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.
Pool boiling with high energy conversion efficiency is critical to steam generation, electronics cooling, water purification, and other energy applications. Great efforts have been taken to the ...design and fabrication of high-performance pool boiling systems to meet the requirements of high heat flux applications. Three-dimensional complex structures (3D-CS) that can suppress wall superheat at the onset of nucleate boiling, enhance the heat transfer coefficient and critical heat flux are taken as an effective means to enhance the pool boiling performances. Relatively recent advances in the fabrication of 3D-CS have led to exciting enhancements in pool boiling performances and a better understanding of the underlying science. The superiority of 3D-CS in pool boiling heat transfer is mainly attributed to the larger heat transfer area, more nucleation sites, better bubble dynamics, and faster liquid rewetting rate, etc. In this review, the structural characteristics and pool boiling enhancement of 3D-CS are reviewed from the perspective of fabrication methods. Traditional machining methods, special machining methods, and other machining methods used for fabricating 3D-CS are introduced and their advantages and disadvantages are summarized. Emphasis is on the influence of fabrication methods on pool boiling enhancement of 3D-CS, which is beneficial to the application and commercialization of pool boiling systems. Moreover, some challenges and research recommendations of 3D-CS are presented for future studies.
•A review on pool boiling enhancement of three-dimensional complex structures (3D-CS) is presented.•The structural characteristics of 3D-CS are reviewed from the perspective of fabrication methods.•Almost all the fabrication methods used to fabricate 3D-CS are summarized.•The influence of fabrication methods on pool boiling enhancement of 3D-CS is discussed.
With recent advances in micro- and nanofabrication, superhydrophilic and superhydrophobic surfaces have been developed. The statics and dynamics of fluids on these surfaces have been well ...characterized. However, few investigations have been made into the potential of these surfaces to control and enhance other transport phenomena. In this article, we characterize pool boiling on surfaces with wettabilities varied from superhydrophobic to superhydrophilic, and provide nucleation measurements. The most interesting result of our measurements is that the largest heat transfer coefficients are reached not on surfaces with spatially uniform wettability, but on biphilic surfaces, which juxtapose hydrophilic and hydrophobic regions. We develop an analytical model that describes how biphilic surfaces effectively manage the vapor and liquid transport, delaying critical heat flux and maximizing the heat transfer coefficient. Finally, we manufacture and test the first superbiphilic surfaces (juxtaposing superhydrophobic and superhydrophilic regions), which show exceptional performance in pool boiling, combining high critical heat fluxes over 100W/cm2 with very high heat transfer coefficients, over 100kW/m2K.
•Pool boiling tests on microporous coated surfaces were performed.•Microporous surface gives low incipience superheat, 50–270% enhancement in HTC and 33–60% in CHF.•Bubble site density, departure ...diameter and departure frequency were measured from high speed videos and a heat flux partition model was used to obtain contributions of various modes.•On the plain surfaces, both evaporative and quenching components contribute nearly equally to the total heat flux.•On the microporous surfaces, the evaporative component contributes ∼70% of the total heat.
Nucleate pool boiling experiments were performed on microporous copper surfaces and plain surfaces using saturated HFE-7100 as the working fluid. Quantitative measurements of the bubble dynamics, such as the nucleation site density, bubble diameter at departure, and bubble departure frequency, were obtained using high-speed visualization. The microporous surfaces, with coating thicknesses in the range of 100–700μm, porosity of 55–60%, and cavity sizes in the range of 0.5–5μm, showed a significantly lower boiling incipience temperature, which enhanced the heat transfer coefficient by 50–270% and enhanced the critical heat fluxes by 33–60% when compared to the plain surface. At low heat flux levels, the surface with a thicker microporous coating showed better performance than the thinner one. However, the thinner microporous coating resulted in higher critical heat flux than the thicker surface. The site density, departure diameter, and departure frequency were compared against the predictions using various correlations from the literature. Based on a heat flux partition model, using the measured values of the active site density and bubble departure diameter and frequency, and neglecting the single-phase heat transfer effects of bubble coalescence, the individual modes of heat transfer (evaporative, quenching, and convective) were computed. Reasonably good agreement between the partition model results and the experimental data was obtained. On the plain surfaces, the evaporative and quenching components were approximately equal. On the microporous surfaces, the evaporative component was found to be significantly higher.
