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The rapid progress of electronic devices has necessitated efficient heat dissipation within boiling cooling systems, underscoring the need for improvements in boiling heat transfer coefficient (HTC) and critical heat flux (CHF). While different approaches for micropillar fabrication on copper or silicon substrates have been developed and have shown significant boiling performance improvements, such enhancement approaches on aluminum surfaces are not broadly investigated, despite their industrial applicability. This study introduces a scalable approach to engineering hierarchical micro-nano structures on aluminum surfaces, aiming to simultaneously increase HTC and CHF. One set of samples was produced using a combination of nanosecond laser texturing and chemical etching in hydrochloric acid, while another set underwent an additional laser texturing step. Three distinct micropillar patterns were tested under saturated pool boiling conditions using water at atmospheric pressure. Our findings reveal that microcavities created atop pillars successfully facilitate nucleation and micropillars representing nucleation site areas on a microscale, leading to an enhanced HTC up to 242 kW m−2 K−1. At the same time, the combination of the surrounding hydrophilic porous area enables increased wicking and pillar patterning, defining the vapor–liquid pathways on a macroscale, which leads to an increase in CHF of up to 2609 kW m−2. [ABSTRACT FROM AUTHOR]

The rapid advancement of engineering systems has spurred the search for innovative thermal management solutions. Boiling, as a phase‐change heat transfer method, has shown promise in heat dissipation, but non‐functionalized surfaces struggle with increasing cooling demands. To improve heat dissipation efficiency across different heat loads, functionalized surfaces with tailored wettability have been proposed. Separately, superhydrophilic and superhydrophobic surfaces each offer benefits and drawbacks in boiling applications but combining them on a single "biphilic" surface simultaneously harnesses their advantages. In this study, laser‐functionalized copper surfaces with spatially tailored wettability are developed by combining two‐step laser texturing with a self‐assembled monolayer coating, while focus is placed on the impact of the size and pitch of superhydrophobic spots. The developed functionalized surfaces exhibit exceptional boiling performance with heat transfer coefficients up to 299 kW m−2 K−1, a 434% enhancement over untreated surfaces. Optimal ratios of superhydrophilic and superhydrophobic areas and optimal spot pitch are identified. Additionally, varying behavior at different heat flux levels is observed, emphasizing the importance of considering thermal loads when determining the optimal surface pattern. This advancement in performance, along with the rapid and cost‐effective functionalization process, represents a significant breakthrough for enhanced thermal management applications. [ABSTRACT FROM AUTHOR]

Prior studies have evidenced the potential for enhancing boiling heat transfer through modifications of surface or fluid properties. The deployment of nanofluids in pool boiling systems is challenging due to the deposition of nanoparticles on structured surfaces, which may result in performance deterioration. This study addresses the use of TiO2–water nanofluids (mass concentrations of 0.001 wt.% and 0.1 wt.%) in pool boiling heat transfer and concurrent mitigation of nanoparticle deposition on superhydrophobic laser-textured copper surfaces. Samples, modified through nanosecond laser texturing, were subjected to boiling in an as-prepared superhydrophilic (SHPI) state and in a superhydrophobic state (SHPO) following hydrophobization with a self-assembled monolayer of fluorinated silane. The boiling performance assessment involved five consecutive boiling curve runs under saturated conditions at atmospheric pressure. Results on superhydrophilic surfaces reveal that the use of nanofluids always led to a deterioration of the heat transfer coefficient (up to 90%) compared to pure water due to high nanoparticle deposition. The latter was largely mitigated on superhydrophobic surfaces, yet their performance was still inferior to that of the same surface in water. On the other hand, CHF values of 1209 kW m−2 and 1462 kW m−2 were recorded at 0.1 wt.% concentration on both superhydrophobic and superhydrophilic surfaces, respectively, representing a slight enhancement of 16% and 27% compared to the results obtained on their counterparts investigated in water. [ABSTRACT FROM AUTHOR]

