Boiling Heat Transfer Mechanism Research Articles

Bubble behavior parameters extraction and analysis during pool boiling based on deep-learning method

The nucleate pool boiling plays an important role in thermal and chemical engineering applications. Analyzing bubble dynamics at nucleation site is crucial to improve the understanding of boiling heat transfer mechanism. Quantitative extraction of bubble parameters from high-speed visualized images is a labor-intensitive and time-consuming task making it necessary for automatically detect single bubble growth and measure boiling characteristic parameters.In the present work, we proposed a deep learning based self-adaptive statistical algorithm for extraction of bubble behavior parameters quickly and automatically from numerous high-speed visualization images looking from the side view of a boiling chamber. A dataset was constructed for training and performance evaluation based on experimental data of saline solution pool boiling. The StarDist and U-Net convolutional neural network were combined in the algorithm so that more exact segmentation of the bubbles can be identified. Based on the segmentation results, a post-processing program was developed to extract the sequential variation of bubbles during consecutive cycles at nucleation sites. The dynamic characteristic parameters that affect heat transfer, such as nucleation density, bubble departure diameter, departure frequency, and wait time under different heat flux were obtained quantitatively. The comparison of automatic extraction algorithm and manual processing proves the reliability and superiority of our method. This work indicates that the proposed method has great potential to be widely applied as an efficient and universal tool for processing different types of bubble shadowgraph images.

Comparison between pool boiling system of graphene quantum dots and nitrogen-doped graphene quantum dots suspended in binary base fluids

Using nanofluids instead of conventional heat transfer fluids as a passive method is a well-established and widely used technique by researchers to increase the rate and thermal performance of engineering equipment. In this study, the hydrothermal method with a bottom-up approach was used for the synthesis of graphene quantum dots and nitrogen-doped graphene quantum dots. Then, nanofluid samples were prepared in a two-step process, by adding nanoparticles to binary base fluids of deionized water and ethylene glycol in volume concentrations of (50:50) and (60:40), in four concentrations of 100, 200, 500, and 1000 ppm. In order to better understand the boiling heat transfer mechanism and measure its characteristics such as critical heat flux and heat transfer coefficient, an experimental system was designed and built. Nanofluids based on graphene quantum dots have unique features such as compatibility with the environment, economic efficiency, high stability and suitable heat transfer capability. For this reason, their selection in the pool boiling heat transfer process, in addition to saving energy, is introduced as one of the most effective options for improving CHF and HTC. The tests were performed under saturated conditions, atmospheric pressure, and on a vertical flat and polished copper thermal plate. Prepared nanofluids GQDs and N: GQDs based on DI-water maintained their apparent stability for two months. For GQDs nanofluids at an optimal concentration of 500 ppm with a volume ratio of (60:40) DI-water and EG, compared to DI-water, the greatest increase in CHF and HTC is 90.69, 85.011 % and for N: GQDs at a concentration of 500 with a volume ratio (50:50) DI-water and EG, 75.37 and 78.17 % compared to DI-water.

Numerical study on the growth characteristics of bubble in a petal-shaped fuel rod channel

Based on the VOF multiphase flow model and bubble growth model, a numerical analysis model of bubble growth of subcooled flow boiling in a petal-shaped fuel rod channel was established. The growth characteristics of a single bubble and the influence of thermal parameters were analyzed. Firstly, the growth rate, shape, and trajectory of the bubble were investigated under typical working conditions. Secondly, the influences of inlet subcooling, inlet velocity, and wall superheat on the bubble growth characteristics were clarified. Finally, the existing bubble growth rate prediction correlation was evaluated based on the results of numerical calculations. The results show that the bubble growth rate is fast at the early stage of bubble growth when the evaporation heat flow from the microlayer plays a major role. With the disappearance of microlayer evaporation, the bubble diameter increases slowly. The bubble diameter decreases continuously when the evaporation heat flow in the overheating layer is less than the condensation heat flow after the bubble lift-off. Compared to the rectangular channel, the bubble diameter in a petal-shaped fuel rod channel has a larger equivalent diameter and more obvious lateral movement. With the increase of inlet flow velocity and inlet subcooling, the total evaporation heat flow of the bubble decreases, while the condensation heat flow increases, which results in the decrease of bubble diameter. The influence of increasing wall superheat on bubble diameter is opposite to that of increasing inlet flow velocity and inlet subcooling. Based on the numerical results, a new prediction correlation for bubble growth rate in a petal-shaped fuel rod channel was developed, whose prediction error is within ±20 %. The related research results help to further reveal the subcooled boiling heat transfer mechanism in a petal-shaped fuel rod channel.

