Pool Boiling Heat Transfer Enhancement Research Articles

Enhanced pool boiling heat transfer characteristics on microstructured copper surfaces coated with hybrid nanofluid

Enhanced pool boiling heat transfer characteristics on microstructured copper surfaces coated with hybrid nanofluid

Hierarchical sintered porous surfaces with enhanced pool boiling heat transfer performance for high-power cooling applications

Porous surfaces fabricated by sintering maintain good pool boiling heat transfer performances and have the advantages of high processing efficiency and low cost. Hierarchical sintered porous structures (HS) composed of copper mesh and porous column array are fabricated by a sintering process. Chemical oxidation and high-temperature reduction are used to deposit micro/nano coating on HS (NHS). The porous column array induces the capillary value effect and enhances the capillary performances of HS and NHS. The micro/nano coating provides more nucleation sites and further enhances the capillary velocity. The faster capillary velocity and higher volumetric flow rate of HS and NHS result in much higher critical heat flux (CHF) than sintered mesh surfaces (MS). The CHF of NHS reaches 2422.9 kW/m2, which is 43.7 % and 21 % higher than that of MS and HS, respectively. A good linear relationship between the volumetric flow rate and the CHF is observed. Besides, the porous structures greatly enhance the heat transfer coefficient (HTC). The micro/nano coating of NHS has a more obvious enhancement effect on HTC at low heat fluxes. The pool boiling heat transfer performance of HS and NHS is comparable to or even better than those in literature and has the feasibility of application in high-power cooling scenarios.

Pool boiling heat transfer enhancement induced by capillary wicking from radial gradient structures: A lattice boltzmann study

With the aid of a three-dimensional lattice Boltzmann model built in the lab, the boiling state on the surface with gradient groove structure is investigated. The emphasize is placed on the impact brought by the wicking capability from the gradient structure on boiling performance. Besides the roles of distribution, number, width and depth of the grooves in the boiling process discussed in detail, the effects of wall superheat and wettability on boiling heat transfer are also explored. The results show that the wicking force generated by the gradient grooves can improve the boiling performance to an extent. Adjusting the geometry of grooves can enhance heat flux by up to 27.46% compared to a flat competitor. However, there exists a limit for the gradient defined in this study. Pursuing larger gradient values can extend the pit region at the center of surface. Upon TPCL being trapped within the pit region, it causes a negative effect on the boiling performance. The wicking capability has a positive relationship with surface wettability at a given groove number and size. In regards to the parametric effects of groove, the heat flux first increases but then declines with increasing the groove width at a fixed Jacob number. Despite the enhanced boiling performance with increasing the groove number and depth, the enhancement has an upper limit. But, the groove distribution has a minor effect on the boiling performance if TPCL is outside the pit region. This study provides quantitative evidence for the fact that wicking can promote boiling from the numerical view.

Experimental study of pool boiling performance of Fe3O4 ferromagnetic nanofluid on a copper surface

Nanofluids significantly enhance the critical heat flux of boiling heat transfer. This paper experimentally investigates the pool boiling performance and the influence mechanism of Fe3O4 nanofluids. Compared with deionized water, the 0.001 vol% nanofluid increases a maximum enhancement in critical heat flux by 47.90%. During nanofluid boiling, Fe3O4 nanoparticles are deposited on the surfaces. The nanoparticle deposition surfaces are physically characterized to explain the influence mechanism of Fe3O4 nanoparticles on boiling heat transfer. Nanoparticle deposition modifies the surface micro-morphology, which increases roughness and improves wettability. The changes are essential factors for the enhancement of the critical heat flux. This paper further analyses the boiling results of deionized water on the nanoparticle deposition surfaces. Compared with a polished surface, the critical heat flux and heat transfer coefficient of the nanoparticle deposition surface show maximum increases of 52.39% and 56.19%. Due to the similar enhancement of critical heat flux using the Fe3O4 nanofluid and the nanoparticle deposition surface, it is found that the increased critical heat flux of the nanofluids is attributed to the improvement of surface wettability and roughness by nanoparticle deposition. This study analyzes the mechanism of Fe3O4 nanofluid for enhancing pool boiling heat transfer from the perspective of modifying boiling surface characteristics by nanoparticle deposition, especially in wettability and roughness, which advances the understanding of enhanced boiling heat transfer by nanofluids.

