Boiling Heat Transfer Enhancement Research Articles

Investigation of the Mechanism in Film Boiling Heat Transfer Enhancement by a Honeycomb Porous Plate

Investigation of the Mechanism in Film Boiling Heat Transfer Enhancement by a Honeycomb Porous Plate

Flow boiling characteristics in plain and porous coated microchannel heat sinks

Flow boiling heat transfer enhancement using porous coatings in microchannels has been experimentally investigated with HFE-7200. Results of the coated microchannel heat sink were compared to baseline results in a plain, micro-milled copper microchannel heat sink at similar operating conditions, namely inlet pressure of 1 bar, mass flux of 200 – 400 kg/m2 s and inlet subcooling of 10 K at wall heat fluxes between 24.5 kW/m2 to 206.6 kW/m2. Flow visualisation results and SEM surface analysis are presented. The coated surface was densely populated with well-defined cavities between 0.6 µm to 3.3 µm wide. The plain channels had fewer cavities for a given area and these were larger, i.e. up to 6 µm. Bubble generation frequency in the coated channels is significantly higher than in the plain channels due to the presence of more favourable nucleation sites on the coated surface. Flow pattern development occurred similarly in both heat sinks, namely bubbly to slug, churn and annular flow with increasing heat flux, with earlier flow pattern transitions in the coated heat sink. Enhancement in microchannel flow boiling heat transfer was shown to be influenced by mass velocity and may reach up to 44% at low heat fluxes, where the nucleate boiling mechanism is dominant. It diminishes with increasing heat flux, corresponding to nucleate boiling suppression following flow regime transition. Pressure drop increase posed by the coated heat sink is relatively small in terms of overall system power consumption but appears to be influenced by mass flux, pressure oscillations as well as channel rewetting behaviour.

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).

Boiling heat transfer enhancement over copper tube via electrolytic and electrostatic effects

• An active technique was used to study pool boiling heat transfer over a copper tube. • Higher HTC and earlier ONB were achieved with the electrolytic boiling and the assistance of attached surfactants. • More nucleation sites and higher bubble detachment rate were obtained by the active method. • An optimal electrolytic current was observed to achieve the best boiling heat transfer performance. The effects of electrolytic pool boiling with and without the application of charged surfactants on heat transfer performance were experimentally investigated. The experiments were conducted at the saturated state under atmospheric pressure. This is the first study of the heat transfer mechanism in pool boiling using electrolysis with charged surfactants on copper tubes. The results indicate that heat transfer performance is not proportional to the applied electrolytic current. An optimal current was obtained for the enhancement of the heat transfer coefficient during electrolytic boiling. Supported by high-speed images, an analysis of bubble behaviors under different conditions was conducted. Compared with when water was used as the working fluid without the application of an electric field, the use of charged surfactants in electrolytic boiling with a current of 3 mA resulted in a 1.68 times higher heat transfer coefficient due to the number of bubble nucleation sites and the bubble departure frequency. Furthermore, the circumferential surface temperature distributions during boiling under different experimental conditions were elucidated by the analysis of bubble dynamics.

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%.

Effect of Nanofluids on the Enhancement of Boiling Heat Transfer: A Review

Boiling heat transfer can play a vital role in the two-phase flow applications. The analysis of the boiling hat transfer enhancement is of importance in such applications and the enhancement can be mostly conducted by using various active and passive techniques. One type of passive techniques is the enhancement of heat transfer by nanofluids. This article presents an extensive review on the effect of different nanofluids on the enhancement of heat transfer coefficient (HTC) and critical heat flux (CHF) for both pool as well as flow boiling. Nanoparticles addition to a working fluid is done arbitrarily to improve the thermophysical properties which in turn improves heat transfer rate. Numerous works have been done in the studies on nanofluid boiling. Among various nanoparticles, the most frequently used nanoparticles are Al2O3 and TiO2. In the case of binary nanoparticles, the most commonly used combination is Al2O3 and TiO2. After reviewing the relevant literatures, it is found that for pool boiling, the maximum HTC is increased to 138% for TiO2 nanoparticles and the maximum CHF is increased to 274.2% for MWCNTs. Conversely, in flow boiling the maximum HTC is increased to 126% for ZnO nanoparticles and the maximum CHF increased to as 100% for GO nanoparticles. In addition, when two or more nanoparticles in succession or binary nanofluids are used the CHF in pool boiling increased up to 100% for Al2O3 and TiO2 as well as the CHF in flow boiling increased up to 100% for Al2O3, ZnO, and Diamond. Though the information of the coefficient of heat transfer and the critical heat flux varied for different nanofluids and vary from experiment to experiment for each of the nanofluids. This variation happens because the coefficient of heat transfer and the critical heat flux in boiling is dependent upon several factors.

