Boiling heat transfer enhancement on titanium through nucleation-promoting morphology and tailored wettability

Surface engineering aimed at tuning the wettability and morphology of the boiling surface is a facile approach to moderate and enhance the nucleate boiling process. Key issues include control over the active nucleation site density, bubble departure frequency and liquid replenishment of active nucleation sites while simultaneously reducing the bubble nucleation temperature. In this study, we fabricated speartype (ST) and cavity-type (CT) TiO2 nanostructures on 25 mu m titanium foils via hydrothermal etching in an alkaline solution. High-speed IR and video cameras were used to detect local phenomena in terms of temperature and heat flux fluctuations and observe the bubble dynamics during saturated pool boiling of water. Intrinsically hydrophilic ST and CT surfaces provided a moderate overall enhancement of the heat transfer coefficient compared to an untreated surface due to increased nucleation site density and bubble frequency. The CT surface also decreased the bubble nucleation temperature due to effective vapor-entrapping and nucleation-promoting cavities. In a further step, both surfaces were hydrophobized through chemical vapor deposition of a fluorinated silane to tailor the wettability of the surface into a superhydrophobic state. This further reduced the average surface superheat by at least 40%, while the nucleation frequencies exceeded 200 Hz on the hydrophobized CT surface. In comparison with the untreated reference surface, the heat transfer coefficient on hydrophobized ST and CT surfaces was enhanced by 89% and 237% at 100 kW m(-2), respectively. Moreover, the full width at half maximum (FWHM) value of the surface temperature distribution was reduced by 73% and 95% at the same heat flux, respectively. The study confirms that hydrophobic surface treatment can significantly enhance the nucleate boiling process when combined with an appropriate surface structure. Despite the affinity between the vapor and the hydrophobic layer, the cavity-type and spear-type TiO2 structures are able to maintain active nucleation sites well-separated, which prevents the undesirable vapor spreading that possibly leads to an early onset of critical heat flux. (C) 2022 The Author(s). Published by Elsevier Ltd.

Automated summary

Surface engineering is an effective approach to improve the nucleate boiling process by tuning the wettability and morphology of the boiling surface. In this study, TiO2 nanostructures were fabricated on titanium foils, and the effects of hydrophobic surface treatment on nucleate boiling were investigated. The results show that hydrophobic surfaces increase the nucleation site density and bubble frequency, reduce the bubble nucleation temperature, and significantly enhance the heat transfer coefficient.