Heat transfer coefficient (HTC) was experimentally measured for saturated and subcooled pool boiling of binary mixtures of water and glycerin. Saturated boiling was studied for mixtures with water mass fractions ω w from 100 % to 60 % on horizontal flat nickel-plated surfaces at heat fluxes from 50 to 650 k W m − 2 at atmospheric pressure. Subcooled boiling was investigated in the range of subcooling from 0 to 30 K at heat fluxes of approximately 250, 450 and 650 k W m − 2 . It was found that mixture effects have a significant impact on saturated boiling HTC even for mixtures with very low content of glycerin as significant drops of HTC were observed for subtle changes in composition for mixtures of high ω w . Measured HTC was successfully correlated with the combination of Yagov (1999) and Inoue and Monde (2009) correlations with a mean relative error of 12 %. A simple empirical HTC correlation is also proposed. For subcooled boiling, developed subcooled boiling regime was reached for all investigated heat fluxes. For this regime, correlations, which were able to predict HTC for saturated boiling, were employed to predict subcooled boiling HTCs for all investigated concentrations, heat fluxes and subcoolings. Effect of subcooling and effect of liquid composition on total HTC were of the same importance for mixtures with higher water content. With the increase in concentration of glycerin in the mixture, decrease in total HTC with increasing subcooling became more significant. [ABSTRACT FROM AUTHOR]

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]

The enhancement of boiling heat transfer has been extensively shown to be achievable through surface texturing or fluid property modification, yet few studies have investigated the possibility of coupling both enhancement approaches. The present work focuses on exploring the possibility of concomitant enhancement of pool boiling heat transfer by using TiO2-water nanofluid in combination with laser-textured copper surfaces. Two mass concentrations of 0.001 wt.% and 0.1 wt.% are used, along with two nanoparticle sizes of 4–8 nm and 490 nm. Nanofluids are prepared using sonification and degassed distilled water, while the boiling experiments are performed at atmospheric pressure. The results demonstrate that the heat transfer coefficient (HTC) using nanofluids is deteriorated compared to using pure water on the reference and laser-textured surface. However, the critical heat flux (CHF) is significantly improved at 0.1 wt.% nanoparticle concentration. The buildup of a highly wettable TiO2 layer on the surface is identified as the main reason for the observed performance. Multiple subsequent boiling experiments using nanofluids on the same surface exhibited a notable shift in boiling curves and their instability at higher concentrations, which is attributable to growth of the nanoparticle layer on the surface. Overall, the combination of nanofluids boiling on a laser-textured surface proved to enhance the CHF after prolonged exposure to highly concentrated nanofluid, while the HTC was universally and significantly decreased in all cases. [ABSTRACT FROM AUTHOR]

• Laser-textured open microchannel surfaces are designed for pool boiling. • Three depths of the microchannels were investigated. • Structured microchannel surfaces show enhancement up to 210 % (CHF) and 320 % (HTC). • Increased rewetting and nucleation site density led to improved boiling performance. Technological advancements in various electronic devices, consumer or industrial power electronics, as well as higher demand for energy efficiency, have in recent years presented the need for advanced thermal management to enable the realization of their increased performance. Research in this field has taken up pace in the last decade, with various surface engineering techniques being proposed. This paper investigates enhancement of pool boiling performance with hierarchical microchannel copper surfaces, augmented with additional laser texturing and selective hydrophobization. The surfaces were fabricated with either straight or segmented microchannels of varying depths, while laser texturing was applied to either the base of the channels or the entire surface. Multiple families of surfaces with mini-, micro- and nanoscopic surface structures were created through different combination surface treatments, including milled microchannels, laser-induced surface structures and a hydrophobic coating. Pool boiling heat transfer performance tests were carried out with twice-distilled water in saturated state at atmospheric pressure. All engineered surfaces achieved an increase in the heat transfer coefficient (HTC) and the critical heat flux (CHF) values. The highest CHF value of 3142 kW m −2 was recorded on a laser-textured surface with deep microchannels, with an improvement over the reference surface of 210 %, and a corresponding HTC of 132 kW m −2 with enhancement of 214 %. On the other hand, the highest HTC value of 174 kW m −2 was achieved on a hydrophobized laser textured surface shallow microchannels, with an improvement of 314 %, while its CHF value was 1963 kW m −2 with an enhancement of 94 %. Laser-textured microchannel surfaces exhibited higher CHF values over their reference counterparts due to the fabricated microcavities on the microchannels, which facilitates improved liquid supply and nucleation. Fully superhydrophobic surfaces exhibit an HTC compared to surfaces characterized by mixed superhydrophobic and hydrophobic regions, which is ascribed to the larger surface area featuring a reduced energy barrier, thereby promoting a higher density of active nucleation sites. Additionally, the results of this study show that CHF increases with increasing channel depth, while HTC deteriorates with increasing channel depth. In general, non-hydrophobized surfaces with microchannels and laser-induced microcavities presented the highest improvements in CHF values, while still achieving notably enhanced HTC values, representing a very favorable combination for industrial applications. [Display omitted] [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]