Effect of change in surface condition induced by oxidation on transient pool boiling heat transfer of vertical stainless steel and copper rodlets

In the present experimental study, the effect of change in surface condition induced by oxidation on the transient pool boiling heat transfer was investigated using vertical SS (Stainless Steel) and copper rodlets. The quenching method was applied, and pure water was used as a quenchant. Various kinds of surface characterizations (i.e., photograph, microscopic image, surface roughness, and water contact angle) together with boiling visualizations were provided and discussed in detail. The surface conditions of the copper test specimen were more sensitively influenced by the repeated quenching tests and 2 h-oxidation, as compared with those of a SS test specimen. The copper test specimen showed a shorter quenching duration than the SS test specimen, owing to its smaller heat capacity. Through a 2 h-oxidation of the test specimens, the surface conditions and quenching curves of the SS test specimen were not altered much. On the other hand, the copper had a very rough surface with unique feather-like structures, which considerably enhanced the quenching performance. This may be because the flaky feather-like structures kept disrupting the vapor film. This result implies that the transient pool boiling heat transfer can be improved and controlled by the functionally well-designed surface structure. Through the boiling visualizations of 2 h-oxidized SS and copper test specimens, the vapor film collapse modes were observed to be different. In SS, the propagative collapse mode was mainly observed. On the other hand, in copper, the vapor film seemed to begin to be collapsed through a coherent collapse mode. Based on this study, it was found that the quenching performance and vapor film collapse mode are strongly influenced by changes in the surface conditions induced by oxidation. (C) 2014 Elsevier Ltd. All rights reserved.