Accurate and robust predictions of pool boiling heat transfer with micro-structured surfaces using probabilistic machine learning models

The accurate and reliable prediction of enhanced heat transfer performance of micro-structured surfaces is crucial to optimally design and operate pool boiling systems. However, the existing empirical models predict the enhanced pool boiling heat transfer with very large errors up to ±81 % even using the experimental data from the same study, mainly due to the complex nature of the pool boiling processes. More importantly, the existing models predict only limited coolant types, surface geometries, and operating conditions. To overcome these challenges, this study examines three deterministic and two probabilistic machine learning (ML) models for accurate and reliable enhanced pool boiling prediction, while using carefully selected key six dimensionless numbers. The models were trained and tested using 1,241 data from 20 experimental studies with 80/20 % of train/test ratio, and the pre-trained models were tested for additional 519 data from 6 studies to evaluate the models reliability. The predicted mean absolute percentage error (MAPE) shows that Bayesian, deep, and 1-D convolutional neural network (1D-CNN) models outperform the random forest and natural gradient (NG) boost models due to their extended hidden layers. The machine learning models improve the MAPE by up to 30 % compared to the existing correlations. Furthermore, a parameter sensitivity analysis is performed using explainable artificial intelligence showing that the boiling Reynolds number is the most critical parameter followed by the kinetic Reynolds and Bond numbers. The probabilistic ML models can also provide the uncertainties to improve prediction reliability compared to the deterministic ones.

Corrosive effect on saturated pool boiling heat transfer characteristics of metallic surfaces with hierarchical micro/nano structures

Surface modification is of critical interest to enhance boiling heat transfer in terms of heat transfer coefficient or critical heat flux (CHF), which is significantly affected by distinct surface morphology and wettability and it can improve the efficiency and safety of equipment. Furthermore, actual service environment may cause severe corrosion to the processed structured surfaces while its consequence on boiling heat transfer is still obscure. In this article, comprehensive researches are conducted to unravel corrosive effect on metallic samples made of stainless steel (SS) and Inconel materials with microstructures. Different constructions (i.e., microgroove, microcavity and micropillar array) and characteristic dimensions (∼20, 50 μm) of microstructure, various duration time (up to 300 days) and pH values (∼7.0–8.5) of corrosive environment are compared thoroughly. Conclusions can be drawn that not all microstructures can enhance pool boiling heat transfer characteristics, especially in terms of CHF values. More importantly, CHF value of SS microgroove sample firstly increases from 60.94 to 94.09 W·cm−2 in 50 days, then decreases to 47.77 W·cm−2 in 300 days, which can be attributed to competition result between formation of hierarchical micro/nano structure with enhancing wicking capability and chemistry condition with increasing contact angle. In addition, distinct bubble dynamics during pool boiling is also analyzed. The insights obtained from this article can be used to guide surface modification method and to reveal evolvement rule of engineered metallic surface in highly corrosive and harsh boiling heating transfer environment.

An Overview of the Recent Advances in Pool Boiling Enhancement Materials, Structrure, and Devices.

This review attempts to provide a comprehensive assessment of recent methodologies, structures, and devices for pool boiling heat transfer enhancement. Several enhancement approaches relating to the underlying fluid route and the capability to eliminate incipient boiling hysteresis, augment the nucleate boiling heat transfer coefficient, and improve the critical heat flux are assessed. Hence, this study addresses the most relevant issues related to active and passive enhancement techniques and compound enhancement schemes. Passive heat transfer enhancement techniques encompass multiscale surface modification of the heating surface, such as modification with nanoparticles, tunnels, grooves, porous coatings, and enhanced nanostructured surfaces. Also, there are already studies on the employment of a wide range of passive enhancement techniques, like displaced enhancement, swirl flow aids, and bi-thermally conductive surfaces. Moreover, the combined usage of two or more enhancement techniques, commonly known as compound enhancement approaches, is also addressed in this survey. Additionally, the present work highlights the existing scarcity of sufficiently large available databases for a given enhancement methodology regarding the influencing factors derived from the implementation of innovative thermal management systems for temperature-sensitive electronic and power devices, for instance, material, morphology, relative positioning and orientation of the boiling surface, and nucleate boiling heat transfer enhancement pattern and scale. Such scarcity means the available findings are not totally accurate and suitable for the design and implementation of new thermal management systems. The analysis of more than 100 studies in this field shows that all such improvement methodologies aim to enhance the nucleate boiling heat transfer parameters of the critical heat flux and nucleate heat transfer coefficient in pool boiling scenarios. Finally, diverse challenges and prospects for further studies are also pointed out, aimed at developing important in-depth knowledge of the underlying enhancement mechanisms of such techniques.