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

Flow boiling heat transfer enhancement under ultrasound field in minichannel heat sinks

The enhancement of the heat transfer assisted by ultrasound is considered to be an interesting and highly efficient cooling technology, but the investigation and application of ultrasound in minichannel heat sinks to strengthen the flow boiling heat transfer are very limited. Herein, a novel installation of ultrasound transducers in the flow direction of a minichannel heat sink is designed to experimentally study the characteristics of heat transfer in flow boiling and the influence of operation parameters (e.g., heat flux, mass flux rate) and ultrasound parameters (e.g., frequency, power) on the flow boiling heat transfer in a minichannel heat sink with and without ultrasound field. Bubble motion and flow pattern in the minichannel are analyzed by high-speed flow visualization, revealing that the ultrasound field induces more bubbles at the same observation position and a forward shift of the onset of nucleation boiling along the flow direction, as ultrasonic cavitation produces a large number of bubbles. Moreover, bubbles hitting the channel wall on the left and right sides are found, and the motion speed of the bubbles is increased by 31.9% under the ultrasound field. Our results demonstrate that the heat transfer coefficient obtained under the ultrasound field is 53.9% higher than in the absence of the ultrasound field under the same conditions, and the enhancement ratio is decreased in the high heat flux region due to the change of the flow regime with increasing heat flux. This study provides a theoretical basis for the application of an ultrasound field in minichannel heat sinks for the enhancement of flow boiling heat transfer.

Flow Boiling Heat Transfer Enhancement Under Ultrasound Field in Minichannel Heat Sinks

The enhancement of the heat transfer assisted by ultrasound is considered to be an interesting and highly efficient cooling technology, but the investigation and application of ultrasound in minichannel heat sinks to strengthen the flow boiling heat transfer are very limited. Herein, a novel installation of ultrasound transducers in the flow direction of a minichannel heat sink is designed to experimentally study the characteristics of heat transfer in flow boiling and the influence of operation parameters (e.g., heat flux, mass flux rate) and ultrasound parameters (e.g., frequency, power) on the flow boiling heat transfer in a minichannel heat sink with and without ultrasound field. Bubble motion and flow pattern in the minichannel are analyzed by high-speed flow visualization, revealing that the ultrasound field induces more bubbles at the same observation position and a forward shift of the onset of nucleation boiling along the flow direction, as ultrasonic cavitation produces a large number of bubbles. Moreover, bubbles hitting the channel wall on the left and right sides are found, and the motion speed of the bubbles is increased by 31.9% under the ultrasound field. Our results demonstrate that the heat transfer coefficient obtained under the ultrasound field is 53.9% higher than in the absence of the ultrasound field under the same conditions, and the enhancement ratio is decreased in the high heat flux region due to the change of the flow regime with increasing heat flux. This study provides a theoretical basis for the application of an ultrasound field in minichannel heat sinks for the enhancement of flow boiling heat transfer.

Numerical study of flow reversal during bubble growth and confinement of flow boiling in microchannels

The flow boiling instability caused by confined bubbles generated during the flow boiling process in parallel microchannels leads to flow reversal and earlier critical heat flux (CHF), which is a major problem with the extensive application of two-phase flow in microchannel heat sinks. In this study, single bubble growth in one of the two nearby microchannels is investigated through numerical methods. The VOF method, Hardt's phase-change model, conjugate heat transfer between solid and fluid domains were adopted within a self-developed OpenFOAM solver. After the growing bubble being confined by the sidewalls, the flow path of the channel is blocked thus the inlet liquid flow tends to flow into the other channels instead of pushing the confined bubble, which caused the confined bubble to extend towards upstream and flow reversal. By increasing the mass flux, the flow reversal is suppressed, but the flow boiling heat transfer enhancement is minimized due to the less evaporation area. The method of combination of microchannel and microgap (CMC) has little effect on flow reversal until the bubble enters the microgap. And its thermal performance is worse than the original heat sink. The confined bubble in the finned microchannel (FMC) heat sink would flow into the nearby channel through the secondary channel between the fins thus prevent flow reversal. Furthermore, the evaporation area between the bubble and solid walls increases, thus significantly lower thermal resistance (for about 40%) can be obtained. Considering the overall flow stability and heat transfer performance, the finned microchannel is recommended to be fabricated in industrial applications.