Multiwalled carbon nanotubes (MWCNTs) exhibit outstanding physical properties, including high thermal conductivity, excellent mechanical strength, and low electrical resistivity, which make them ...suitable candidates for a variety of applications. The work presented in this paper focuses on the pool boiling performance of refrigerant R-134a on microporous Cu-MWCNT composite surface layers. A two-stage electrodeposition technique was used to fabricate Cu-MWCNT composite coatings. In order to achieve variation in the surface properties of the Cu-MWCNT composite surface layer, electrodeposition was carried out at various bath temperatures (25 °C, 30 °C, 35 °C, and 40 °C). All surfaces coated with the Cu-MWCNT composite demonstrated superior boiling performance compared to the uncoated surface. Heat transfer coefficient (HTC) values for Cu-MWCNT composite surface layers, prepared at bath temperatures of 25 °C, 30 °C, 35 °C, and 40 °C, exhibited improvements of up to 1.75, 1.88, 2.06, and 2.22, respectively, in comparison to the plain Cu surface.
In this study, a stable hydrophilic thin layer resembling SiOx is formed on the copper surface by combining plasma polymerization using hexamethyldisiloxane (HMDSO) and Ar plasma activation. The ...effect of coating on the Heat Transfer Coefficient (HTC) and Critical Heat Flux (CHF) at two different subcooling temperatures is investigated through pool boiling experiments. It is found that the HTC and CHF of the modified surface improved by 42 % and 97 %, respectively. The chemical composition of the coating, as well as changes in surface roughness, wettability, and porosity, are studied using the Scanning Electron Microscope (SEM), Energy Dispersive X-ray spectrometer (EDX), Fourier Transform Infrared (FT-IR) spectroscopy, and contact angle measurement. The boiling/cooling experiments for the plasma-coated surface show good stability, demonstrating that the surface characteristics remain stable even after three boiling/cooling cycles.
•Study explores pool boiling of HFE-7100 on copper surfaces.•Heat transfer coefficient improves with increasing roughness.•An optimal hole diameter and pitch for artificial nucleation sites gives ...best heat transfer.•Combination of roughness and hole pattern further improves the heat transfer.
This study explores pool boiling of HFE-7100 on copper surfaces. The key objective of this study was to examine the effects that surface modifications have on nucleate boiling performance. The surface enhancements studied are roughness, artificial nucleation sites, and a combination of both. Observing roughness between 0.480 μm to 7.564 μm shows that the heat transfer coefficient improves with increasing roughness. Observing hole diameters from 1 mm to 3 mm and hole pitch, or spacing to diameter ratio, from 1.75 to 3.5; a configuration with a hole diameter of 1 mm and pitch of 2.5 provides the best improvement to heat transfer coefficient compared to a bare surface with a roughness of 0.480 μm, while the configuration with a hole diameter of 1 mm and pitch of 3.5 provides worse heat transfer coefficient compared to a bare surface with a roughness of 0.480 μm. Applying a roughness to a hole pattern also improves the heat transfer coefficient with increasing roughness compared to both a bare surface with a roughness of 0.480 μm, as well as to the hole pattern alone. The majority of the surface enhancement modes yield overall improvements in heat transfer coefficient. The introduction of surface enhancement decreases critical heat flux across all samples.
•Pool boiling on pillar-structured surfaces with different wettability patterns is investigated.•The combined effects of surface structure and mixed wettability are experimentally explored.•MPoPi ...mixed pattern always leads to a leftward shift of the boiling curve but gives a lower CHF.•The performance of MPiPo mixed pattern is strongly dependent on the channel-to-pillar width ratio.
