Critical Heat Flux Research Articles

Effect of graphene oxide coating on bubble dynamics and nucleate pool boiling heat transfer

Materials coating has been proved to be an effective mean to increase the number of active nucleating sites, and therefore generate more vapor bubbles and lead to better poolboiling heat transfer performance. In this work, graphene oxide (GO) is coated on a boiling surface by self-assembly method, to enhance critical heat flux (CHF). The pool boiling is carried out on a smooth copper surface to study the effect of GO coating using distilled water as the working fluid along with bubble dynamic visualization. GO coating facilitates bubble nucleation by providing numerous microscale cavities. The visualization investigation of bubble dynamic behavior shows that the CO-coated surface exhibits a higher bubble departure frequency, a smaller bubble departure diameter and smaller bubble diameters in the pool, indicating greatly enhanced heat transfer effects. Meanwhile, the GO-coated surface exhibited a smaller contact angle than the copper surface, revealing that surface becomes more hydrophilic after GO coating. Consequently, GO-coated surface with a coating time of 4 h provides a CHF of 224.3 W/cm2 and a heat transfer coefficient (HTC) of 8.79 W/(cm2·K), representing an improvement of 94.0% in CHF and 83.5% in HTC compared to smooth copper surface.

Liquid Super-Spreading Boosted High-Performance Jet-Flow Boiling for Enhancement of Phase-Change Cooling.

Enhanced boiling heat transfer via surface engineering is a topic of general interest for its great demand in industrial fields. However, as a dynamic interfacial phenomenon, deep understanding of its process and mechanism, including liquid re-wetting and vapor departure, is still challenging. Herein, we design a micro-/nanostuctured Cu surface containing periodic micro-groove/pyramid array with rich nano-wrinkles, where super-spreading (<134.1ms) of organic cooling agents highly boosts liquid re-wetting process, causing a discontinuous solid-liquid-vapor three-phase contact line and ultra-low under-liquid bubble adhesion force (∼1.3 μN). Therefore, a characteristic, ultra-fast jet-flow boiling (bubbles rapidly ejected in multiple strips) is obtained on this surface, giving a priority to nucleation (superheat ∼1.5°C) and simultaneously enhancing the critical heat flux and heat-transfer coefficient by up to 80% and 608%, respectively, compared with a flat surface. In situ observation and analysis of the nucleation, growth, and departure of micro-sized jet-flow bubbles reflects that micro-grooves/pyramids with nano-wrinkles promote the latent heat exchange process by super-spreading induced ultra-fast liquid re-wetting and constant vapor film coalescing. Based on the designed structures, high-performance phase-change cooling for CPU heat management in supercomputer centers is accomplished with an ultra-low power usage effectiveness (PUE<1.04). This article is protected by copyright. All rights reserved.

Some recent progress in reactor core thermal-hydraulics modelling

In the last more than ten years the research groups of the present authors have been working on some of the key phenomena or parameters widely used in sub-channel thermal-hydraulics (SCTH) codes, e.g. (a) transversal exchange between sub-channels, (b) circumferential non-uniform heat transfer in fuel assemblies with tight or semi-tight lattice (pitch-to-diameter ratio smaller than 1.25), (c) PDO heat transfer and rewetting and (d) CFD analysis of two-phase flow and heat transfer, incl. critical heat flux. Among others, three transversal exchange phenomena were studied, i.e. turbulent mixing coefficients, void drift and wire wrap induced sweeping flow. New models for various turbulent mixings, void drift and wire wrap induced cross flow were developed based on analytical analysis and CFD simulations. Importance of the non-uniform heat transfer behavior inside single sub-channel was identified in fuel assemblies with tight or semi-tight lattice. Correlations for predicting the non-uniform heat transfer coefficient were proposed, which were implemented into SCTH codes, to accurately determine the local hot spot on the fuel pin surface. A large experimental data base containing more than 60,000 data points of PDO heat transfer and rewetting was established. Based on the data base a new empirical correlation of PDO heat transfer was proposed. In addition, dynamic behavior of droplets (droplet size and velocity) and the thermal non-equilibrium were experimentally measured, which can be used for the validation and improvement of mechanistic models of PDO heat transfer and rewetting, also proposed by the research groups of the present authors. For the CFD prediction of CHF, a new and robust method for wall boiling heat transfer was developed to calculate the near wall void fraction and its connection to CHF.

