Pool Boiling Heat Transfer Research Articles

Saturated Boiling Enhancement of Novec-7100 on Microgrooved Surfaces with Groove-Induced Anisotropic Properties

The effects of the anisotropic properties (wettability and roughness) of microgrooved surfaces on heat transfer were experimentally investigated during pool boiling using Novec-7100 as a working fluid. The idea for introducing the concept of anisotropic wettability in boiling experiments draws inspiration from biphilic surfaces. The investigation is also motivated by two-phase immersion cooling, which involves phase-change heat transfer, using a dielectric liquid as a working fluid. Very few studies have focused on the effects of surfaces with anisotropic properties on boiling performance. Thus, this study aims to examine the pool-boiling heat transfer performance on surfaces with microgroove-induced anisotropic properties under the saturation condition. A femtosecond-laser texturing method was employed to create microgrooved surfaces with different groove spacings. The results indicated that anisotropic properties affected the heat transfer coefficient and critical heat flux. Relative to the plain surface, microgrooved surfaces enhanced the heat transfer performance due to the increased number of bubble nucleation sites and higher bubble detachment frequency. An analysis of bubble dynamics under different surface conditions was conducted with the assistance of high-speed images. The microgrooved surface with a groove spacing of 100 μm maximally increased the BHTC by 37% compared with that of the plain surface. Finally, the CHF results derived from experiments were compared with related empirical correlations. Good agreement was achieved between the results and the prediction correlation.

Numerical investigation on pool boiling heat transfer with trapezoidal/inverted trapezoidal micro-pillars using LBM

Pool boiling with high-efficient heat transfer process is crucial to chemical engineering, nuclear engineering, electronic cooling, and other energy conversion applications. Surface modification technology has attracted more attention in recent years as a simple and reliable boiling enhancement technique. Most of the studies using surface modification technology have focused on the enhancement effect of rectangular micro-structures, only a few researchers have adequately investigated the enhancement effects on pool boiling heat transfer and mechanisms of other complex micro-structures (like circle, trapezoid, or inverted trapezoid). In the present study, the pool boiling processes on surfaces with trapezoidal/inverted trapezoidal micro-pillars were numerically investigated using lattice Boltzmann method (LBM). The bubble dynamics and heat transfer characteristics during the pool boiling processes on surfaces with different pillar bottom angles (30° ≤ θ ≤ 135°) were compared under different wall-superheat conditions. The orthogonal tests were adopted on micro-pillars to optimize the geometric parameters, including the bottom angle of the micro-pillar, the bottom width of the micro-pillar, the number of the micro-pillar, and the height of the micro-pillar. A new CHF correlation on the mixed wettability surface with trapezoidal/inverted trapezoidal micro-pillars was proposed. It can be found that the main mechanism of nucleate boiling heat transfer on the micro-pillars is the evaporation of the microlayer, and the pillar bottom angle reveals a remarkable effect on the pool boiling performance on the mixed wettability surfaces. The surface of θ = 135° has the minimum wall-superheat at the onset of nucleate boiling (ONB). Within a wide range of wall-superheat conditions, the heat transfer efficiency of pool boiling firstly increases with the increasing of the bottom angle of the micro-pillar and subsequently decreases. The surface of θ = 60° has the highest critical heat flux (CHF). The surface of θ = 90° has the highest heat transfer efficiency in the film boiling process. The results of orthogonal tests suggested that the bottom angle of the micro-pillar is the most significant influential factor, and the optimal geometrical combinations were proposed. The new CHF correlation has acceptable accuracy in predicting the CHF on surfaces with trapezoidal/inverted trapezoidal pillars, which provides guidance for the design and optimization of micro-structured surface modification in various pool boiling applications.

Entropy generation and thermodynamic analysis of pool boiling heat transfer on doubly enhanced tubes

To improve the efficiency of the heat exchanger tube, one smooth tube and four doubly enhanced tubes (EX1, EX2, EX3 and EX4) have been investigated for the pool boiling heat transfer experiments in this paper. The indicate that the pool boiling heat transfer coefficient of the doubly enhanced tubes increased visibly with the augmentation of heat flux through the bubble behavior. Heat transfer reinforcement effect of the doubly enhanced tubes are significantly better than that of the smooth tube. Additionally, pool boiling heat transfer coefficient can be further enhanced by raising the saturate temperature. Entropy generation minimization analysis demonstrates that the heat transfer characteristics of like T-shaped tubes (EX1 and EX2) are superior to that of low fin tubes (EX3 and EX4). Particularly, tube EX1 exhibits higher pool boiling heat transfer efficiency. It is observed that a reasonable fin pitch is more advantageous for improving heat transfer characteristics. The utilization of entropy generation minimization analysis provides theoretical support for the design and optimization of doubly enhanced tubes.

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.