The synergetic effects of the surface wettability and the patterned nanostructure on boiling heat transfer enhancement

Engineering nano-structured surfaces with mixed/thermo-responsive wettability offer a new approach to improve the boiling performances of advanced thermal systems, such as the solar system and the heat dissipation systems in nuclear power plants, where more efficient cooling and higher safety limits are extremely desirable. In this study, five groups of surfaces: a) plain surfaces, b) nanofilm coated surfaces, c) patterned surfaces with superhydrophilic nanograss, d) patterned surfaces with superhydrophobic nanograss, f) patterned surfaces with thermo-responsive wettable nanograss are investigated for their boiling performances. It is found that the nanofilm coated surfaces show improved maximum heat transfer coefficient (HTCmax) as well as critical heat flux (CHF) compared with the plain surface. The patterned surfaces shift the boiling curves to left, and the CHF increases with increasing nanograss cover density. The surfaces with thermo-responsive wettability, which responses to the external heating/cooling stimuli by gradually increasing or decreasing the wettability, show the most optimal CHF enhancement. This study serves as a proof-of-concept for efficient heat transfer through carefully fabricated nano-structured wettability-enhanced surfaces.

Nucleate boiling enhancement by structured surfaces with distributed wettability-modified regions: A lattice Boltzmann study

In this paper, we conceive a novel pillar-structured surface for enhancing nucleate boiling heat transfer, namely a pillar-structured surface with distributed wettability-modified regions on the top of each pillar. A three-dimensional thermal multiphase lattice Boltzmann model with liquid–vapor phase change is employed to investigate the boiling performance on the pillar-structured surface with distributed wettability-modified regions and the associated mechanism of nucleate boiling heat transfer enhancement. According to the distribution of the wettability-modified regions, the bubble dynamics on the newly conceived pillar-structured surface can be classified into three regimes, i.e., regimes I, II, and III, among which the regime II shows relatively better boiling performance than the other two regimes due to the synergistic effects of surface structure and mixed wettability. It is found that in the regime II the bubbles nucleated at the wettability-modified regions do not coalesce with each other, which therefore elongates the length of the triple-phase contact lines on the pillar top in comparison with the pillar-structured surface with a unified wettability-modified region. Meanwhile, in the regime II the bubbles on the pillar top receive a strong bubble-wake effect supplied by the bubbles generated at the bottom substrate, which shortens the bubble growth cycle and promotes the departure of the bubbles on the pillar top, and also reduces the area of dry spots on the pillar top. The influences of the pillar width and the width of the wettability-modified regions are also studied. It is shown that the best boiling performance is always achieved in the cases that fall into the regime II.

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.

Enhancement of Pool Boiling Heat Transfer over Plain and Rough Cylindrical Tubes

This study investigates the pool boiling heat transfer of water over cylindrical heating tubes for different orientations and surface roughness of the tubes. First, two orientations of a smooth heating tube, horizontal and vertical, were used in the boiling chamber. For a given heat flux, the heat transfer coefficient achieved with the horizontal tube was always higher than that for the vertical tube. To investigate the influence of surface roughness, a rough heating tube with a fully rough outer surface was developed through a metal etching process. Under the same range of wall superheat, the rough tube enhanced the heat transfer rate significantly compared to the smooth tube. Finally, a modified heating tube (MHT) was developed by axially roughening half of the surface of an originally smooth tube. The orientation angle of the rough surface of this MHT was varied from 0° (horizontal-upward) to 180° (horizontal-downward) in the chamber. The heat flux increased significantly with the increase of orientation angles from 0° to 90° (the maximum of 80 kW/m2 at 90°), whereas the same decreased as the orientation angle is further increased from 90° to 180°. Results revealed that the bubble dynamics over the heating tubes play a vital role in pool boiling performance.