A review of nucleate pool-boiling heat transfer in different liquids and nanofluids

The goal of this review is to examine the current state of the art in nucleate pool boiling heat transfer in a variety of different fluids. The review is divided into many sections that discuss heat transfer in pool boiling, such as pool boiling of nanofluids, boiling behavior of water–glycerin combinations, and operational parameters. With the appropriate mixes of hydrocarbons and other commercial liquids, higher heat transfer coefficients may be produced. Coatings of nanoparticles with varying layer thicknesses applied to the heater surface may be optimized to improve heat transfer from the pool to the surrounding water. The heat transfer hypothesis elucidates the peculiarities of each pool’s boiling regime. It is also possible to expand it to flow boiling by combining pool boiling liquid motion with external mechanical force. Other phase transitions, such as condensation, solidification, and melting, can also be described using boiling heat flow processes. Pool boiling performance can be improved by making a variety of adjustments to the heating surfaces as well as by using pure liquids in the water. Improvements can be made to boiling parameters such as the heat flux, the critical heat flux, the heat transfer coefficient, bubble development and departure, and so forth. A nanoparticle addition to a pure liquid or a surface coating on a heating surface can improve heat transfer and boiling properties by increasing the surface area of the liquid. Pool boiling critical heat flux was enhanced with Al2O3-water nano fluid. Authors used three different powder sizes of Al2O3 which were 0.05, 0.3 and 1.0 μm. Addition of alumina particle in water increases the boiling heat flux. Critical heat flux (CHF) was significantly enhanced using Titania and Alumina nano particles in water as compared to pure water. Average size of nano particle used was 85 nm measured by scattering electron microscope. Enhancement in Critical heat flux is due to nano particle coating on heating surface. Characteristics of nucleate boiling are greatly affected by the operating pressure. Miniature flat heat pipe (MFHP) with evaporator having micro grooved heat transfer surface gives 50% increment in critical heat flux at atmosphere pressure whereas this value increases up to 150% at 7.4 kPa pressure. The addition of CNT (carbon nanotube) to the base liquid increases the critical heat flux. Transmission electron microscopy confirms the average size of a nanoparticle as 15 nm. Authors found that by decreasing pressure from atmosphere condition critical heat flux increases to 200% with CNT/water nano fluid as compared to deionized water. SiC-water nanofluids of 100 nm size were experimented with at volume concentrations of 0.001%, 0.001%, and 0.01%. The size of the nanoparticle was confirmed by a scattering electron microscope. Authors concluded that at 0.01% of nano particle enhances critical heat flux to 105%.

Sensitivity Analysis of Transient Critical Heat Flux by RIA Under High-Pressure Flow Boiling Conditions in TRTL

A reactivity-initiated accident (RIA) is a design-basis accident under which critical heat flux (CHF) is likely to be exceeded. The operational margin for RIAs is currently determined using steady-state CHF lookup tables, which provide conservative estimates relative to transient CHF phenomena. The Transient Reactor Test Loop (TRTL) facility at Oregon State University is capable of performing out-of-pile rapid heating experiments representative of a RIA at conditions representative of a pressurized water reactor (PWR). To further our understanding of and ability to predict transient CHF under PWR conditions, we performed a sensitivity analysis on a RELAP5-3D model of the TRTL facility coupled to the RAVEN code framework to define a proposed experimental test matrix to be performed at the TRTL facility. We then implemented a flow boiling CHF correlation into RELAP5-3D and performed a secondary sensitivity analysis inspecting the impact of the built-in RELAP5-3D CHF and heat transfer multipliers on both the prediction of CHF and key safety parameters, such as peak cladding temperature and heat flux. The results show that the multiplier with the highest influence toward the prediction of CHF occurrence and the safety parameters is the transient CHF multiplier. Operational performance envelopes have been developed for each of the test matrix cases and will be used for validation once the experiments are performed. The TRTL facility is currently performing shakedown testing to verify system performance prior to proceeding with the experimental campaign. Restart testing results include pump curve restart testing, pressure tests, and heater rod thermocouple transients.

