Pool Boiling Heat Transfer Enhancement Research Articles

The Role of Pin Fin Array Configurations and Bubble Characteristics on the Pool Boiling Heat Transfer Enhancement

Pool boiling surface modification by pin fin array has demonstrated a practical but effective cooling enhancement. Whilst the main challenge is to determine the best configuration of the pin fin dimensions and positioning, it is known that the result varies among the working conditions. To obtain a more general formulation, it is utterly vital to obtain a deep understanding of the pool boiling mechanism during the phase-change process. This study aimed to analyze the role of pin fin array configuration on the pool boiling phenomenon by generating visualization and non-dimensional analysis. The results illustrated three different regimes occurring during the boiling process: natural convection, isolated bubble, and merged bubble. For all regimes, the average heat transfer performance was enhanced as the fin gap rose to 17% and 63% for circular and rectangular pin fins, respectively. Furthermore, Grashof (Gr) and Bond (Bo) numbers were calculated to quantitatively describe the effect on the bubble dynamics. From these approaches, it was found that the insulated bubble regime provides a better means of cooling by around 60% due to the better bubble dynamics.

Spontaneously grown boehmite structures improve pool boiling heat transfer on aluminium surfaces

Superhydrophilic nanostructures that function to improve wettability have been proven to enhance boiling heat transfer. Herein, the mechanism for enhancing pool boiling heat transfer on aluminium surfaces by spontaneous growth of boehmite layers in boiling water was comprehensively explored. The morphology and chemical composition of the nanolayers formed on boiling water-treated (BWT) aluminium surfaces were characterized. The surfaces were covered with abundant cross-linked nanosheets and dispersed microclusters after BWT, which were verified to be boehmite. In addition, the effect of BWT time on the thickness and wettability of the boehmite layer was quantitatively studied. The results showed that the aluminium surfaces became superhydrophilic after BWT for 30 minutes, and the thickness and morphology of the boehmite layer achieved near stable after processing beyond 3 hours. Further, BWT samples exhibited excellent thermal performance and reproducibility during pool boiling tests. Compared to the plain copper surface, a 49% higher critical heat flux and a 79.6% higher heat transfer coefficient were demonstrated on BWT aluminium alloy surfaces. The stable and dense boehmite layer was proven to significantly enhance the boiling heat transfer of the substrate by increasing the effective nucleation sites, specific surface area, wettability, and capillary rewetting ability.

Pool boiling heat transfer enhancement of aqueous solution with quaternary ammonium cationic surfactants on copper surface

Pool boiling is a widely encountered heat transfer phenomenon and the enhancement of boiling heat transfer performance is of vital importance for real applications. Adding surfactant into distilled water is regarded to be a promising and effective approach for heat transfer improvement. The present study selected four kinds of quaternary ammonium cationic surfactants with an increased molecular weight, namely, decyltrimethylammonium bromide (DeTAB), dodecyltrimethylammonium bromide (DoTAB), myristylmethylammonium bromide (MTAB) and cetyltrimethylammonium bromide (CTAB), to investigate the pool boiling performance of their aqueous solutions on a copper heater with and without electric field under different concentrations. Their dynamic surface tensions and viscosities were also measured. The bubble dynamics during pool boiling and apparent contact angle on copper surface before and after boiling test were evaluated. Experimental results reveal that the aqueous surfactant solution can significantly enhance the heat transfer coefficient (HTC) compared with distilled water. The HTC exhibits an increment with concentration until critical micelle concentration (CMC) and then levels off. The maximum relative HTC enhancement can reach up to 209.3%, which is ascribed to the increased nucleate site and bubble departure frequency. In addition to the reduction in surface tension, a repulsion force caused by electrostatic interaction is also responsible for the changes in nucleate site and bubble departure frequency. However, the critical heat flux (CHF) shows a continuous decrement with surfactant concentration. In a low concentration surfactant solution, the CHF is even slightly higher than that of water, which is caused by an improvement in surface wettability after boiling test. The CHF in moderate and high surfactant concentration will be lower. The surfactant with a smaller CMC tends to have a higher HTC and a lower CHF under a low concentration. However, for a relatively high concentration comparative to CMC, the four kinds of quaternary ammonium cationic surfactant solutions tend to have pretty similar boiling curves. The electric field is found to have a negligible influence on the pool boiling performance in aqueous surfactant solutions. Results in present study can provide valuable guidance for surfactant selection during the design of highly effective heat exchangers.

