Surface roughness variation effects on copper tubes in pool boiling of water

Abstract

Results showing the effects of varying the surface roughness on copper tubes in pool boiling of water are presented in this study. To obtain different surface roughness values of each sample, the copper tubes were rotated with an electric rotor and sanded using sandpaper of different grit sizes. The average surface roughness values of the plain copper tubes were in the range 0.032–0.544 µm. All experimental samples were horizontally oriented, and experiments were carried out in ambient conditions up to a moderate heat flux regime (450 kW/m2). Moreover, for a comparative analysis, a sample with a rough surface and hydrophobic patterns was included in this study. Compared with the smoothest surface, the aforementioned rough sample exhibited a heat transfer coefficient that was up to a factor 1.5 higher for the highest evaluated heat flux. These findings show that even small increments in the surface roughness along with the addition of hydrophobic patterns can significantly lower the wall superheat temperature and increase the heat transfer coefficient of copper tubes. Furthermore, supported by high-speed imaging of the experiment, it was observed that increasing the surface roughness caused bubbles to depart when their diameter was larger, and the nucleation site density and bubble departure frequency increased. In contrast, the rough surface with hydrophobic patterns exhibited the best overall enhancement, including the characteristics mentioned above of the rough surfaces along with a uniform distribution of the bubbles around the surface.