In-situ hierarchical micro/nanocrystals on copper substrate for enhanced boiling performance: An experimental study

Micro/nano structures on the Cu substrate are normally used for the enhancing the heat transfer capacity for many boiling-related applications. Conventional methods for fabricating these structures, however, require additional processing equipment and involve relatively complex processes. In this study, a simple immersion method for generating in-situ micro/nanocrystal structures on Cu substrates is developed for simplifying the fabrication with reduced cost and improving the boiling performance in the meantime. Using characterization analytical instruments including SEM, AFM, XRD, EDS and XPS, the surface morphology and chemical contents of the micro- and nanocrystals generated on the Cu substrate were examined. The experimental results showed that the hierarchical micro/nanocrystals enabled simultaneous enhancements in critical heat flux (CHF) and heat transfer coefficient (HTC), indicating the superiority of the hierarchical micro/nanocrystals in facilitating the boiling performance compared to conventional structured surfaces. It is found that the nanosheet and micro-flowers (NSMF) surface provides the largest enhancement amongst other micro/ nano structures including the nanograss forests (NG), nanograss forests and micro-petals (NGMP), and nanograss forests and micro-flowers (NGMF). Comparatively, the CHF and HTC could achieve 65.7 W/cm2 and 4.9 W/cm2K, showing an increase of the CHF and HTC by 56.5 % and 170 %, respectively, using the smooth surface in the same condition as the benchmark. It is implied that the liquid circulation is apparently promoted through the separation of vapor–liquid pathway and the bubble blanket formation is remarkably inhibited due to the special structure and morphology of the hierarchical surface. However, vital factors including the surface wettability and the experimental accuracy need to be considered for optimizing the boiling performance in diverse scenarios.

Electric field inspired alternating surface wettability for enhancing pool boiling heat transfer performance: A lattice Boltzmann method study

Electric field and surfactants have been proven to potentially alter surface wettability on demand. In this work, alternating wettability inspired by an electric field has been used to enhance boiling heat transfer performance in terms of critical heat flux (CHF) over a rough surface by using two-dimensional lattice Boltzmann simulations. In alternating wettability, the surface wettability changes with time between hydrophobic and hydrophilic. The effects of the parameters, including alternating wettability time-period and wettability contrast on the boiling process have been revealed. The simulation results show that for the alternating wettability: when surface is hydrophobic bubble expansion is faster, and then wettability transition to hydrophilic quickly retracts three-phase contact line leading to faster bubble shrinking as compared to constantly hydrophobic and constantly hydrophilic surfaces. Furthermore, the rapid shrinking of the bubble tends to disintegrate vapor film on the surface at high wall superheats, resulting in a 53% enhancement in CHF compared to a constantly hydrophilic surface. The bubble departure takes much longer time for too small/large time-periods of alternating wettability. The CHF increases with time-period, becomes maximum at a certain value and then reduces. The CHF also increases with wettability contrast, becomes largest for hydrophilic and hydrophobic contact angles 55o and 107o, respectively, and decreases with further increase in wettability contrast due to high potential for vapor film formation.

High mass flux spray evaporative quenching on hot moving plate: A novel methodology for achieving high heat flux and continuous production criteria

The attainment of very fast quenching rate and continuous production criterion has been a challenging task for the current generation researcher as the open literature does not address the above mentioned. The role of various additives for the dynamic condition of the plate has also not been disclosed in case of high mass flux spray quenching. Therefore, in the current work, the above stead is tried to mitigate by performing quenching with various additives on a hot moving plate as the discussed process minimises the chances of coolant accumulation and also reduces the drag created by the vapour on the evaporating droplet. The information on the above stated has also not been revealed in the literature. For the above-stated purpose, systematic experiments were conducted, and the obtained results have been compared with the data obtained for the static condition of the plate. The analysis ensures attainment of enhancement (38%) in the case moving condition of the plate. For all coolants such as Ethanol, SDS, CTAB, Tween 20, Nanofluid, PEG 200, and PEG 600 the boiling curve depicts the same trend, i.e., transition boiling behaviour at higher temperature and nucleate boiling behaviour at a lower temperature. The achieved average heat flux, critical heat flux and initial heal flux values are higher than the data obtained in the case of the static condition of the plate.