Femtosecond laser-treated copper sintering surface to enhance pool boiling heat transfer

Surface properties have a crucial effect on advancing boiling heat transfer (BHT). This study proposes a porous microgroove-structured surface fabricated via copper powder sintering and femtosecond laser processing to enhance pool boiling heat transfer. Copper powder sintering creates a porous layer, and the application of a femtosecond laser subsequently induces the formation of a microgroove structure atop the porous layer. Eight porous microgroove-structured surfaces were fabricated under different laser surface energy densities. Laser surface energy density was controlled by laser power and scanning speed. Pool boiling experiments were conducted for each sample, including smooth, porous, microgroove, and porous microgroove structures. Interestingly, a distinctive displacement was identified within the heat flux curve of porous microgroove-structured surfaces. The wall superheat decreased with increasing heat flux, causing the graph to shift to the left. To elucidate this phenomenon bubble visualization on the boiling surfaces was performed for each sample. It was observed that a porous microgroove-structured surface could effectively activate several bubble nucleation sites. Additionally, secondary pool boiling on the porous microgroove-structured surface was observed leading to the unique “hook back” shift in the boiling curve and the nonlinear increase of HTC. A pool boiling enhancement mechanism was proposed based on experimental data and bubble visualization. This study offers a pathway for advancing heat dissipation efficiency and understanding of surface engineering strategies.

Enhancing pool boiling heat transfer of modified surface by 3-D Lattice Boltzmann method

In this study, pool boiling from micro-pillar modified surface has been simulated numerically by a 3-D lattice Boltzmann method. Effects of geometries and wettability of micro-pillar on boiling heat transfer performance were also systematically evaluated. Result showed that compared within micro-pillar surface, heat flux of cubic micro-pillar surface was the highest with the lowest wall temperature. In addition, compared to hydrophilic condition, Heat flux of cubic micro-pillar surface with hydrophobic wettability increased by 98.3%. This is because hydrophobic wettability influenced nucleation site density, vapor-liquid-flow field and heat transfer performance much more than cubic shaped geometry. Finally, heat flux of cubic micro-pillar surface with hybrid wettability increased by 430.7% compared to pure hydrophilic wettability. That is due to optimal hybrid wettability surface could control nucleate site location, restrict bubble growth, and increase obviously heat transfer performance.

A mesoscopic numerical method for enhanced pool boiling heat transfer on conical surfaces under action of electric field

The saturated pool boiling heat transfer on a conical structure surface under the action of an electric field is numerically investigated by using the lattice Boltzmann (LB) model coupled with an electric field model. A comparison study of boiling heat transfer phenomenon smooth surface and conical surface without the action of an electric field is first conducted in order to quantitatively analyze the mechanism of the electric field effect on boiling heat transfer on the conical structure surface. It is discovered that the conical structure has more active nucleation sites during the nucleate boiling regime, improving the boiling heat transfer efficiency and enhancing the critical heat flux (CHF). However, in the transition boiling stage and film boiling stage, the conical structure increases the flow resistance of the fluid on the fin surface, hindering heat transfer between the vapor and liquid and producing lower heat transfer performance than smooth surface. Based on the aforementioned findings, the boiling heat transmission on the conical structure surface is enhanced by applying an electric field. Numerical results indicate that the effect of the electric field on the boiling heat transfer performance on the conical structure surface is related to the boiling regime. In the earlier stage of the nucleation boiling regime, when an electric field is present, the onset time of bubble nucleation is slightly delayed, bubble size decreases a little, and boiling is slightly suppressed. However, the combination effect of electric field and conical structure, especially the tip effect, prevents the spread and diffusion of dry areas on the heating surface, thereby enhancing boiling heat transfer in the fully developed nucleate boiling stage. The tip effect grows more evidently in the transition boiling regime and film boiling regime, and increasing electric field intensity causes boiling to continue in the nucleate boiling regime at a higher superheat level. As a result, boiling heat transfer performance is greatly improved, and CHF steadily rises.

Subcooled pool boiling heat transfer enhancement on hierarchically biomimetic laser-structured surfaces with gradient pores

Subcooled pool boiling heat transfer enhancement on hierarchically biomimetic laser-structured surfaces with gradient pores

Micro/nanostructuring of metal additively manufactured aluminum alloy for enhanced pool boiling of dielectric fluids