POOL BOILING HEAT TRANSFER CHARACTERISTICS OF POROUS NICKEL MICROSTRUCTURE SURFACES

Pool boiling is an effective heat dissipation approach in electronic cooling, battery thermal management, etc. This study used the electrochemical deposition method to fabricate one smooth nickel specimen (named Ni-smooth) and three specimens with a porous nickel-stacked structure. The three porous specimens were created with deposition current densities of 0.5 A·cm<sup>-2</sup> (named Ni-0.5), 2.0 A·cm<sup>-2</sup> (names Ni-2.0), and 5.0 A·cm<sup>-2</sup> (named Ni-5.0), respectively. The four samples underwent microstructural characterization via scanning electron microscopy. The increasing current density led to the porous nickel surface exhibiting a more distinct pore structure, and the nickel sphere grains became more refined, developing a loose "mound-like" structure. A marked increase in the nickel film thickness was also observed. Through visual experiments, we evaluated their wettability, and through pool-boiling experiments, we tested their boiling heat-transfer properties. Our findings suggest that samples incorporating a porous nickel structure consistently outperform unmodified samples regarding heat-transfer efficiency. Specifically, sample Ni-0.5A demonstrated the most optimal boiling heat-transfer performance, evidenced by a 32.2% reduction in temperature at the onset of boiling, a 19.9% increase in critical heat flux density, and a 78.6% larger maximum heat-transfer coefficient compared to the smooth nickel sample. These marked improvements are intrinsically linked to the specific characteristics of the porous nickel structure. The higher performance of samples Ni-0.5 can be attributed to the presence of additional nucleation sites within the porous structure and the formation of smaller micro-crystalline dendritic constructs due to the specific current density applied during electrodeposition. Understanding this relationship between surface characteristics and electrodeposition is essential in maximizing heat-transfer efficiency.

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.

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

PRESSURE EFFECT ON POOL BOILING HEAT TRANSFER CHARACTERISTICS OF HIGH-VOLATILE LIQUIDS WITH STRUCTURED SURFACE

The study of enhancement of the heat transfer during boiling and an increase of the critical heat fluxes (CHF) have a great importance in design of the modern and cost-effective heat transfer devices for thermal management of microelectronics, cryogenics and refrigeration, and the power electrical engineering industry. In this work we present the results of the experimental studies of the heat transfer and CHF during the pool boiling of refrigerants R113, RC318, and dielectric liquid HFE-7100 on a structured surface under saturation conditions in the range of reduced pressures 0.03-0.36. The effect the heat flux and pressure on the boiling heat transfer coefficient was studied. It is shown that the correlation of Yagov predicts with an accuracy of ± 35% the CHF for pool boiling of highly volatile liquids on structured surfaces in a wide range of reduced pressure.

Simulation Study of Porosity Effects of Porous Media on Pool Boiling Heat Transfer Performances

Simulation Study of Porosity Effects of Porous Media on Pool Boiling Heat Transfer Performances

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.

Effects of cover plate wettability on pool boiling heat transfer in a narrow gap

An experimental study is conducted to evaluate the effects of cover plate wettability (hydrophobic to hydrophilic) on boiling heat transfer in a narrow horizontal gap (0.64mm to 5mm thick). When the gap thickness is greater than 1mm, the cover plate wettability plays a significant role. A hydrophobic cover plate causes vapor bubbles to move from the heating surface and get attached to the cover plate, maintaining liquid water on the heating surface. When the vapor layer does not reach the heating surface, the nucleate boiling heat transfer coefficient is hardly affected by the gap. However, the critical heat flux (CHF) is reduced relative to the unconfined case as vapor fills the narrow gap. Contrarily, when a hydrophilic cover plate is used with a gap thickness greater than 1mm, a vapor slug forms and hovers over the heating surface with transient lateral movement. Thin liquid film evaporation occurs under the slug, causing an enhanced heat transfer coefficient at low heat flux (q'' < 250kW/m2). As the heat flux increases, the vapor slug movement causes fluctuations in the heater temperature due to partial dryout at the heating surface. Consequently, a rapid decrease in the heat transfer and a reduced CHF are observed relative to the unconfined case. When the gap thickness is reduced to below 1mm, a film boiling-like behavior is observed regardless of the cover plate wettability. However, there is an increase in the heat transfer coefficient at low heat flux (q'' < 250kW/m2).

Pool Boiling of CNT + GO Nanomaterial–Coated Copper Substrate: An Experimental Study

Abstract Nanoparticle coating on copper substrates like carbon nanotubes (CNT) and graphene oxide (GO) is a promising method to enhance the surface properties as well as improve the boiling heat transfer characteristics. The main objective of the present investigation is to study the influence of the nanocomposite coating on the performance of pool boiling heat transfer. CNT + GO nanomaterials are coated on copper substrates via the dip coating method by varying the concentration of the nanomaterial. Morphological analysis, surface roughness, and wettability behavior of the coating are also observed. The result shows that CNT + GO increases the surface roughness of the samples, and the coated samples are superhydrophilic in nature. Compared with the uncoated sample, the coated sample shows the maximum increase in critical heat flux and heat transfer coefficient is 145.76% and 259.08%, respectively. A high-speed camera is used to study the bubble dynamics. Bubble diameter, departure frequency, and site density are also calculated and presented.