Studies of liquid metal boiling in fuel assemblies of fast reactors in accident conditions

The paper presents the results of experiments carried out at IPPE JSC on heat transfer and circulation stability at sodium–potassium alloy boiling on models of single fuel assemblies and in a system of parallel fuel assemblies, taking into account the influence of various factors on the boiling process at coolant natural convection.The obtained experimental data have shown that the stable nucleate boiling in the simulated fuel assemblies is observed only in a restricted region of heat fluxes, its transition to unstable pulsating slug boiling is determined by various factors, the transition boundary from nucleate boiling to slug, annular-dispersed and dispersed regimes of two-phase liquid metal flow in fuel rod assemblies are approximated by simple dependences.The onset of an oscillation process at coolant boiling in one of the parallel fuel assemblies leads to an antiphase oscillation process in another fuel assembly, the hydrodynamic interaction of the loops in various fuel assemblies may cause a significant increase in the amplitude of coolant flow rate oscillations (“resonance” of flow rate oscillations) and “choking” or inversion of the coolant flow rate in the loops, the temperature rise in the coolant and fuel rod cladding (the effect of inter-channel instability) and, eventually, a critical heat flux (CHF).In the assembly with low surface roughness of the rod simulators, evolution of an unstable (slug) regime with sharp coolant flow rate oscillations and overheating of the simulator wall results in a CHF, . For the fuel rod simulators with industrially-manufactured surface roughness, due to the appearance of a liquid film on the surface of the simulators, a transition from the unstable slug regime to the stable annular-dispersed one is observed.For sodium boiling in the fuel assembly model with a sodium “cavity“ located above the reactor core, which is designed for compensation of the positive sodium void reactivity effect in case of boiling onset, the feasibility of long-term sodium cooling of fuel rod simulators in the fuel assemblies has been demonstrated for these conditions.The data on liquid metal boiling heat transfer in the assemblies were generalized a cartogram of the flow regimes for a liquid metal two-phase flow in the assemblies was constructed, which significantly differs from a cartogram of the flow regimes for water. The paper presents the results of the calculated and experimental data comparison.

HADES - high heat load testing - facility at CEA-IRFM

A new facility based on an electron beam for the testing of plasma facing components (PFC) has been built at CEA-IRFM (France). The aim of this facility, named HADES (High heAt loaD tESt), is to assess heat removal capabilities of PFCs exposed to high heat flux. The facility can cover thermal fatigue tests, critical heat fluxes and thermal shocks. This facility has been designed in order to provide the maximum performance and flexibility for the qualification of PFCs.This facility has been built collecting a former high heat load platform, named FE200 [1], and located in Le Creusot-France resulting from collaboration between CEA and Framatome in the 90′s. All the systems have been refurbished except the e-beam gun, which has been replaced by a brand new one with 150 kW and 45 kV of maximum power and accelerating voltage, respectively.The HADES e-beam gun will permit to simulate steady state experiments with thermal flux up to a few tens MW/m² and transient events with thermal energy up to 1 GW/m² during a few milliseconds. A programmable sweep system allows several kinds of energy repartition (uniform and peaked), on a ± 25° shooting angle with the normal to the surface.This paper will present in details the technical specifications related to the e-beam gun as well as to the auxiliary systems. Finally, the main results of the commissioning tests and first campaign, achieved at the end of 2020 will be presented.