A method to partition boiling heat transfer mechanisms using synchronous through-substrate high-speed visual and infrared measurements

Heat transfer during boiling involves a variety of transport mechanisms. Available experimental techniques cannot yet fully delineate these mechanisms which has contributed to a long-standing, persistent challenge of constructing accurate mechanistic models for boiling. In this work, we develop a method to identify and distinguish between the individual heat transfer mechanisms that occur during boiling using synchronous, through-substrate, high-speed visual and infrared measurements. Local heat fluxes are deduced from temperature measurements and a synchronized set of binarized phase maps are obtained from processing high-speed visual measurements. Experimental pool boiling investigation of HFE-7100 fluid on an indium tin oxide surface revealed four distinct heat transfer signatures in the heat flux maps corresponding to liquid convection, contact line evaporation, vapor convection, and local microconvection due to rewetting during boiling. To classify these regions, pixel-wise binary and morphological operations are performed on the phase maps of the entire surface. In contrast with prior partitioning techniques which use standalone heat flux measurements, our synchronous high-resolution visualization enables the region classification around fine bubble footprints that otherwise would not be detected with heat flux maps alone. The heat fluxes and superheats are then partitioned into their underlying mechanisms using classified regions from the phase maps. Analysis of the nucleate boiling regime data shows that different mechanisms contribute in varying degrees to the overall heat transfer at different points along the boiling curve: for the surface-fluid combination studied, single-phase liquid heat transfer is the primary contributor at low heat fluxes while at high heat fluxes, contact line evaporation contributed the most. We further employ the experimental approach to partitioning dynamic processes resulting from step increases in heat input demonstrating its potential for investigating transient phenomena such as dryout. The experimental and post-processing method introduced in this study is the first to delineate partitioning between different heat transfer mechanism regions in the presence of multiple interacting bubbles over the entire surface throughout the boiling curve in both steady and transient operating conditions.

Bubble dynamics and their effects on interfacial heat transfer in one single microchannel

Bubble dynamics are fundamental to understand boiling heat transfer mechanism at microscale. In this study, bubble dynamics in one single confined microchannel was experimentally investigated through vision-assisted technique. Experimental studies were performed to understand the effects of hydraulic diameter (Dh) of channel, flow rate and heat flux on bubble dynamics, which highly affect the boiling heat transfer performance. First of all, experimental results show that Dh has a vital effect on the volume change rate of confined bubble, which can be used to compute the heat flux transported by single bubble. The liquid-vapor (L-V) interfaces were tracked by non-rigid image registration and the velocity and acceleration of L-V interface were estimated through the analysis of recorded videos using vision-based technique. To quantitatively evaluate the impact of bubble dynamics, transient heat transfer rates were presented at different flow rates. Furthermore, the interfacial heat transfer (Nuc) influenced by bubble dynamics was evaluated. Nuc changes with Reb during bubble growth and bubble shrinking. Finally, an analysis of energy distributions by bubble dynamics is conducted. Compared to energy transported by bubble and kinetic energy of bubble, heat flux through interface is significant, dissipating over 96% of total input power.

Preliminary physical analysis of a single-bubble pool-boiling experiment in space: Effect of subcooling and possible non-condensable residuals

The purpose of this work is to study the boiling heat transfer mechanisms in microgravity. Single bubbles growing without departure up to a centimetric size on a single artificial nucleation site are investigated under well-controlled pool-boiling conditions in the framework of the RUBI experiment on the International Space Station. Subcooling is one of the important parameters defining the bubble growth process. In this work, we conduct a first physical analysis of the experiment. The bubble diameter is evaluated in time using a developed algorithm. Several stages are highlighted. It is shown that subcooling has a significant effect on all stages of bubble life and can even lead to a partial collapse. A numerical model is developed to help a better understanding of the phenomenon. Some observations couldn't be explained without evoking the presence of non-condensable residuals. The amount of those is estimated.

