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•Aging of untreated and enhanced boiling surfaces is underreported.•Two surfaces are exposed to water and pool boiling for eight days.•Untreated surface exhibits two-stage aging with ...increased final performance.•Laser-textured surface exhibits stable behavior with slight HTC degradation.•Universal prediction of the effect of aging on boiling performance is not possible.
Despite its importance for practical applications, investigation of aging of enhanced boiling heat transfer surfaces, which occurs during exposure to the boiling process, is often neglected. This study explores the boiling-induced aging behavior of an untreated and a laser-textured copper boiling surface during an eight-day testing period under pool boiling conditions using saturated water at atmospheric pressure. During the test, approx. 40–50 h of intermittent boiling operation were simulated. Boiling curve measurements are used to quantify boiling performance and measurements during steady state operation periods are used to analyze the heat transfer coefficient variations. SEM imaging, contact angle measurements and Raman spectroscopy are used to analyze the surface morphology, chemistry and wettability before and after exposure to hot water and boiling. The results show that no universal prediction of aging effects is possible. A distinct two-stage aging behavior resulting in increased performance was observed on the reference surface, while the boiling performance of the laser-textured surface changed very little during the test. The results also indicate that functionalized surfaces, which might primarily rely on surface microtopography for boiling enhancement, are presumably less prone to boiling behavior changes resulting from oxidation since slight changes in surface wettability will only have a minor effect on the overall boiling performance. Furthermore, evaluation of long-term performance of enhanced surfaces is necessary since their aging behavior differs from untreated surfaces.
•Superbiphilic surfaces are fabricated by CVD hydrophobization and laser texturing.•Influence of spot size, pitch and scale on boiling performance is investigated.•Optimal fraction of ...superhydrophobic areas is found to be approx. 23%.•Spot pitch has a greater influence on boiling performance than spot diameter.•Both heat transfer coefficient and CHF are enhanced using superbiphilic surfaces.
In this study, the optimal surface pattern of low and high wettability regions for enhanced boiling heat transfer is investigated using aluminum superbiphilic surfaces. Samples are fabricated by combining chemical vapor deposition of a fluorinated silane to turn them superhydrophobic and nanosecond laser texturing to render selected areas superhydrophilic. Triangular lattice pattern of superhydrophobic circular spots is utilized with spot diameters between 0.25 mm and 1.0 mm and pitch values of 0.5–2.5 mm. Pool boiling heat transfer performance of superbiphilic surfaces is evaluated using saturated water at atmospheric pressure. A strong wettability contrast is shown to be important in ensuring high heat transfer performance of wettability-patterned surfaces. Highest heat transfer performance is achieved using 0.5 mm diameter spots with a spot pitch of 1 mm and a corresponding superhydrophobic area fraction of approx. 23%. The optimal pitch value will provide a high density of potentially active nucleation sites but still allow for the development of the thermal boundary layer thus not inhibiting the activation of neighboring spots. The size of (super)hydrophobic spots appears not to have a major influence on the boiling performance when using the optimal spot pitch. The developed superbiphilic surfaces increase the CHF and provide greatly enhanced heat transfer coefficients especially at medium and high heat fluxes, making them suitable especially for high-heat-flux applications.
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•Discrepancies in reported CHF values on smooth copper surfaces are investigated.•A total of 54 data points from 47 sources from literature are analyzed.•Roughness and sample ...fabrication type statistically significantly influence CHF.•Thermal conductivity and gradient calculation method influence heat flux up to 30%.•Recommendations for reduction and fair evaluation of uncertainty are made.
This study investigates the effect of experimental setup design factors on pool boiling CHF, compares spatial temperature gradient calculation methods and analyzes the uncertainty of heat flux and surface superheat. Reported CHF values on smooth copper surfaces, measured for saturated pool boiling of water at atmospheric pressure on flat horizontal samples, are highly scattered, which cannot be explained solely by the measurement uncertainty or the randomness of the boiling process. CHF data for 54 experiments from 47 publications is analyzed using regression analysis and ANOVA to determine which experimental setup design factors influence the CHF value. Methods for estimating the axial temperature gradient in a heating stem are compared using the Monte Carlo method and analytical nonlinear gradients. Heat flux values calculated using temperature measurements in a cylindrical copper heating stem together with either constant or temperature-dependent thermal conductivity and various temperature gradient calculation methods are compared. Overall heat flux and surface superheat measurement uncertainties are analyzed and the impact of contributing uncertainties including that of the thermal conductivity, temperature measurement and distance between thermocouples is reported.
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.
Functionalized interfaces enhancing phase-change processes have immense applicability in thermal management. Here, a methodology for fabrication of surfaces enabling extreme boiling heat transfer ...performance is demonstrated, combining direct nanosecond laser texturing and chemical vapor deposition of a hydrophobic fluorinated silane. Multiple strategies of laser texturing are explored on aluminum with subsequent nanoscale hydrophobization. Both superhydrophilic and superhydrophobic surfaces with laser-engineered microcavities exhibit significant enhancement of the pool boiling heat transfer. Surfaces with superhydrophobic microcavities allow for enhancements of a heat transfer coefficient of over 500%. Larger microcavities with a mean diameter of 4.2 μm, achieved using equidistant laser scanning separation, induce an early transition into the favorable nucleate boiling regime, while smaller microcavities with a mean diameter of 2.8 μm, achieved using variable separation, provide superior performance at high heat fluxes. The enhanced boiling performance confirms that the Wenzel wetting regime is possible during boiling on apparently superhydrophobic surfaces. A notable critical heat flux enhancement is demonstrated on superhydrophobic surfaces with an engineered microstructure showing definitively the importance and concomitant effect of both the surface wettability and topography for enhanced boiling. The fast, low-cost, and repeatable fabrication process has great potential for advanced thermal management applications.
