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.
•The apparent contact angle is influenced by nano-bubbles at the bottom of the porous coating. As heat flux density increases, these nano-bubbles gradually form thickening vapor films.•This study ...proposes a CHF prediction model applicable to biphilic surfaces based on the two-dimensional distribution of Helmholtz instability at the interface.•This study presents a CHF prediction model tailored for biphilic surfaces based on the influence of Taylor instability on the formation location of mushroom bubbles and the impact of Helmholtz instability on the liquid layer beneath the bubbles.
This study demonstrated that combination hydrophilic copper and hydrophobic nano-silver surfaces simultaneously enhanced the critical heat flux (CHF) and heat transfer coefficient (HTC) in pool boiling heat transfer (BHT) with deionized water as the employed working fluid. To further investigate the influence of geometric factors on heat transfer, this study designed three patterns denoted as squares/stripes/networks, ranging from 50 μm to 3000 μm. The effects of pattern size d, spacing p, and pitch ratio p/d on heat transfer results were obtained. Based on bubble visualization, the study further analyzed the reasons for inflection points in the boiling curve and the mechanisms caused by porous hydrophobic coating. Finally, the study proposed a numerical model suitable for predicting CHF of biphilic surfaces based on Helmholtz instability and Taylor instability. The model began with vapor columns escaping toward far-field and the fluid supplied to the heating surface. Concrete model derivation and adjustments are categorized according to the relationship between pattern size and bubble detachment diameter. For surfaces where the bubble detachment diameter exceeds the pattern size, this study originated from the perspective of Helmholtz instability on the vapor column surface. The resulting model is anticipated to quantitatively solve for the former and modulate the wavelength of Taylor instability on biphilic surfaces. Regarding surfaces with bubble detachment diameters smaller than the pattern size, this research begins with the existence time of the micro-liquid layer. The derived model validates the predominant role of p/d in the CHF values on micron-scale biphilic surfaces.
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•A biphilic surface consisting of superhydrophilic TiO2 and superhydrophobic Teflon was introduced.•The superhydrophilic TiO2 layer was deposited by the room temperature aerosol deposition ...method.•The superhydrophobic Teflon dots were deposited by the atmospheric supersonic cold spraying method.•The superhydrophobic Teflon dots facilitated efficient bubble release.•Sufficient flow passage with the hydrophilic wetting area was secure.
The ever-increasing power density overloads the heat removal capability of miniaturized power electronic devices and thus an efficient liquid cooling method needs to be strategized. Herein, we introduce a biphilic surface consisting of both superhydrophilic TiO2 and superhydrophobic Teflon. The purpose of this unique combination is to decrease the superheat (Tsat) by efficiently releasing vapor bubbles via the superhydrophobic surface and to increase the critical heat flux (CHF) using efficient wetting supplied by the superhydrophilic surface. The superhydrophilic TiO2 layer was deposited by the room temperature aerosol deposition method, whereas the superhydrophobic Teflon dots were deposited by the atmospheric supersonic cold spraying method. Each Teflon dot was 1 mm in diameter and the dots were patterned in the form of n×n arrays, where n varied from 1 to 5. The optimal CHF (187 kW·m−2), superheat, and effective heat transfer coefficient (4.6 kW·m−2·k−1) were observed for the 4 × 4 sample, beyond which the cooling effect deteriorated because of the excessive area covered by the Teflon dots. While the Teflon dots facilitated efficient bubble release, securing sufficient flow passage with the aid of a hydrophilic wetting area was also critical; thus the combination of the superhydrophobic and superhydrophilic surfaces was optimally balanced.
In this paper, pool boiling on micro-pillar structured surfaces is studied by using a three-dimensional pseudo potential phase-change lattice Boltzmann method (LBM). The joint enhancing effects on ...bubble nucleation and boiling performance are discussed in detail regarding various surface wettability and pillar geometrical parameters. Results show that on neutral and hydrophobic surfaces, enlarging the spacing of micro pillars delays the nucleation but can reduce the temperature inside vapor film and improve heat conduction. Despite that increasing the pillar height can improve heat flux, it is adverse to nucleation due to the increased cooling effect on the roots of micro pillars. In contrast, on hydrophilic surfaces, the impact of pillar spacing on nucleation is not monotonous and relatively much complicated. The heat flux is enhanced with increasing pillar spacing because of the extended three-phase contact line. And, the nucleation positions differ significantly by varying pillar geometrical parameters on hydrophilic surfaces.