Comprehensive study of synergistic effects of phase separation structures and electric fields for enhancing the heat transfer performance of minichannel heat sinks

In this paper, the effects of the phase separation structure on the enhanced boiling heat transfer mechanism and bubble dynamics were investigated. Two types of phase-separation structures and one type of conventional minichannel were manufactured by ultrafast laser technology. Flow boiling experiments of a glycerol-water solution were conducted under various heat flux and electric field conditions. Entropy generation analysis was employed to comprehensively analyze and assess energy losses and irreversibility within the system. Response surface analysis method was introduced to optimize operational and design parameters for achieving optimal experimental design. Moreover, the coupling effect of the phase-separation structure and electric field was observed to have a significant impact on the enhancement of heat transfer. The visualization results indicate that, compared to conventional parallel minichannels, the countercurrent minichannel with phase-separation structure leads to a gradual decrease in the aspect ratio of confined bubbles downstream (in contrast to the increasing aspect ratio of confined bubbles downstream in general minichannels), and smaller bubbles at the upstream of adjacent minichannels gradually disappear and diminish due to the condensation effect. The experimental results demonstrate that the porous phase-separation structure plays a significant role in enhancing heat transfer performance and can reduce the increase of two-phase friction pressure drop per unit length with the increase of heat flux. Under electric field-free conditions, the porous phase separation structure exhibits a maximum increase in heat transfer efficiency of 26.2% compared to conventional structures. Furthermore, the maximum reduction in two-phase frictional pressure drop per unit length reaches 19.4%. In the presence of the synergistic effect between an electric field and the phase separation structure, the maximum heat transfer efficiency is enhanced by 65.5%. The above analysis provides an effective means for investigating the comprehensive performance of phase separation structures and the bubble dynamics of confined bubbles in heat sinks.

Investigation of bubble dynamics and boiling heat transfer characteristics under vertically heating walls via lattice Boltzmann method

Boiling heat transfer characteristics greatly depend on bubble dynamics behaviors. Whereas the influence of bubble dynamics on boiling heat transfer mechanism under the vertical heating wall condition is scarce. In this work, lattice Boltzmann (LB) method is implemented to investigate the relationship between bubble dynamics and boiling heat transfer on the vertical heating wall. Bubble nucleation, growth and departure processes under different wettability conditions are systematically studied. The results demonstrate that the vapor phase covering the vertical surface increases at a greater contact angle. The bubble detachment becomes difficult, leading to the reduction of bubble volume. Because of the interaction of buoyancy force and bubble growth under the vertical heating wall, physical properties of the three-phase contact line and bubble dynamics show strong coupling characteristics. In addition, the heat flux on the hydrophilic and hydrophobic surfaces can be enhanced by 98% and reduced by 75% compared to that on the neutral surface.

Experimental Investigation and CFD Simulation of Cryogenic Condenser

In this research, experimental investigation and the computational fluid dynamic (CFD) simulation of a cryogenic condenser for oxygen liquefaction was carried out. The liquid nitrogen was used as a cooling fluid. In the simulation section, a three-dimensional model with a structured mesh with high mesh quality for aspect ratio and skewness was considered. The multi-phase flow inside the condenser was studied numerically, using the volume of fluid (VOF) method. This work also examined the assessment of the vapor generation rate during the condensation of oxygen, based on the boiling heat transfer mechanism and the unique physical characteristics. The experiment was conducted to examine the simulation results. The effect of liquid nitrogen height on the oxygen mass flows was investigated using computational fluid dynamics (CFD). The average deviation of the CFD predictions from the available experimental oxygen mass flows was 17%.

Numerical simulation of single bubble growth and heat transfer of R32+R1234yf binary mixtures during saturated pool boiling

As a promising alternative refrigerant for meeting carbon reduction targets, R32+R1234yf binary mixtures combine the thermal performance of R32 with the environmental performance of R1234yf. Since the degree of heat dissipation during the boiling process is closely related to bubble behavior, the bubble growth and heat transfer characteristics of R32+R1234yf mixtures must be understood in depth. Accordingly, this study proposed a numerical model for single bubble growth during nucleate pool boiling of mixtures. Numerical simulations of single bubble growth in R32, R1234yf, and four binary mixtures thereof were conducted. Owing to the preferential evaporation of the more volatile component, a low concentration region of R32 appeared in the liquid near the heating surface, while the concentration of R32 in the vapor was generally higher than that in the liquid. The evaporation heat flux at the bubble interface in the mixtures was significantly lower than in pure R32, indicating that the existence of mass transfer resistance considerably reduced the heat transfer capacity of mixtures. In contrast to a pure fluid, the microlayer evaporation in a mixture might terminate owing to an increase in the local bubble point temperature, and two modes of microlayer evaporation—called “bubble-point-controlled mode” and “thickness-controlled mode”—were identified accordingly. The total heat transferred during bubble growth first decreased and then increased with an increase in the bulk concentration of R32; a minimum value was observed at an R32 mol fraction of approximately 0.4. This result is consistent with the observed variation in the difference between the vapor and liquid phase concentrations. Overall, this study provides a detailed description of the mass and heat transfer characteristics during single bubble growth in binary mixtures, contributing to a deeper understanding of the associated boiling heat transfer mechanisms.