Surface micro/nanotextures have the potential to increase bubble nucleation and promote capillary liquid wicking to enhance pool boiling heat transfer significantly. Despite past studies implementing surface micro/nanofabrication strategies to enhance boiling, the role of structure length scale and morphology on boiling heat transfer coefficient (h) and critical heat flux (CHF) of low-surface-tension dielectric fluids remains unclear. In this study, we systematically tune the surface microstructures on additively and conventionally manufactured aluminum alloys (AM AlSi10Mg and Al6061) from 1 µm to 5 µm to study the influence of microstructure length scale on boiling performance. In addition, to further understand the effect of submicron structures on boiling, nanostructures of 300 nm length were generated on a plain AM surface and hierarchically incorporated atop the microstructures. Dielectric fluid, HFE-7100, was used to investigate the effects of surface morphology on pool boiling heat transfer coefficient (h) and critical heat flux (CHF). Our results show that the 5 µm microstructured AM surface, AM-H(400)E(5), attained a CHF of 19.44 W/cm2 and a maximum heat transfer coefficient (hmax) of 2.89 W/cm2·K, which represents a reduction in CHF of 28.5 % and an enhancement in hmax of 103.8 %, as compared to the plain Al6061 surface. In addition, AM-H(400)E(5) achieved a large enhancement ratio of 2.2 as compared to the plain Al6061 at a heat flux of 20 W/cm2, and the highest h value amongst all structured surfaces, indicating its cavity size is optimal for bubble nucleation. Through the systematic tuning of micro/nanostructures length scale and morphology, this study found that the micro/nanostructures of 1 µm size and below are too small to serve as bubble nucleation sites. Additionally, micro/nanostructure wickability plays a negligible role in affecting pool boiling performances of highly wetting dielectric fluids, while surface morphology plays a dominant role in bubble nucleation to enhance boiling. Lastly, high-speed immersion microscopy was performed to understand the effects of micro/nanostructures on bubble characteristics such as bubble departure diameter and growth period. In summary, this work not only reports the first micro/nanostructured AM surfaces utilizing scalable fabrication techniques for enhanced boiling, but it also demonstrates the potential of tunning the micro/nanostructure length scale to optimize the bubble nucleation site density, resulting in significantly improved pool boiling performances.

Combined Effect of Structured Surface and Biosurfactant in Pool Boiling Heat Transfer Enhancement

An attempt has been made to explore the combined effect of structured surfaces (segmented finned surface and uniform cross-section surface) and Rhamnolipid biosurfactant to improve heat transfer rate in pool boiling. Experimental investigations were performed at normal atmospheric condition and till reaching critical heat flux. The results shows that the segmented finned surface has shown better heat transfer than plain surfaces, with a 200% increase in heat transfer coefficient. In contrast, a uniform cross-section surface showed an improvement in heat transfer coefficient of 107% over a plain surface. The reasons for better thermal performance in segmented finned configuration can be associated to the reduced surface tension of the working fluid increasing more nucleation sites. Additionally, better rewetting phenomena and bubble evolution from the segmented finned surface have also assisted in improving the heat transfer rate. Moreover, the pool boiling results of all the heating surfaces with surfactant solution is compared with pure water. It has been observed that all heating surfaces have performed better with surfactant solution compared to pure water. However, the CHF obtained on all heating surfaces with surfactant solution were lesser than pure water. Low CHF is due to the foamability of the surfactant solution.

Lattice Boltzmann method study of pool boiling on an electrowetting substrate

Electrowetting has attracted significant attention in condensation, but its application in enhancing pool boiling heat transfer is very limited. In this paper, the lattice Boltzmann method is adopted to investigate the pool boiling heat transfer on electrowetting surfaces with different electrode configurations. The wettability of the baseline hydrophobic surface above the electrode will change to hydrophilicity once a voltage is applied. The differences in heat transfer and boiling phenomenon between the hydrophilic and hydrophobic surfaces without electrodes are illustrated firstly by the density contours and boiling curves. Then, the possibility of electrowetting in improving the heat transfer coefficient (HTC) and critical heat flux (CHF) is verified by comparing with baseline hydrophobic and patterned-coating surfaces, and different boiling heat transfer regimes are illustrated in detail. Furthermore, based on the heat transfer characteristics of the electrowetting surfaces with different electrode configurations, a surface with temperature-dependent electrodes is obtained to comprehensively enhance heat transfer from the onset of nucleate boiling (ONB) to CHF.

Pool Boiling Heat Transfer Enhancement through Biphilic Stepped Microchannel

In present study, pool boiling heat transfer (PBHT) from biphilic stepped microchannel comprising: hydrophobic fin top and hydrophilic channel region is investigated. The biphilic stepped microchannel is prepared by mechanical polishing and thermo-catalytic etching. The improved liquid supply pattern, increased nucleation site density, retarded bubble coalescence between the adjacent channel and decreased wall forces acting on the bubble meniscus resulted in the PBHT enhancement. Contact angle of the water droplet on hydrophobic and hydrophilic surface is 74.02° and 22.5°, respectively. Enhancement in critical heat flux and heat transfer coefficient by the biphilic stepped microchannel is 195.52% and 367.00%, respectively.