Flow Boiling Heat Transfer Enhancement Using Tuned Geometrical Contact-Line Pinning

Wettability patterning is a promising method to manipulate bubble dynamics in microscale boiling systems, allowing the transfer of large heat fluxes at low wall temperatures. Herein, we experimentally investigate the enhancement of flow boiling through exploitation of contact-line pinning using superbiphilic wettability patterns with a range of geometries and orientations. We compare the boiling performance on symmetrical (i.e., circular, square, and diamond-shaped) and asymmetrical (i.e., triangular) superhydrophobic patches and also create rings and chevrons through insertion of self-similar, recursive superhydrophilic cut-outs. Two main principles for boiling heat transfer enhancement are demonstrated: first, the ease of bubble departure from the superhydrophobic patches is shown to depend upon the interaction between the local contact angle and the bubble's tilt due to hydrodynamic drag; second, we find that ring-shaped superhydrophobic patches may trap droplets inside the forming bubbles, thus supplementing the heat transfer coefficient and critical heat flux through latent heat. Through application of these principles, the heat transfer coefficient and critical heat flux of heterogeneous surfaces was enhanced over the homogeneous analogues by 62% and 24%, respectively. Finally, we establish and validate a general model to estimate the ease of bubble departure through the use of geometric arguments.

Boiling heat transfer study of Micro-Diamond coated surface using underwater explosion shock wave method

Pool boiling heat transfer experiments are performed on micro-diamond particles coated heating surface (DPC heating surface) and no-coated smooth surface on copper substrates. Here DPC heating surface is fabricated with the aid of underwater explosion shock wave. Experiments are conducted on the substrate surface area of 15 × 15 mm2 using distilled water as the working fluid. The substrate is fixed tightly on the heater block, which is embedded with ceramic cartridge heater of 1500 W. The influence of pool boiling heat transfer is experimentally studied over a range of surface roughness values with particles coated heating surface at atmospheric pressure. The obtained results of DPC surfaces are compared with that of the smooth bare copper surface. A remarkable enhancement of the critical heat flux (CHF) at lower wall superheat is observed on the DPC surfaces. The DPC surface is found to be effective in increasing the nucleate boiling and CHF

Bubble-induced oscillating flow in microbubble emission boiling under highly subcooled conditions

Microbubble emission boiling (MEB), a phenomenon that occurs under highly subcooled boiling conditions, can achieve a high heat flux beyond critical heat flux. MEB has been observed to differ from nucleate boiling and is always accompanied by the emission of microbubbles from an oscillating bubble. The heat transfer mechanism of MEB differs from that of nucleate boiling and remains elusive. In this study, we measure the behaviour of a vapour bubble and its induced liquid flow simultaneously during subcooled boiling using the high-speed two-phase particle image velocimetry method. Different from nucleate boiling, we observe a rapid oscillating flow outside the bubble that constantly attaches on the heating surface in MEB. The spatially and temporally averaged velocity magnitude values under this oscillating flow, and their dependency on liquid subcooling and heat flux, are quantified. Then we derive a scaling law for the heat transfer of MEB as a function of Péclet and Jacob numbers. With such correlations, we suggest that the oscillating flow induced by bubble oscillations is important to MEB, thus elucidating a different heat transport process from nucleate boiling.

Numerical Investigation of the Heat Transfer Characteristics and Wall Film Formation of Spray Impingement in SCR Systems

This work established a numerical model to investigate the heat transfer characteristics and wall film formation of spray impinging on the wall in the selective catalytic reduction (SCR) system. The model is developed by the Eulerian–Lagrangian approach, where the Lagrangian approach is used to represent the spray generated by a commercial non-air-assisted pressure-driven injector and the Eulerian approach is adopted to represent exhaust gas. The Stochastic Kuhnke Model is applied to spray/wall interaction. The model considers relevant processes, which include mass transfer, momentum transfer, heat transfer, droplet phase change, spray/wall interaction, and wall film formation. The numerical results compared with that of the experiment indicate that the model can accurately estimate the heat transfer characteristics of the wall surface during the spray impingement. Based on the numerical results, the causes of the spray local cooling effect and the rapid cooling effect are analyzed. The correlation between the critical transition temperature and the critical heat flux temperature for wall film formation is derived from the trends of wall temperature and heat flux. In this work, the Stochastic Kuhnke Model is applied and compared with the Kuhnke Model, which proves that it can improve the disadvantage of sudden change during the wall film formation. When the wall temperature is below the critical transition temperature, the wall film mass is sensitive to the wall temperature and increases as the wall temperature decreases.