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

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

Automated bubble analysis of high-speed subcooled flow boiling images using U-net transfer learning and global optical flow

Capturing and analyzing the bubble dynamics is crucial to improving the understanding of boiling heat transfer mechanisms and predicting boiling heat transfer coefficient and boiling crisis. High speed video (HSV) imaging has been used for decades towards this end. Still, there is no universal approach to quantitatively analyze bubble dynamics from HSV images.In this study, we propose a data-driven post-processing approach to segment, track, and identify wall-attached vapor bubbles from HSV images of the boiling process in subcooled flow conditions. Firstly, we employ a transfer learning framework with a U-Net-based convolution neural network (CNN) architecture to detect and segment bubbles in HSV images of diverse contrast and surface texture using very little data (e.g., 10 images) for training. Then, we evaluate the trained CNN model with 100 ground-truth images, and the validation results show that the model accuracy and precision in detecting the optical footprint of bubbles are higher than 90%. Finally, we suggest a criterion to identify a condensing bubble based on the divergence of the bubble displacement, which is calculated from sequential segmented bubble images using a global optical flow code. Using this combination of machine learning and optical flow, we can identify nucleation sites and track the growth of bubbles nucleating at each site to quantify nucleation site density, nucleation frequency, and other fundamental boiling parameters. The proposed system is validated using results obtained on a special heater, which enables both infrared (IR) thermometry and HSV imaging on a metallic surface. We compare the fundamental boiling parameters obtained by the two different diagnostics. The results show good agreement. The difference between the measurements of nucleation site density, averaged nucleation frequency, and averaged growth time performed with the two techniques is always within ± 20% and mostly ± 10% of the values measured with IR thermometry.

Flow boiling heat transfer mechanisms and flow characteristics of pump-driven two-phase flow systems used in data center cooling

Pump-driven two-phase flow systems are commonly used in data center cooling owing to their outstanding heat-transfer performance. The evaporator is a key component of the cooling system, and its design significantly affects the overall cooling efficiency of the system. The boiling heat transfer process in the evaporator is primarily affected by the boiling heat transfer mechanism. Such a mechanism is mainly influenced by the type of coolant, inner wall structure of the tube (threaded tube or smooth tube), heat flux, and mass flow rate. In this study, we experimentally investigated the heat transfer mechanisms, capacity, and flow characteristics of the boiling process in a pump-driven two-phase flow system commonly used in data center cooling. The heat transfer capacity involved in this experiment was 140–1150 W, and the mass flow rate was 40–270 kg/h. Different coolants (R134a and R410A) and tube structures (threaded and smooth) were tested. The main novelty of this study is as follows. First, under the working conditions in this experiment, the inside-tube flow boiling mechanism was nucleate boiling for both the smooth and threaded tubes, and the boiling heat transfer coefficient remained constant as the mass flow rate decreased and vapor quality increased. Second, although it is commonly believed that a threaded pipe can significantly enhance the heat transfer ability, our study found that under data center cooling applications, the threaded tube had little strengthening effect on the heat transfer performance. Third, commonly used coolants, including R134a and R410A, were tested and the above conclusions were verified.

Transient boiling heat transfer mechanism of droplet impacting heated cylinder

The spray on the heated cylindrical surface for boiling widely exists in the industrial application. In order to discover the transient boiling heat transfer mechanism, the cases of droplet impacting on the cylindrical surface were experimentally carried out. In this experiment, the droplet hydrodynamics were captured by high-speed camera. The effects of impact velocity and wall temperature on the hydrodynamics and boiling heat transfer performance were studied in detail. The results show that, for the droplet impacting on the heated cylindrical surface, four types of phase change heat transfer regimes, i.e., film evaporation regime, nucleate boiling regime, transition boiling regime and film boiling regime are found. From the built map of relationship between wall temperature and Weber number, it is demonstrated that when the wall superheat is low, increasing the droplet impact velocity enlarges the solid-liquid contact area, boosting the heat transfer performance in the film evaporation and nucleate boiling regimes. However, when the wall superheat is high, increasing the droplet impact velocity or Weber number makes the Leidenfrost point be lower and is unfavorable for heat transfer due to the easy generation of vapor layer, which strongly differs from the conclusion of droplet impacting on the heated flat surface according to others’ investigations. The underlying mechanism can be revealed from two aspects, i.e., the formation mechanism of Leidenfrost state and the effect of surface geometry. Finally, the quantitative relationship between Leidenfrost point and Weber number is given.