In this paper, we experimentally investigate the pool boiling heat transfer on pillar-structured surfaces with different wettability patterns and different values of the channel-to-pillar width ratio. Five types of wettability patterns are compared, i.e., homogeneous hydrophilicity (HPi), homogeneous hydrophobicity (HPo), homogeneous near-superhydrophilicity (SHPi), a mixed pattern with hydrophilic bottom and hydrophobic pillar top (MPiPo), and another mixed pattern with hydrophobic bottom and hydrophilic pillar top (MPoPi). The effects of homogenous wettability patterns on the boiling performance of pillar-structured surfaces are basically consistent with those on plain surfaces, but the differences between the HPi and HPo patterns are found to be gradually enlarged with the decrease of the channel-to-pillar width ratio. Moreover, compared with the base HPi pattern, the MPoPi mixed pattern leads to a leftward shift of the boiling curve and an upward shift of the heat transfer coefficient (HTC) curve but gives a lower value of the critical heat flux. Such a tendency of the MPoPi mixed pattern is not affected when the channel-to-pillar width ratio is decreased from 2.0 to 0.5. In contrast, the boiling performance of the MPiPo mixed pattern is found to be significantly affected by the channel-to-pillar width ratio as it performs best among the five types of wettability patterns when the channel-to-pillar width ratio γ⩾1, but it deteriorates the boiling heat transfer at high heat fluxes when the ratio γ is reduced to 0.5.
The dielectric fluid of GaldenR HT 55 (Perfluoropolyether) is regarded as a potential candidate for two-phase immersion cooling since its boiling point at 1 atm is 55 °C which is quite suitable for ...electronics cooling. The fluid is of special interest since the main vender of dielectric fluid will no longer produce FGAS after 2024. However, very rare nucleate boiling performance data for HT55 were reported in the literature. The objective of this study aims to provide nucleate boiling performance data for various heat sinks applicable for nucleate boiling. The test samples include a smooth reference surface (#1), 2 pin fins (the pin diameters are 1 mm (#2) and 2 mm (#3), and 3 pin fin with various sintered coatings (powder size: 150–200 μm (#4), 200–250 μm (#5), and 250–350 μm (#6)). Heat flux spans from 80 to 600 kW/m2. HTC for small pin fin (#2) is marginally higher than that of smooth surface, and large pin fin (#3) shows about 40 % enhancement compared to smooth surface at 140 kW/m2. HTCs for sintered pin fins are much higher than those of pin fin or smooth surface with approximately 90–95 % enhancement relative to the smooth surface at low heat flux. The visual observations indicate that sample #6 yields the largest bubble departure diameter and departure frequency. Yet bubble generations are especially pronounced on top of the pin fin. The CHF of the tested surfaces are following the sequence: #5 > #6 > #4 > #3 > #2 > #1. The sample #5 contains a CHF over 600 kW/m2 which is about four times higher than that of the smooth surface.
•Pool boiling performance for six heat sinks subject a dielectric fluid of GaldenR HT 55.•Test samples are a smooth surface (#1), 2 pin fins, and 3 pin fin with various sintered coatings.•The coating of powder size: 150–200 μm (#4), 200–250 μm (#5), and 250–350 μm (#6)).•HTCs for sintered pin fins exceed pin fin or smooth surface by 90–95 %.•The CHF sequence: #5 > #6 > #4 > #3 > #2 > #1. The CHF of sample #5 is 4 times #1.
We study the role of surface topology, surface chemistry, and wall superheat temperature on the onset of boiling, bubble nucleation and growth, and the possible formation of an insulating vapour film ...by means of a novel setup for large-scale MD simulations. To minimise the effects of the system size on the bubble growth and the formation of the vapour film, we perform simulations in a box larger than those previously considered. The effect of the system pressure on bubble nucleation and growth is isolated by imposing a constant force on a moving piston and mechanically controlling the pressure. The simulations reveal that the presence of a nanostructure determines the nucleation site and facilitates the energy transfer from the hot substrate to the water. The surface chemistry, on the other hand, governs the shape of the formed bubble. A hydrophilic surface accelerates the bubble nucleation, however, decelerates the bubble expansion, thus postponing the formation of the film of vapour. Hence, a hydrophilic surface provides better energy transfer from the hot wall to the water. By analysing the system energy, we show that irrespective of wall topology and chemistry, there is a wall temperature for which the amount of transferred energy from the wall is maximum.
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