Convective Flow Boiling Heat Transfer Enhancement with Aqueous Al2O3 and TiO2 Nanofluids: Experimental Investigation

Convective Flow Boiling Heat Transfer Enhancement with Aqueous Al2O3 and TiO2 Nanofluids: Experimental Investigation

Experimental investigation of a new hybrid structured surface for subcooled flow boiling heat transfer enhancement

A new scheme with hybrid submillimetre/micro/nano-structured surfaces providing both high subcooled flow boiling performance and relatively low flow resistance was proposed to develop highly efficient and reliable water cooling schemes for plasma facing components (PFCs) in future nuclear fusion engineering. Both comparative microscopic surface structure pattern observations and macroscopic mockup subcooled flow boiling experiments under fusion reactor like heat flux conditions were conducted. The proposed technique can create hybrid structures favourable for subcooled flow boiling enhancement: including increasing bubble generation sites and multiscale instability etc. We demonstrated that the heat transfer coefficients of manipulated surfaces in the subcooled flow boiling state were almost doubled and that the critical heat flux (CHF) values were promoted by 80–200% under the tested flow conditions compared with under smooth surface conditions, with the flow path pressure drop deteriorating by less than 12%, indicating better capacities of high heat flux removal and low flow path resistance than those of existing PFC cooling schemes. The results encourage further optimization and development of this new scheme for practical application in nuclear fusion engineering.

Experimental investigation on boiling heat transfer enhanced by gradient aperture porous copper

To ensure the working stability of electronics, highly-efficient heat dissipation technology such as pool boiling is attracting more and more attention. The three-dimensional porous surface used in pool boiling enhancement will be subject to the vapor leaving resistance, which will lead to steam accumulation and thus weaken the pool boiling heat transfer performance. To facilitate the vapor releasement and also the liquid replenishment in the three-dimensional porous surface, we developed multi-layer gradient aperture open-cell porous copper in this work. Then, the pool boiling curves and the corresponding bubble dynamics of these porous copper samples were experimentally investigated. Results show that the onset of nucleate boiling (ONB) of a 4-layer gradient sample (about 0.9 ℃) is only about 8.3% that of the smooth surface (about 12 ℃), and the maximum heat transfer coefficient (about 8.2 × 104 W/m2·K) is about 8 times higher than that of the smooth surface, resulting from the porous structure which could disturb the flow field and provide massive nucleation sites. We also found that the bubble departure diameter of the 4-layer sample is almost 50% times smaller than that of the smooth surface while the frequency is 2 times higher. This phenomenon is due to the gradient aperture structure which could match the different pore-aperture requirements of both vapor escape and liquid replenishment. Considering the simple fabrication and excellent pool boiling performance, we conclude that the gradient aperture porous copper could provide an effective way for boiling heat transfer enhancement.

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.

Microtube Surfaces for the Simultaneous Enhancement of Efficiency and Critical Heat Flux during Pool Boiling.

Boiling is an essential process in numerous applications including power plants, thermal management, water purification, and steam generation. Previous studies have shown that surfaces with microcavities or biphilic wettability can enhance the efficiency of boiling heat transfer, that is, the heat transfer coefficient (HTC). Surfaces with permeable structures such as micropillar arrays, in contrast, have shown significant enhancement of the critical heat flux (CHF). In this work, we investigated microtube structures, where a cavity is defined at the center of a pillar, as structural building blocks to enhance HTC and CHF simultaneously in a controllable manner. We demonstrated simultaneous CHF and HTC enhancements of up to 62 and 244%, respectively, compared to those of a smooth surface. The experimental data along with high-speed images elucidate the mechanism for simultaneous enhancement where bubble nucleation occurs in the microtube cavities for increased HTC and microlayer evaporation occurs around microtube sidewalls for increased CHF. Furthermore, we combined micropillars and microtubes to create surfaces that further increased CHF by achieving a path to separate nucleating bubbles and rewetting liquids. This work provides guidelines for the systematic surface design for boiling heat transfer enhancement and has important implications for understanding boiling heat transfer mechanisms.