Experimental investigation of pool boiling heat transfer enhancement using electrodeposited open-cell metal foam

Pool boiling is specified as boiling stemming from a heated surface that is submerged in a volume of liquid resulting in a phase transition to vapor. The subject of pool boiling heat transfer heat has continuously received attention due to its various applications among researchers. One of the avenues used to improve pool boiling is the employment of open-cell metal foams. In this study, an attempt has been made to study the effect of electrodeposited open-cell metal foam coating on pool boiling heat transfer under atmospheric conditions and deionized water. The foams have three pore densities of 5, 10, and 20 ppi and a constant porosity of 0.95. In each pore density of the open-cell foams with five different thicknesses, copper is deposited on the foams through electrodeposition, and thereafter pool boiling test is performed. Also, to evaluate the number of electrodeposition steps, 4 and 8-step electrodeposition is implemented on the best pore density foam sample from the preceding section. In addition, untreated foams in five different thicknesses and also nickel foams are tested for comparison with electrodeposited copper. The results reveal that 20 ppi 6-step electrodeposited copper open-cell foam with a thickness of 4 mm improves the pool boiling heat transfer in medium and high heat fluxes by up to 160% and 130%, respectively, and thence has the best performance among all the sample foams. However, at a pore density of 5 ppi, the 8-and 6-step electrodeposited open-cell foam with 8 mm thickness achieves the best performance and improves the pool boiling heat transfer at medium and high heat fluxes by approximately 150% and 100%, respectively. In addition, at a pore density of 10 ppi, the 6-step electrodeposited open-cell foam with 4 mm thickness, attained the best results and improved the pool boiling heat transfer at medium and high heat fluxes by 103% and 120%, respectively. SEM, microscopic, and aging images of the foams were carried to compare the structure and strength of the number of electrodeposition steps.

Enhancing Pool Boiling Heat Transfer by Structured Surfaces– A Lattice Boltzmann Study

The structured surface-enhanced pool boiling process and associated heat transfer enhancement characteristics are numerically investigated by using the pseudopotential multiphase flow lattice Boltzmann (LB) model coupled finite difference method (FDM). In the current study, the effects of different microstructure geometries(square structures, triangular serrated structures, triangular-raised structures) and varying spacing in triangular-raised structures (d = 0, 22, 44, 66, 88, 110 l.u. (lattice units)) on boiling heat transfer (BHT) characteristics and bubble dynamics behavior are studied in detail. The results showed that microstructure can accelerate bubble nucleation. Among the three microstructures, the heat transfer performance of triangular-raised structures was significantly better than that of square and triangular serrated structures in the nucleate boiling (Ja number is 0.124-0.145). The oscillation and deformation of bubbles led to the lateral migration of bubbles, the continuous nucleation of small bubbles. The phenomenon of re-wetting of heating surface occurred in the process of bubble migration, necking, deformation and detachment was found, which enhances heat transfer in nucleate boiling. At the same time, the growth, oscillation and detachment of bubbles also perturb the liquid and enhance the natural convection around the bubbles.Therefore the main BHT mechanism of nucleate boiling on the three kinds of structured surface is the combined action of transient heat conduction and micro-convection. The variation of the spacing between microstructures showed an important effect on the BHT performance of the heating surface and the generation of activated nucleation sites in the nucleate boiling. Triangular-raised structures can enhance transient heat conduction and micro-convection, with the strongest enhancement effect at d = 66 l.u when the Ja number is 0.124.

