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

Može, Matic (author), Hadžić, Armin (author), Zupančič, Matevž (author), Golobič, Iztok (author)

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 spear-type (ST) and cavity-type (CT) TiO$_2$ nanostructures on 25 μm 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 TiO$_2$ 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.

• Language: English
• Keywords: pool boiling, heat transfer enhancement, IR thermography, titanium nanostructures, hydrophobicity, functionalized surfaces
• Work type: Article
• Typology: 1.01 - Original Scientific Article
• Organization: FS - Faculty of Mechanical Engineering
• Publication status: Published
• Publication version: Version of Record
• Year: 2022
• Number of pages: 17 str.
• Numbering: Vol. 195, art. 123161
• PID: 20.500.12556/RUL-137561
• UDC: 536.24
• ISSN on article: 0017-9310
• DOI: 10.1016/j.ijheatmasstransfer.2022.123161
• COBISS.SI-ID: 112393987
• Publication date in RUL: 22.06.2022

Record is a part of a journal

• Title: International journal of heat and mass transfer
• Shortened title: Int. j. heat mass transfer
• Publisher: Elsevier
• ISSN: 0017-9310
• COBISS.SI-ID: 3064335

Licences

• License: CC BY 4.0, Creative Commons Attribution 4.0 International
• Link: http://creativecommons.org/licenses/by/4.0/
• Description: This is the standard Creative Commons license that gives others maximum freedom to do what they want with the work as long as they credit the author.

Secondary language

• Language: Slovenian
• Keywords: vrenje v bazenu, izboljšan prenos toplote, IR termografija, titanove nanostrukture, hidrofobnost, funkcionalizirane površine