Alteration of pool boiling heat transfer on metallic surfaces by in situ oxidation

The critical heat flux during pool boiling has been investigated for a range of applications including electrical power generation and thermal management. Reported experimental CHF values during pool boiling of water on flat metallic surfaces, however, show a large discrepancy across studies. Here, we address this discrepancy in CHF values by accounting for oxidation of metallic surfaces during boiling. We studied the effect of in situ oxidation on flat Cu and Ni surfaces by changing the duration that samples were held in saturated water before conducting boiling experiments. The morphology and chemical composition of surfaces after the boiling experiments were analyzed by atomic force microscopy and X-ray photoelectron spectroscopy, respectively. Cu surfaces showed gradually increasing CHF values as the duration in saturated water increased, which could be attributed to the increase in roughness due to the formation of Cu2O nanostructures. Conversely, Ni surfaces showed relatively stable CHF and morphology as a nearly flat layer of NiO formed, with one exception: formation of a highly wetting hydroxide, Ni(OH)(2), on a Ni coupon held in saturated water for 24 h resulted in a uniquely high CHF value, signifying the importance of surface chemistry in addition to morphology. The fundamental mechanisms resulting in the wide spread of CHF values on metallic surfaces elucidated in this work will lead to more accurate estimation of CHF as well as a deeper mechanistic understanding of CHF values on engineered surfaces. (c) 2021 Elsevier Ltd. All rights reserved.

Automated summary

This study investigates the effect of surface oxidation on the critical heat flux (CHF) values during boiling. It was found that the CHF values on copper surfaces gradually increased with the duration in saturated water, attributed to the formation of Cu2O nanostructures. On the other hand, the CHF values on nickel surfaces remained relatively stable, except when a highly wetting hydroxide, Ni(OH)2, formed after 24 hours of immersion. These findings provide insights into the fundamental mechanisms causing the wide spread of CHF values on metallic surfaces.