Search

Evaluating boiling heat transfer enhancement depends on reliable reference values in the form of boiling curves and critical heat flux (CHF) values. Typically, the evaluation is performed in pool boiling conditions with water at atmospheric pressure. Literature includes a wide scatter in reference values, prompting this study to comprehensively evaluate boiling performance and CHF on reference surfaces to investigate the scatter's origin and set a definitive reference value. The study recorded 125 boiling curves and CHF values on nominally identical bare copper surfaces, establishing an average boiling curve and mean CHF value. Despite consistent experimental conditions, the recorded CHF values displayed significant variability with a mean CHF of 1112 ± 102 kW m−2 and a scatter from 902 kW m−2 (−19 % of average CHF) to 1339 kW m−2 (+25 % of average CHF). Using Rohsenow's correlation on the average boiling curve, a C sf factor of 0.0151 was obtained. The acquired CHF data is proposed to serve as a foundational benchmark for future research in enhancing pool boiling heat transfer. [Display omitted] • Measurement of 125 boiling curves on nominally identical copper samples. • Average CHF determined as 1112 kW m−2 ± 102 kW/m−2. • Determination of the average boiling curve for water boiling on bare copper. • Fitting of Rohsenow's correlation, yielding average C sf = 0.0151. • Compilation of a CHF database as a foundational benchmark for future studies. [ABSTRACT FROM AUTHOR]

• Cavity- and spear-type TiO 2 nanostructures are prepared by hydrothermal treatment. • Nucleation-promoting cavity morphology is superior to spear-like microstructure. • Hydrophobic surface with microcavities (CTH) provides HTC enhancement of over 200%. • High nucleation site density on CHT surface narrows surface temperature distribution. • ONB at 2.5 K and D b of 0.55 mm are observed on hydrophobic microcavity surface. 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. [Display omitted] [ABSTRACT FROM AUTHOR]