Enhanced nucleate pool boiling heat transfer on CNTs-Cu nanoparticles-coated surfaces: Effects of sintering temperature and CNTs composition on pool boiling behavior

The present study involves the experimental investigation of pool boiling heat transfer characteristics on CNTs-Cu nanoparticles-coated heating surfaces. The advantages of coating the mixture of infinitesimal CNTs with Cu particles on the copper surface are to generate the active nucleation site density by porous structures and enhance the thermal conductivity of the heating surfaces. To explore the effects of sintering temperatures and CNTs composition on pool boiling behavior, we fabricated the boiling test sections under the sintering temperatures of 700, 750, and 800 °C for four different CNTs compositions of 0.0CNTs-100Cu, 0.2CNTs-99.8Cu, 0.3CNTs-99.7Cu, and 0.5CNTs-99.5Cu in wt.%. We first analyzed the surface morphologies, that is, the average pore size, porosity, and pore density, to characterize each tested sample. Pool boiling experiments were later conducted under atmospheric pressure using water to obtain the pool boiling curves and bubble dynamics for different sintering temperatures and CNTs composition. Based on the experimental analysis, nucleate boiling heat transfer enhancement was interpreted in relation to the sintering temperature with surface morphologies and CNTs composition in relation to improved pore density. Compared to the bare copper surface, the achieved maximum enhancement for boiling heat transfer coefficient and critical heat flux were 3.86 and 1.49, respectively, when a 0.5CNTs-Cu-coated surface sintered at 800 °C was applied.

Experimental investigation of nucleate pool boiling heat transfer characteristics on modified copper surface via laser-structured microstructures

This study aims to evaluate the performance of pool boiling heat transfer on a structured surface subjected to constant heat flux. A nanosecond laser ablation was used to create different surface profiles on copper samples. Specifically, a series of step-like microstructured surfaces with varying secondary groove widths were fabricated to investigate their effect on pool boiling heat transfer performance of distilled water. The results indicated a significant enhancement in heat transfer performance for the laser-structured surfaces compared to the smooth surface at low heat flux. This improvement was attributed to the increased surface area, nucleation frequency, and nucleation site density. However, at higher heat flux, the surface with a smaller secondary groove width (LS 2) exhibited a decline in heat transfer performance, which was likely due to larger bubble escaping resistance. In contrast, the surface with a larger secondary groove width (LS 1) demonstrated the best heat transfer performance. The current work would help in finding an optimum surface structuring design for gaining higher boiling heat transfer performance which benefits industries dealing with thermal management processes.

Saturated Pool Boiling Heat Transfer of R-141b on Al2O3 Nano-Structured Surfaces Fabricated by Dip-Coating Method

Saturated Pool Boiling Heat Transfer of R-141b on Al2O3 Nano-Structured Surfaces Fabricated by Dip-Coating Method

Synergistic Effects of Hydrophilic-Hydrophobic Porous Structures for Enhancing Nucleate Pool Boiling Heat Transfer

Synergistic Effects of Hydrophilic-Hydrophobic Porous Structures for Enhancing Nucleate Pool Boiling Heat Transfer

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 of ammonia outside a single tube with fin structures: Hysteresis phenomena and boiling enhancement

Pool boiling heat transfer of ammonia outside a single tube was investigated at three saturation pressures of 6.15, 8.57, and 11.7 bar (saturation temperature of 10, 20, and 30 °C) at the heat fluxes ranging from 1.2 to 72.9 kW/m2. Hysteresis in a boiling curve and boiling enhancement were examined on both the smooth tube with machined roughness and two enhanced tubes with low fins and re-entrant cavities. All three tubes have outermost diameters from 15.68 to 15.87 mm and the same length of 796 mm. A large hysteresis between increasing the heat flux and decreasing the heat flux was observed on the smooth tube, and the hysteresis increased as the saturation pressure decreased. The boiling hysteresis of ammonia was reasonably explained from the thermodynamic analysis based on Clausius-Clapeyron and Young-Laplace equations. Heat transfer coefficients on the enhanced tubes were increased by 47% to 105% (Low-fin) and 103% to 159% (Turbo-E) compared with the smooth tube at the saturation pressure of 8.57 bar. However, the hysteresis itself was increased on the enhanced tubes owing to the wall temperature decrease along the fin. Hydrophobic coating and the enhanced tube having re-entrant cavities with cavity mouths of a few micrometers were suggested to mitigate the hysteresis phenomena in the pool boiling of ammonia.

Effect of dense packed micro-/nano-porous thin film surfaces developed by a combined method of etching, electrochemical deposition and sintering on pool boiling heat transfer performance

Effect of dense packed micro-/nano-porous thin film surfaces developed by a combined method of etching, electrochemical deposition and sintering on pool boiling heat transfer performance