Heat Transfer Characteristics of Pool Boiling with Scalable Plasma-Sprayed Aluminum Coatings

To ensure adequate reliability in two-phase cooling systems involving boiling, it is essential to enhance the heat transfer coefficient and maximize the critical heat flux (CHF) limit. A key technique to avoid surface burnout and increase the CHF limit in pool boiling is the frequent coolant supply to the probable dry-out locations. In the present work, we have explored the plasma-spray coating as a surface modification technique for enhancing heat transfer coefficient and CHF value in pool boiling applications. Three plasma-coated aluminum surfaces (C-15, C-20, and C-25) are fabricated on a copper substrate at three different plasma powers of 15, 20, and 25 kW, respectively. Detailed surface morphologies of the plasma-sprayed coatings are presented, and their roles in pool boiling heat transfer mechanisms are analyzed. Plasma-coated surfaces exhibit wickability characteristics and enhanced wettability compared to the plain copper surface. Saturated pool boiling experiments are performed with DI (deionized) water at atmospheric pressure. Plasma spray-coated surfaces show favorable boiling incipience with less wall superheat and more active nucleation sites than the plain copper surface. Compared to the plain copper surface, enhancement values of nearly 68, 60.7, and 55.5% in the heat transfer coefficient are observed for C-15, C-20, and C-25 plasma-coated surfaces, respectively. Experiments could not be performed beyond the heat flux of 197 W/cm2 due to repeated failure of the cartridge heaters. Based on the experimental measurement of wickabilities, the CHF values of plasma-coated surfaces have been theoretically calculated. Compared to the plain copper surface, a maximum 2.39 times higher CHF value is observed for C-15 plasma-coated surface. Improved wettability and wickability are responsible for CHF enhancement in the case of plasma-coated surfaces. At higher heat flux, capillary wicking and frequent rewetting of the dryout locations delay the burnout phenomenon, enhancing CHF in plasma-coated surfaces. The plasma-spray coating is a robust and scalable process, which can be a potential candidate for high heat flux dissipation in various industrial applications.

CHF enhancement of CNT-COOH nanofluids in the boiling experiment with a downward-facing heating surface

The effects of carboxylated carbon nanotube (CNT-COOH) nanofluids prepared with various surfactants were researched on the pool boiling heat transfer coefficients (HTC) and critical heat fluxes (CHF). Sodium dodecyl sulfate (SDS), and gum arabic (GA) were used to improve the dispersion and stability of CNT-COOH nanofluids. Then, preparing different weight concentrations of each nanofluid (CNT-COOH/GA-water and CNT-COOH/SDS-water nanofluid) and investigated for their thermal performance. The result shows that the CHF of both nanofluids was significantly increased in a specific concentration range. Furthermore, the CNT-COOH/SDS-water nanofluids have lesser pool boiling HTC compared with reverse osmosis (R.O.) water, and CNT-COOH/GA-water nanofluids slightly increase at 0.005 g/L and 0.02 g/L. By conducting roughness and hydrophilia of the heating surface tests, it is speculated that the CNT-COOH deposition produced during the boiling process is the main reason for affecting HTC and CHF. The deposition has a complex hydrophilic microstructure, which makes the water droplets entering the interior can take away a large amount of heat on the heating surface and then enhance CHF.