•Surfaces with periodically changed wettability were produced by a ns marking laser.•Heat transfer was investigated on uniformly and non-uniformly wettable surfaces.•Microporous surfaces with ...non-uniform wettability enhance boiling heat transfer.•The most bubble nucleations were observed in the vicinity of the microcavities.•Results agree with the predictions of the nucleation criteria.
Microstructured uniformly and non-uniformly wettable surfaces were created on 25-μm-thin stainless steel foils by laser texturing using a marking nanosecond Nd:YAG laser (λ=1064nm) and utilizing various laser fluences and scan line separations. High-speed photography and high-speed IR thermography were used to investigate nucleate boiling heat transfer on the microstructured surfaces. The most pronounced results were obtained on a surface with non-uniform microstructure and non-uniform wettability. The obtained results show up to a 110% higher heat transfer coefficients and 20–40 times higher nucleation site densities compared to the untextured surface. We show that the number of active nucleation sites is significantly increased in the vicinity of microcavities that appeared in areas with the smallest (10μm) scan line separation. Furthermore, this confirms the predictions of nucleation criteria and proves that straightforward, cost-effective nanosecond laser texturing allows the production of cavities with diameters of up to a few micrometers and surfaces with non-uniform wettability. Additionally, this opens up important possibilities for a more deterministic control over the complex boiling process.
Stability of functionalized surfaces is an often-neglected topic in phase-change heat transfer research. Here, we examine the chemical and morphological changes of textured surfaces on the molecular ...and atomic level after the critical heat flux incipience during saturated pool-boiling of water. SEM imaging, EDS, AES and XPS analyses are used to examine the surface changes. Copper samples were laser textured via ablation using a nanosecond fiber laser under air or argon atmosphere. Multiscale microcavities, which serve as preferential nucleation sites, were produced on the samples, which exhibited significantly enhanced heat transfer performance in pool-boiling tests. Repeated formation of a vapor film and accompanying temperatures of up to 320 °C during the tests resulted in changes of the surface chemistry and nanomorphology. It was determined that Cu (II) oxide and hydroxide transform into Cu (I) oxide and Cu metal as a result of repeated low-temperature annealing of the surface when a vapor film is formed during the transition towards film boiling. This additionally causes a wettability transition of the functionalized surfaces from hydrophilic towards hydrophobic. Both effects importantly influence the solid-liquid-vapor interface during phase-change heat transfer. Overall, surfaces functionalized via laser texturing exhibited significantly enhanced stability and boiling heat transfer performance.
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•Copper surfaces are modified using nanosecond laser-texturing•Pool boiling heat transfer on textured surfaces is significantly enhanced•A shift in boiling curves after the first CHF incipience is observed and investigated•Surface chemical composition is analyzed and correlated with boiling behavior•Conversion of CuO and Cu(OH)2 into Cu2O and Cu as a result of CHF onset is detected
Abstract 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.
Abstract Comprehensive grasp of heat and mass transfer, particularly in applications involving liquid-vapor phase change, hinges on management of nucleation, growth, and detachment of vapor bubbles. ...Various parameters influence the dynamics of phase-change heat and mass transfer and thus dictate the interactions between the surface, the liquid, and the vapor, profoundly impacting the underlying processes. To tailor these phenomena and harness them for technical applications involving high heat flux densities and intense mass transfer, such as boiling and electrolysis, surface functionalization is under intense development. By designing structured surfaces and creating preferential nucleation sites that promote heterogeneous nucleation, it is possible to exert control over the location and density of active nucleation sites on the surface. This, in turn, enables the regulation of bubble growth and detachment from the surface. With the aim of identifying optimal surface treatments for functionalizing surfaces and enhancing their performance in phase-change applications, we evaluated the nucleation, growth and detachment of a single bubble in a liquid-vapor phase change on untextured and laser-textured surfaces during water electrolysis. Platinum was chosen as the preferential material due to its favourable electrochemical properties for the hydrogen evolution reaction in acidic media. Electrolysis was performed at various voltages and the bubble dynamics were evaluated through high-speed imaging to investigate the intricacies of bubble growth and their mutual interactions. At 3.5 V using the laser textured surface, hydrogen bubbles detaching from the electrode surface had on average 40% smaller diameter, while the frequency of their detachment was 2,5-times higher compared to the untextured surface. This opens the possibility for further research that could lead to improving the efficiency of electrolysis.
A vast majority of heat exchangers suffer from unwanted deposition of material on the surface, which severely inhibits their performance and thus marks one of the biggest challenges in heat transfer. ...Despite numerous scientific investigations, prediction and prevention of fouling remain unresolved issues in process engineering and are responsible for large economic losses and environmental damage. This review article focuses specifically on crystallization fouling, providing a comprehensive overview of the state-of-the-art of fouling in heat exchangers. The fundamentals of the topic are discussed, as the term fouling resistance is introduced along with distinct fouling behaviour, observed in laboratory and industrial environments. Insight into subsequent phases of the fouling process is provided, along with the accompanying microscale events. Furthermore, the effects of fluid composition, temperature, flow velocity, surface condition, nucleate boiling and composite fouling are comprehensively discussed. Fouling modelling is systematically reviewed, from the early work of Kern and Seaton to recently used artificial neural networks and computational fluid dynamics. Finally, the most common fouling mitigation approaches are presented, including design considerations and various on-line strategies, as well as off-line cleaning. According to our review, several topics require further study, such as the initial stage of crystal formation, the effects of ageing, the interplay of two or more fouling mechanisms and the underlying phenomena of several mitigation strategies.
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