•Three-dimensional pseudo potential lattice Boltzmann model is built up to study boiling on micro-pillar surface.•Pillar parameters pose significant impact on vapor bubble nucleation sites and boiling performance aside from wettability.•Bubble nucleation time, departure period and heat flux are assessed quantitatively.•Three types of bubble nucleation behaviors are detected on hydrophilic surface regarding different pillar parameters.
•Biphilic pattern enables vapor bubbles to move on the surface during boiling.•On-surface bubble movement affects boiling heat transfer.•Biphilic surface shows boiling enhancement for low heat ...fluxes.•Biphilic surface increases critical heat flux.
In pool boiling on a horizontal surface, the flow of vapor is mainly the departure of vapor bubbles from the boiling surface. If the surface is composed of specially laid-out areas with varied wettabilities, vapor can move on the surface and out of the boiling region. In the present work, a silicon surface with such a function is fabricated using laser ablation and silanization grafting methods, and pool boiling on the surface is experimentally studied. On the biphilic surface, the boiling region is super-hydrophilic (SHI), and two opposite sides of the SHI region each is in connection to a wedge-shaped SHO (super-hydrophobic) track. The two sides of the SHO track are bounded by HO (hydrophobic) areas. Tests with air bubbles underwater show that the SHO tracks enable the directional spontaneous movement of the air bubbles on the surface, and the driving force for the natural motion is analyzed. Boiling heat transfer on the surface is compared with a SHI surface without the SHO tracks. The biphilic surface shows noticeable enhancement of heat transfer for low heat fluxes (< ∼150W/cm2) but reduces heat transfer for high heat fluxes in nucleate boiling. In addition, the biphilic surface extends the heat flux and surface temperature ranges of nucleate boiling, showing ∼14% increase of critical heat flux. The effects on boiling heat transfer are related to the observations of vapor transport by the SHO tracks.
•A commercial CNT buckypaper is utilized to enhance boiling of dielectric liquids.•CHF and maximum HTC were increased by 74.2 % and 102 % compared to the flat surface.•Surface characteristics, ...wickability and bubble dynamics were investigated.•Predicted CHF based on a modified model agrees with the experimental result.•Low-cost and mass-producible buckypapers is promising for immersion cooling.
To further develop the two-phase immersion cooling technology with dielectric liquids and accelerate their applications in industrial practices, a comparative experimental study on saturated pool boiling heat transfer of FC-72 on the carbon nanotube (CNT) buckypaper and the smooth indium tin oxide (ITO) film is performed at atmospheric pressure in the present work. The commercially available buckypaper is selected as the nanostructured reinforced surface because of its low-cost, highly-scalable and mass-producible advantages. It achieves enhancements of CHF and maximum HTC up to 74.2 % and 102 %, compared to the smooth ITO surface. The underlying mechanism is well elucidated from the perspectives of surface characteristics, capillary wicking capability and bubble dynamics. The interconnected nanoporous network structure of CNT buckypaper can significantly improve bubble dynamics behavior and induce a marked nanoscale capillary wicking effect. The outstanding wickability can provide additional liquid replenishment on the boiling surface, facilitating the rapid generation and departure of bubbles as well as delaying film boiling. Interestingly, in terms of dielectric fluid (FC-72), it is found that the markedly increased bubble departure frequency on the CNT buckypaper surface primarily arises from a substantial reduction in the bubble waiting period. The present work not only provides an in-depth insight into the enhanced pool boiling characteristics of dielectric liquids on nanostructured surfaces, but also opens opportunities for the industrial applications of state-of-the-art two-phase immersion cooling technology based on nanostructures.
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The present investigation is focussed on the surface associated qualitative analysis for nucleate pool boiling heat transfer characteristics of alumina-water based nanofluids. An ...optimal ANN design has been developed with various training algorithms and hidden layer based on experimental results. The nanofluids were prepared using distilled water and alumina nanoparticles of 40 nm size. The particle concentrations varied form 0.01 wt.% to 1 wt.%. The characterisation of nanoparticles and nanofluids was done using FESEM, EDAX, zeta potential stability test and thermophysical analysis. The pool boiling setup validation was done by comparing experimental results with Rohsenow’s correlation outputs. The boiling heat transfer was performed by varying input heat flux and calculating heat transfer coefficients with surface temperatures. The post boiling analysis was also performed on the nanoparticle deposited surfaces. The effect of nanoparticle concentrations and microlayer particle deposition is studied using surface roughness analysis, contact angle measurement and microscopic images. The pool boiling heat transfer with nanofluids showed an enhancement of up to 70 % as compared to distilled water. The improved thermophysical properties of nanofluids and the enhanced surface wettability provided an overall increase in heat transfer coefficients. The particle deposited surfaces showed heat transfer deterioration of up to 40 % with water. The ANN model analysis showed that the LM (Levenberg Marquardt) algorithm provided the most optimised structure. The combination of number of epochs and MSEs (Mean square errors) were taken for selecting the best model.
