Effect of surface roughness on pool boiling heat transfer at a heated surface having moderate wettability

The effect of surface roughness on pool boiling heat transfer coefficient and critical heat flux (CHF) at a copper surface having moderate wettability was studied in saturated water. Copper surfaces were polished with sandpapers of different average surface roughness (Ra), ranging from 0.041 μm to 2.36 μm. Test measurements included static and dynamic contact angles for each of the nine surfaces tested. Although the surface roughness, Ra, moderately influenced the contact angles, pool boiling test results successfully correlated with the coefficient, Csf, in the well-known Rohsenow correlation. The CHF showed noticeably strong dependence and an enhanced performance on the surface roughness as well. The CHF at the roughest surface (Ra = 2.36 μm) was 1625 kW/m2, which is approximately twice as much of that at the smoothest surface (Ra = 0.041 μm). The large increase in CHF with increasing surface roughness is considered to be a consequence of the capillary wicking from the surrounding liquid to the dry spot. A model for the CHF is obtained by modifying an existing correlation for pool boiling with the inclusion of the capillary wicking effect, and a comparison of the results with the experimental data shows good agreement when the wicking effect is included in the correlation.