Effects of silver epoxy reinforcement on graphene nanoplatelets functionalized surface in pool boiling heat transfer enhancement

Silver Epoxy reinforcement is pivotal to enhance the adhesive strength of Graphene nanoplatelets (GNP) on the substrate. This experimental study seeks to scrutinize the effects of Silver Epoxy reinforced GNP functionalized surface on the heat transfer of pool boiling. Three different GNP-based coatings are analyzed: simple GNP, uncured GNP reinforced with Silver Epoxy, and cured GNP reinforced with Silver Epoxy, all applied to copper substrate. For each coating, the surface morphology, chemical bonding, and wettability are examined. Under a heat flux of 550 kW/m2, the experimentally derived heat transfer coefficient improves by 194%, 36%, and 77% respectively on pristine GNP, uncured GNP/Epoxy, and cured GNP/Epoxy compared to the bare copper surface. The epoxy reinforcement weakens the exceptional boiling improvement achieved by a GNP functionalized surface. Although the thermal curing process turns the GNP/epoxy coating from hydrophobic to hydrophilic and recovers partly the level of boiling enhancement, the biphilicity of GNP tends to dominate boiling heat transfer enhancement by enhancing the bubble dynamics. The hydrophilicity has a subordinate effect by re-wetting the dry spots and preventing the vapour blanket. Silver Epoxy reinforcement significantly enhances the adhesive strength of GNP on a substrate but at the expense of boiling heat transfer enhancement.

Simulation on pool boiling heat transfer considering the integrated effect of engineered microchannels and mixed wettability

Comparing with the abundant experimental investigations on the pool boiling heat transfer, this study aims to explore the integrated effect of engineered microchannels and the mixed wettability on the enhancement in pool boiling heat transfer in a wide range of geometric and physical conditions. A two-dimensional transient volume of fluid (VOF) model was adopted to simulate the bubble behavior and thermal performance on different surfaces including the hydrophilic plain, hydrophilic microchannel, hydrophilic microchannel with hydrophobic top, and hydrophilic microchannel with hydrophobic bottom, respectively. The vapor volume fraction, velocity vector field, temperature of solid/liquid interface, and average heat transfer coefficients (HTCs) on different geometric surfaces were obtained based on the numerical model. It was found that hydrophobic areas are necessary to be added in the hydrophilic channels to facilitate the bubble nucleation and detachment due to the merit of the mixed wettability. However, it is crucial to optimize the width and location of the hydrophobic areas and the height of the hydrophilic microchannel for the prevention of early formation of vapor blanket and promotion of liquid replenishment. With the hydrophobic area on the top of the microchannel, the maximum solid/liquid interface temperature was 392.8 K, which decreased by 0.91 % comparing with that on the hydrophilic microchannel. While with a hydrophobic bottom (W1 = 2 and 4 μm), the max average HTCs were 508.2 kW/(m2K) and 503.5 kW/(m2K), which were 16.4 % and 11.4 % higher than that in hydrophilic microchannel (W1 = 2 and 4 μm), respectively. The simulation results indicate that the microchannel with optimized mixing of the wettability can efficiently promote the bubble to move upward and liquid to rewet along with the hydrophilic wall, forming the separation of vapor–liquid pathways to further enhance pool boiling heat transfer.

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.

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.

Pool boiling heat transfer enhancement and bubble visualization on a microporous copper over CuO filmed surface through combination of chemical etching and electrochemical deposition

For high heat flux engineering application, a Microporous Copper over CuO Film Surface (MCCFS) is prepared to find out the Critical Heat Flux (CHF) and Boiling Heat Transfer Coefficient (BHTC). The bare Cu surface is modified into MCCFS through a combination of two chemical processes, i.e., chemical etching and electrochemical deposition. Various properties of the MCCFS, like porosity, wettability, and roughness, are also analyzed. The saturated pool boiling experiment is carried out by taking distilled water as the boiling liquid at atmospheric pressure. Results show that the CHF and the BHTC of the MCCFS are improved by 71% and 78%, respectively, than the bare Cu. This improvement is mainly due to the increased roughness and decreased wettability of the MCCFS. A visualization-based quantitative analysis of the bubble during boiling is also done to correlate the enhanced heat transfer from the MCCFS. Bubble characteristics like nucleation site density, bubble departure diameter, and bubble departure frequency are analyzed. The enhanced bubble site density, decreased departure diameter, as well as the enhanced departure frequency of the bubble supported the improvement of heat transfer from the MCCFS. A comparative analysis of the performance of heat transfer from the MCCFS is also presented.