•Study delves into Nuclear Power Plant emergency condensers' thermal behavior.•Research highlights heat transfer model flaws, for both sides of such heat exchangers.•Coolant pool challenge: temp ...stratification with saturation above, subcooled liquid below.•2D CFD simulations in ANSYS Fluent validated against NOKO test facility data.
Passive residual heat removal safety systems are a modern design concept employed in Nuclear Power Plants (NPPs), and operate based on natural circulation. Several passive safety systems such as the emergency condensers (EC) use heat exchanger tube bundles immersed in large pools of coolant acting as a heat-sink (secondary side). The primary side of the heat exchanger is connected to the reactor core and, when activated, removes the decay heat in case of an accident. Recent research revealed the limitation of the state-of-the-art heat transfer models to capture the heat transfer rates achieved in these systems. The problems can be identified in both – primary and the secondary sides. The main focus of this study is to investigate the thermal hydraulic behavior of the secondary side of an EC. The data from the NOKO test facility has been considered as the validation base of this study. It has been observed that strong temperature stratification establishes in the pool due to the heat transfer process happening in a confined volume zone. Under this condition, the saturation temperature is reached at higher elevations of the tank while lower the liquid remains subcooled. This leads to different thermal behavior of the liquid at different elevations in the tank which is not properly captured by simple models. In addition, due to the complexity of the heat transfer process the required computational power is another challenge to study the thermal hydraulic behavior of the system. Therefore, the main aim of this study was to correctly predict the temperature stratification in the pool and identify boiling onset at different locations specially on the heated surfaces, while maintaining minimum numerical complexity. To achieve these targets, 2D multi-phase CFD simulations of secondary side using two different frameworks was conducted using ANSYS Fluent. Then, the results were validated against experimental data to assess their accuracy to achieve the best simulation approach.
•Review heat transfer/flow parameters from studies on server cooling technologies.•Perform direct quantitative comparison between cooling technologies.•Identify heat transfer limitations, power ...requirements, etc.•Summarize industry implementations of each cooling solution from a design perspective.•Identify the capacity of cooling solution relative to heat loads expected by 2020.
This review quantitatively examines and compares the heat transfer characteristics of several cooling technologies with potential application in the server electronics industry. Strategies that have been examined include traditional air cooling, single and two-phase indirect liquid cooling, heat pipes, pool boiling, spray cooling, and jet impingement. The characteristics that have been examined include heat flux values, coolant temperatures, and coolant flowrates; which serve as indicators of the heat transfer limitations and power requirements of each cooling solution. A direct comparison against anticipated server heat loads has shown that some form of liquid cooling is necessary in high performance computing applications; where individual processor heat loads are expected to reach 300W by the year 2020. While in the case of general purpose computing, where individual processor heat loads are expected to reach 190W, air cooling remains a viable option; although other factors such as operating costs, chip reliability, and waste heat recovery may still encourage the use of liquid cooling.
•Different pool boiling CHF mechanisms and parametric trends are reviewed.•Also reviewed are modifications to mechanisms proposed to improve predictive accuracy.•Available models are shown to focus ...on specific parametric influences but not others.•The study points to the need for better understanding of near-wall interfacial behavior.
Critical heat flux (CHF) is arguably the most important design and safety parameter for any heat-flux controlled boiling application. The present two-part study is focused on CHF for pool boiling from flat surfaces. The first part will review different CHF models and associated mechanisms and parametric trends, while the second part will be dedicated to assessment of CHF models and correlations. Aside from Kutateladze’s 1948 pioneering CHF formulation, which is based on dimensional analysis, five different CHF mechanisms are prevalent in the literature: bubble interference, hydrodynamic instability, macrolayer dryout, hot/dry spot, and interfacial lift-off. Additionally, many modifications to these mechanisms have been proposed to improve predictive accuracy in tackling the parametric influences of pressure, surface size and roughness, surface orientation, and contact angle. Among the five mechanisms, Zuber’s hydrodynamic instability theory has received the most attention because of both its mechanistic formulation and theoretical appeal. More recently, the interfacial lift-off mechanism, which is also theoretically based, has received significant experimental validation, and offers the advantage of tackling different surface orientations. Overall, it is shown that, despite the large body of published pool boiling CHF literature, there are major data gaps in the coverage of relevant parameters. This points to a need for more strategically planned future experiments that would also include microphotographic analysis of near-wall interfacial features, in order to validate or dispute proposed CHF mechanisms.