Effect of thermal contact resistance on the CHF and HTC for pool boiling heat transfer

In recent years, there has been increasing interest in the demand for heat dissipation of electronic devices. Constructing micro/nanostructures and using nanofluid for the pool boiling heat transfer have proved to be a notoriously effective method to increase the critical heat flux (CHF) as well as the heat transfer coefficient (HTC). When the tested samples with micro/nanostructures are soldered to the upper of the heated copper block, the CHF boiling curves of the tested samples always shift right, which may be caused by the thermal contact resistance (TCR) from the soldering interface. However, it is rarely for the research to be reported the effect of the thermal contact resistance on the pool boiling heat transfer performance. This paper highlights the effect of the TCR on the CHF and HTC for pool boiling heat transfer by comparing the boiling curve of the non-soldering, the soldering, and the Rohsenow curve. The relationship between the difference in temperature with increased heat flux is established by measuring the TCR of the soldering interface, and the relationship at the different soldering interface morphology has been explored. The results show that the CHF boiling curve with soldering shift right 106–144 % than that of the non-soldering, and the HTC decreases by 45.6–63.5 %. The shifted boiling curve could be corrected by the relationship between the TCR with increased heat flux. However, the boiling curve cannot be corrected by the relationship when the soldering interface has voids. This work demonstrates that the deviation between the boiling curve of soldering with the boiling curve of non-soldering is universal for engineering applications by comparing similar studies. Regardless of whether soldering is used in pool boiling heat transfer experiments, the Rohsenow curve is still an important reference, but the effect of TCR must be considered in the pool boiling curve of soldering.

A novel surfactant material for performance enhancement on nucleate pool boiling heat transfer

Long-chain surfactants have been extensively studied for boiling enhancement, while the investigation of steroid surfactants is limited. In this study, steroidal bile salts were experimentally attempted for nucleate pool boiling enhancement on flat copper surface. Four types of bile salts were found effective in improving heat transfer coefficient (HTC) in aqueous form, with SDC having the most significant effect, followed by SHDC, SC, and STGC. The STGC deposited surface exhibited a simultaneous improvement of critical heat flux (CHF) and HTC with long-term repeatability. The enhancement mechanism is discussed by means of solution properties, bubble behaviors and surfactant molecular structure. Both aqueous and deposited bile salts show considerable boiling performance with existing materials, with better safety characteristics. The findings of this study can provide a bio-friendly approach to surfactant-aided energy conversion applications and contribute to the development of new-generation green surfactant materials.

Effect of electric field, mass flow rate, and subcooling degree on subcooled flow boiling in a micro-slit channel

This study investigated the design and cooling performance of a micro-slit-channel heat sink for electrical device cooling. The subcooled flow-boiling performance of a dielectric liquid, HFE-7000, was experimentally characterized. A flat electrode made from stainless steel with 10 narrow slits, each 1 mm width and 17 mm long, was placed at 600 μm above the heated surface to generate a high electric field. The flow boiling heat transfer performance of an electroplated diamond surface was evaluated. Four electric field intensities from 0 to −5 kV/mm, three mass flow rates from 1.83 to 4.8 g/s, and four inlet subcooling from 5 to 25 K were tested at an atmospheric system pressure. Four flow boiling patterns were categorized using a high-speed camera: single-phase flow, subcooled nucleate boiling, saturated nucleate boiling, and local dry out boiling. The maximum critical heat flux (CHF) and heat transfer coefficient were 95 W/cm2 and 30 kW/m2K, respectively, for mass flow rate of 4.83 g/s, electric field of −5 kV/mm, subcooling degree of 9 K, and mass flow rate of 4.83 g/s. A semi-empirical prediction of the CHF exhibited good agreement with the experimental results within ±18 %.

A REVIEW OF POOL BOILING HEAT TRANSFER PROPERTIES BY NANOFLUID

Boiling heat transfer has maintained a high degree of interest due to the range of its applications in the energy sector. In recent years, much research has focused on improving the nucleate pool boiling by modifying the fluid properties. In this review article, the basic properties and characteristics of Al2O3 nanofluids and few other nanofluids are explored and discussed through past research findings. Next, previous studies that involved pool boiling heat transfer enhancement using Al2O3 nanofluid and its performance in terms of critical heat flux (CHF) and heat transfer coefficient (HTC) are further highlighted. These studies have employed methods that affected the performance of CHF and HTC such as electric field and surface modification. Maximum enhancement in CHF measured is approximately 200%. On the other hand, usage of prediction models to predict enhancements are also discussed thoroughly. Regardless of boiling performance enhancements with the deployment of nanofluids, several concerns must first be addressed before it is able to be deployed for practical use.