Enhanced Pool Boiling Heat Transfer by Adding Metallized Diamond in Copper Porous Materials

Enhanced Pool Boiling Heat Transfer by Adding Metallized Diamond in Copper Porous Materials

Pool Boiling Heat Transfer Enhancement on the Hybrid Surfaces Coupling Capillary Wick and Minichannels

Pool Boiling Heat Transfer Enhancement on the Hybrid Surfaces Coupling Capillary Wick and Minichannels

Enhanced pool boiling heat transfer by metallic nanoporous surfaces under low pressure

The nano-structure enhanced pooling boiling at standard atmosphere has been fully studied in literature but it is rarely investigated under low pressure. However, the boiling behaviors and bubble dynamics of nano-structures highly depend on the working pressure. This study investigated the enhanced pool boiling (under low pressure) of a nanoporous copper surface (NCS) fabricated by the hot-dip galvanizing & dealloying. The boiling curves and bubble characteristics of the NCS under low-pressure boiling conditions (25 kPa and 65 kPa) were evaluated by a test set up with high-speed visualization. To validate the present experimental plain-wall HTC under low pressure, three predicted models (Labuntsov, modified Rohsenow, and Stephan-Abdelsalam) were applied to obtain the theoretical HTCs for comparison. Besides, the optimization of the galvanization time has been conducted. Experimental results showed that the NCS significantly improved the pool boiling heat transfer and mitigated the negative influence of sub-atmospheric conditions. The enhanced mechanisms are the augmented nucleate site density, decreased bubble departure diameter, and increased bubbles generation frequency induced by the promoted hydrophilicity and rough morphology of the NCS. Among the three analyzed samples, the NCS-500 °C-3 min surface yielded the lowest wall superheat at the onset of boiling (ONB) and the highest heat transfer coefficient (HTC). In addition, the nanoporous surface leads to higher HTC enhancement upon the plain surface at a lower pressure. Specifically, the HTC enhancement percentage was increased from 71.6% (at 65 kPa) to 139% (at 25 kPa) as the pressure decreased.

Unified descriptor for enhanced critical heat flux during pool boiling of hemi-wicking surfaces

Boiling heat transfer is dictated by interfacial phenomena at the three-phase contact line where vapor bubbles form on the surface. Structured surfaces have shown significant enhancement in critical heat flux (CHF) during pool boiling by tailoring interfacial phenomena. This CHF enhancement has been primarily explained by two structural effects: roughness, which extends the contact line length at the bubble base, and wickability, the ability to imbibe liquid through surface structures by capillary pumping. In this work, we show that CHF enhancement on structured surfaces cannot be described by roughness or wickability alone. This result was confirmed using systematically designed micropillar surfaces with controlled roughness and wickability. Further, we performed a scaling analysis and derived a unified descriptor, which represents the combined effects of thin film density and volumetric wicking rate. This unified descriptor shows a reasonable correlation with CHF values with our experiments and literature data. This work provides important insights in understanding the role of surface structures on CHF enhancement, thereby providing guidelines for the systematic design of surface structures for enhanced pool boiling heat transfer.

Experimental studies of the pool boiling heat transfer enhancement capability and mechanisms of the micro-pillared surfaces on copper substrate

ABSTRACT Experimental investigations on the pool boiling heat transfer enhancement and mechanisms for micropillared surfaces are performed. Two square micropillared MP 50–50 (Square) and MP 50–450 (Square)) and a circular micropillared MP 50–250 (Circular)) are fabricated on a copper substrate. Experimental results show that bubbles easily nucleate at the sharp corners of the square micropillars and MP 50–50 (Square) improves the capability of the micropillars, which leads to a higher boiling heat transfer performance. The critical heat flux is 151.41 W/cm2 on MP 50–50 (Square), which is 32% higher than that of a smooth surface (114.78 W/cm2).

Geometrical effects on heat transfer mechanisms during pool boiling in Dual Tapered Microgap with HFE7000

Dual tapered microgap configuration provides a simple external device that can be retrofitted on a boiling surface. It has been shown to be very effective for enhancing pool boiling heat transfer coefficient (HTC) and Critical Heat Flux (CHF) with water. Recent testing showed that the same geometry is not effective with refrigerants and dielectric fluids. A systematic experimental and theoretical study is conducted to bring out the underlying mechanisms and identify the effect of geometrical parameters mainly on HTC, as higher values are desired in electronics cooling application. Five different taper angles of 5°, 10°, 15°, 20°, and 25° and two inlet gap heights of 0.8 mm, and 1.27 mm were experimentally evaluated with dielectric liquid HFE7000. Theoretical analysis has identified two mechanisms from bubble squeezing and pressure recovery for inducing flow in the microgap. These mechanisms are influenced by the fluid properties and the geometrical parameters. The best geometrical configuration was able to achieve a 2X enhancement in HTC over a plain copper test surface. Further enhancements are expected with introduction of additional surface features such as microchannels on the heater surface in the microgap region. The theoretical model was able to estimate the pressure drop, mass flux, and the resulting HTC with high accuracy for different geometric configurations.

