•Cavity- and spear-type TiO2 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 Db 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) TiO2 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 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.
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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.
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 as an innovative thermal management solution. These “biphilic” surfaces exhibit intentionally varied local superhydrophobicity and superhydrophilicity and exceptional pool boiling performance with heat transfer coefficients up to 299 kW m‐2 K‐1, a 434% enhancement over untreated surfaces.
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 Csf 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.
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•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 Csf = 0.0151.•Compilation of a CHF database as a foundational benchmark for future studies.
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•Surface superheat increases during long-term operation in water and salt solutions.•Nucleation activity drops due to wetting, surface chemistry and topography changes.•Boiling curve ...stability is an uncertain indication of surface’s long-term operation.•Aging mechanisms found in water appear in solutions with lower relative importance.•Laser-texturing improves resistance to boiling-induced degradation by up to tenfold.
Long-term boiling stability marks an important and often overlooked characteristic when engineering advanced interfaces for efficient phase-change heat removal. This study presents long-term pool boiling on functionalized smooth and laser-textured copper interfaces in de-ionized water and in an aqueous solution of calcium sulphate. Each surface was exposed to nucleate boiling for several hundred hours and wettability, morphology and surface chemistry were evaluated prior to and after each test. In addition, consecutive pool boiling curves were recorded on all samples. Our findings indicate an increase of wall superheat on all samples, caused by added thermal resistance in the salt solution and by diminished nucleation activity in water. The latter resulted from spatially non-uniform and interconnected mechanisms of wettability transition towards hydrophilicity, surface chemistry changes and topography deformation. These mechanisms also appeared in salt solutions, where their relative importance compared to dominant influence of scaling was linked to crystallization-mitigative ability of the interface. In both liquids, laser-textured surfaces exhibited up to an order of magnitude lower degradation rates. Our results also emphasize the importance of evaluation times of several days to accurately gauge boiling-induced changes to the surface and the ambiguity of evaluating the latter through boiling curve stability.
Boiling is a prevalent phase-change process that plays a vital role in facilitating efficient heat transfer from a heating surface. While this heat transfer mechanism is generally effective, a rapid ...increase in surface temperature can lead to hydrodynamic instabilities, resulting in a boiling crisis. Previous studies have shown that surfactants often improve boiling performance and change the boiling crisis behavior. Conventional wisdom in this field attributes that these changes in boiling behavior are tied to the critical micelle concentration (CMC) of the particular surfactant. However, our work reveals that these changes in boiling behavior are independent of the CMC for three nonionic surfactants across a wide range of molar concentrations. In addition, visual snapshots of the bubbling behavior indicate changes in bubble formation, such as bubble size and nucleation site density, influenced by the molar concentration-dependent diffusion timescale of surfactants. Hence, these findings offer compelling evidence that boiling behavior, encompassing both boiling performance and boiling crisis, is governed by the dynamic adsorption of surfactants rather than dictated by the CMC. This becomes evident when quantifying the heat transfer coefficient (HTC) and critical heat flux (CHF) using the logarithm of molar concentration, as predicted by theory. Building upon these findings, we propose insights for controlling when CHF modification occurs in specific scenarios involving any surfactants. These insights hold significant potential for optimizing heat transfer processes and leveraging surfactants in energy-related applications to maximize boiling efficiency.
•Boiling is influenced by surfactant adsorption dynamics.•Similar bubbling observed for different surfactants at the same molar concentrations.•Molar concentration-dependent diffusion governs boiling behavior.•Boiling trends validated with different boiler and surface type.
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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.
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
K
. 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
.
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
from
to
on horizontal flat nickel-plated surfaces at heat fluxes from 50 to
at atmospheric pressure. Subcooled boiling was investigated in the range of subcooling from 0 to
at heat fluxes of approximately 250, 450 and
. 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
. Measured HTC was successfully correlated with the combination of Yagov (1999) and Inoue and Monde (2009) correlations with a mean relative error of
. 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.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, GIS, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
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 TiO
-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
and 1462 kW m
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.
The advancement in high-power electronic devices coupled with the need to ensure reliable and efficient heat dissipation of two-phase cooling systems underscores the urgent necessity for ...breakthroughs in enhancing boiling performance and especially critical heat flux (CHF) to minimize the risk of system failure. In this field, surfaces with tailored wettability have already demonstrated their potential to enhance boiling heat transfer intensity, while surfaces featuring wickable structures like micropillar arrays have shown significant improvements in CHF. In this study, we investigate the use of aluminium micropillar surfaces with tailored wettability to simultaneously enhance nucleate boiling heat transfer performance and specifically increase of the CHF. We fabricated the micropillar surfaces using a combination of nanosecond laser texturing and chemical etching in hydrochloric acid, while the wettability of selected surfaces was further tailored by application of a fluoroalkyl phosphonic acid and an additional laser texturing step. Three micropillar patterns were tested under pool boiling conditions using saturated twice-distilled water at atmospheric pressure. Importantly, our results revealed that the bottom part of the boiling interface (i.e., the superhydrophilic area) ensured increased liquid supply, while the top parts (i.e., the superhydrophobic area) tend to serve as nucleation sites. When combined, these two effects allowed us to simultaneously improve the CHF and the heat transfer coefficient, resulting in enhancements of up to 113% (2343 kW m−2) and 450% (205 kW m−2 K−1), respectively, compared to the benchmark untreated surface. This research provides a practical and reliable approach to enhancing heat transfer by fabricating hierarchical surfaces, offering potential applications in ultrahigh heat flux thermal technologies.
