. Five aluminum surfaces, with different roughness from 0.45 μm to 9 μm, are employed as test samples. The nucleate boiling phenomenon is examined with the wall-superheat temperature ranging from 5 °C to 30 °C, and surface inclinations of 0°, 90°, and 180°, respectively. SEM images and nucleate boiling bubbles captured by a high-speed camera are utilized for further analyses. For all cases in this study, it is found that the nucleate boiling heat transfer of the sandblasted rough surfaces is superior to those of the polished and machined rough surfaces. For instance, at a wall-superheat temperature of 30 °C and inclination angle of 0°, the heat flux is augmented up to 69.7%, and 139.3% when compared to the polished surface for the sandblasted surfaces with the roughness of 6.2 µm, and 9 µm, respectively. In addition, novel correlations are also proposed for predicting the nucleate pool boiling heat transfer coefficient of HEF-7200 dielectric fluid with different surface roughness, inclinations and heat fluxes.