Pool boiling of nanofluids on biphilic surfaces

This study addresses the combination of customized surface modification with the use of nanofluids, to infer on its potential to enhance pool boiling heat transfer. Hydrophilic surfaces patterned with superhydrophobic regions are prepared and used to act as surface interfaces with nanofluids (water with gold, silver and alumina nanoparticles) and infer on the effect of the nature and concentration of the nanoparticles in bubble dynamics and consequently in heat transfer processes. The main qualitative and quantitative analysis was based on extensive post-processing of synchronized high-speed and thermographic images. The results show an evident benefice of using biphilic patterns, but with well-stablished distances between the superhydrophobic regions. Such patterns allow a controlled bubble coalescence, which promotes fluid convection at the hydrophilic surface between the superhydrophobic regions, which clearly contributes to cool down the surface. The effect of the nanofluids, for the low concentrations used here, was observed to play a minor role.

Pool Boiling Heat Transfer Enhancement Using Perforated Twisted Tape Fins

Abstract The application of twisted tape fins (TTFs) showed a considerable enhancement in pool boiling heat flux. The present study experimentally investigates the effect of solid and perforated TTFs on pool boiling of water by varying the twist ratio (y) and perforation index (PI) from 3 to 4.3 and 5 to 10, respectively. An arrangement of five TTFs with a twist ratio of 3 showed 18.6% enhancement as compared to the plain surface where an arrangement of five perforated TTFs having a PI of 7 and twist ratio of 3 showed a maximum enhancement of 28.7%.

Experiments and modeling of boiling heat transfer on hybrid-wettability surfaces

This paper studies pool boiling heat transfer on the hybrid-wettability surfaces, i.e., hydrophobic dot/stripe patterns fabricated on a superhydrophilic substrate. It is shown that the nucleate boiling heat transfer coefficient for the hybrid surface is improved compared to both the substrate and plain copper reference, but the critical heat flux (CHF) on the enhanced surface is very complex. The pattern-to-substrate contact angle difference is also concerned: CHF for the hybrid surface increases remarkably with increasing the contact angle difference, but the nucleate boiling heat transfer coefficient declines. Combining the results for both dot and stripe patterns, it is revealed that CHF on the hybrid surfaces is closely associated with the pattern-to-surface area ratio and the pattern-to-pattern spacing: it declines generally with increasing the area ratio; its dependence on the pattern spacing is minor at large area ratios; however, at small area ratios, the pattern spacing plays an increasingly important role because it dominates both vapor-liquid instabilities and surface rewetting, and the optimal pattern spacing for maximal pool boiling heat transfer enhancement can be estimated using the capillary length. A modified theoretical model is proposed to predicting CHF on both homogenous- and hybrid-wettability surfaces.

On the effect of silver nanoparticles deposition on porous copper foams on pool boiling heat transfer enhancement: an experimental visualization

On the effect of silver nanoparticles deposition on porous copper foams on pool boiling heat transfer enhancement: an experimental visualization

Modeling of the bubble dynamics and heat flux variations during lateral coalescence of bubbles in nucleate pool boiling

A liquid-vapor phase change lattice Botlzmann method was used to simulate the dynamics of two bubbles during the lateral coalescence on solid surfaces in nucleate boiling. The evolution of bubble shapes and the departure diameter of bubbles were compared with experimental data. Comparisons show good agreement. The heat fluxes under nucleating, coalescing, growing and departing bubbles were simulated and directly related to the bubble dynamics. It was found that a typical ebullition cycle includes nucleation and growth of individual bubbles, coalescence of two bubbles, oscillation and departure of the coalesced bubble. The heat-flux variation under the bubbles has two major peaks, which are due to the coalescence of two bubbles, the departure of the coalesced bubble and nucleation of succeeding bubbles, respectively. The results showed that the heat flux peaks for nucleation and departure are slightly larger than the heat flux peaks for coalescence for the smaller and the larger separation distances, while those for coalescence are larger for the moderate separation distances. It was also found that there exists an optimal separation distance for the coalescing bubbles to achieve the best heat transfer performance, which provides very useful information to enhance pool boiling heat transfer.

Pool boiling heat transfer enhancement by bi-conductive surfaces

The use of the bi-conductive surface provides a promising prospect for the enhancement of the boiling phase change heat transfer. In this study, a theoretical model of two-phase flow and boiling heat transfer on bi-conductive surfaces is developed. With the consideration of thermal conductivity contrast and conjugate heat transfer, the boiling behavior is numerically investigated based on a modified lattice Boltzmann method. The bubble dynamics during the boiling process on the bi-conductive surface are reproduced and the effect of the structural size of these insets on the boiling performance is explored. The results indicate that boiling performance is enhanced on the bi-conductive surface when compared to homogeneous ones. In-plane temperature variation on the surface contributes to the pinning effect on bubble contact lines, and thus promotes boiling heat transfer. Boiling enhancement gets optimized once the embedding pitch space equal to the capillary length of the boiling fluid. Increasing the width of the insets also improves the boiling heat transfer coefficient on the bi-conductive surface under low superheat conditions, but also leads to the advance of CHF. However, changing the depth of the insets shows almost no effect on the boiling heat transfer performance.

Study of Nucleate Pool Boiling Heat Transfer Enhancement on Surfaces Modified by Beam Technologies

A significant part of boiling heat transfer enhancement methods is surface modification and the creation of structures, both regular and irregular multiscale. In this paper, beam technologies for surface modification are presented. Using these technologies, samples are obtained for experimental studies. Experimental data on heat transfer and critical heat flux during R-113 pool boiling on structured surfaces are presented. The possibility of increasing the critical heat flux by up to 25% and significantly expanding the temperature range of the surfaces in heat transfer equipment is shown.

Electrical discharge surface texturing for enhanced pool boiling heat transfer

This research pertains to pool boiling heat transfer characteristics of Aluminum 6061 alloy surfaces that were textured using sink electrical discharge machining (EDM). Prior related work elsewhere utilized EDM as an essential means to roughen a surface or fabricate geometric features such as microchannels and micropillars to investigate boiling. As opposed to this, the present work systematically examined the effect of EDM process parameters viz., the discharge current and the discharge duration to enhance/benchmark boiling performance in the natural convection and nucleate boiling regimes, and comprehend underlying mechanisms. A process-related factor that limits boiling was found to be the growth of a superficial layer on the machined surface, the formation of which is facilitated by the hydrocarbon dielectric fluid used in EDM. This notwithstanding, the highest heat transfer coefficient obtained in this work was roughly five times as that of a reference polished surface. The Data Dependent Systems methodology was adopted to mathematically extract characteristic crater geometry from EDM-textured surface profiles, which highlighted the significance of parameters that are physically relevant such as the crater diameter and the crater volume in correlating boiling performance.

Effect of staggered V-shaped and rectangular grooves copper surfaces on pool boiling heat transfer enhancement using ZrO2 nanofluids

Boiling heat transfer has been a trending research area for the last several decades due to its excellent heat removal capacity and direct applications in the distillation and pharmaceutical industries. The present investigation focuses on the augmentation of efficiency for the boiling requiring applications, as highlighted earlier. The experimental and numerical investigations study the effect of staggered V-shaped and rectangular grooved copper surfaces on pool boiling, compared with the smooth copper surface in the ZrO2 nanofluid media. The authors adopted fiber laser engraving technology to form the highlighted microstructures. Furthermore, three different concentrations, i.e., 0.001%, 0.005% and 0.01% by volume, were investigated in a medium having equal volumes of distilled water and ethylene glycol. The thermophysical properties of nanofluid and peculiar microstructures enhanced the relative heat transfer coefficient by 41.48%. The wall superheat drastically reduced due to the micro-level convection on the boiling heat transfer promoted by the microstructures. Besides, the numerical results fitted well with the experimental results with a little deviation (MAE = 2.04%).