A relatively wide-ranged correlation of saturated flow boiling heat transfer within narrow rectangular channel for water

• Database of boiling heat transfer in narrow rectangular channel over a relatively wide parametric range obtained. • Corresponding transition of two-phase flow regimes with dominant heat transfer mechanisms observed and discussed. • A new, wide-ranged correlation proposed. A campaign of tests is performed for saturated boiling heat transfer characteristics of water in narrow rectangular channel. The test matrix covers a relative wide range of conditional parameters and 3142 test data are acquired, screened and selected for study. Meanwhile, visualization test has been conducted with three typical flow patterns, namely isolated bubbly, confined bubbly and irregular annular flows, being observed and identified, each pattern corresponding to a prevailing or a transitional combined boiling heat transfer mode. Study of the data trend indicates that two boiling heat transfer mechanisms, namely nucleate boiling and forced convective evaporation, are either one dominant or two combined transitional during different phases of saturated flow boiling within the channel. With the basic view obtained from the test campaign, and with related parametric effect and sensitivity study, as well as the existing correlation assessment with the test data, a new correlation for the saturated boiling heat transfer is proposed and regressed, which consists a modification factor composing the two basic boiling mechanisms and the transition function between. A preliminary assessment is performed for its statistical and predictive accuracy, from which a relatively satisfactory agreement is drawn. The new correlation applies to a heat flux range of 100 - 2300 k W / m 2 , a mass flux range of 500 2000 k g / m 2 · s , a system pressure range of 0.2 5.5 M P a and the equilibrium quality range of 0 0.72 . It might be used, in conjunction with the flow pattern and the related heat transfer mechanism, to predict the saturated boiling heat transfer coefficient over a wide range of heat flux, mass flux and pressures with better accuracy than previous correlations.

Experimental studies of liquid immersion cooling for 18650 lithium-ion battery under different discharging conditions

In this study, fluorinated liquid immersion cooling as a new cooling scheme has been tested and discussed for cooling the 18650 lithium-ion battery (LIB). SF33, with the boiling point of 33.4 °C, is chosen as the liquid for the immersion cooling. Comparison of the SF33 immersion cooling and forced air cooling (FAC) for the 18650 LIB under 2C, 4C and dynamic load conditions are made. It is found that the immersion cooling better cools the battery under all these conditions. Under 4C discharging, the maximum cell temperature rise is 14.06 °C for the FAC, while is 4.97 °C for the SF33 immersion cooling. As the temperature of SF33 basically governs the cell temperature, the SF33 temperature should not be too low. This study demonstrates that the LIB has non-negligible power losses under SF33 temperature of 10 °C and 15 °C, compared with the temperatures above 20 °C. Lastly, the two-phase boiling heat transfer mechanisms associated with immersion cooling are discussed with bubble dynamics analysis. It is found under a higher C rate, more aggressive boiling heat transfer is induced. The battery temperature is controlled to be below 34.5 °C even under 7C discharging condition.

Study on boiling heat transfer mechanism based on microlayer evaporation theory

Based on dynamic microlayer evaporation model. Heat transfer mechanism of nuclear boiling and microlayer thickness under single bubble on heated surface. It has a lot to do with the temperature of the wall. Therefore, this paper studies the mechanism of bubble microlayer and makes comparative analysis with its experiments. It is helpful to master nuclear boiling heat transfer more comprehensively. In the study of individual bubble microlayer thickness. The effects of time and radius on wall temperature are considered in this paper. The theoretical formula of initial thickness of microlayer is deduced. The theoretical formula is compared with the experimental data of microlayer evaporation under the nucleation bubble. It was found that the experimental data fell within ±25% of the formula range. It is consistent with the previous experimental research.

Heater size and gravity level effects on nucleate pool boiling heat transfer under controlled wall heat flux condition

With the ability of dissipating large amount of latent heat in nearly isothermal process, pool boiling heat transfer is an efficient solution for cooling problem of different sized electronic devices in different gravities. For the practical heat dissipation of electronic device usually occurs at controlled wall heat flux condition, it is essential to reveal pool boiling heat transfer characteristics at different sizes and gravities under controlled wall heat flux condition. In this paper, boiling heat transfer mechanisms on two sized smooth silicon chips (chip S 1 × 1 and chip S 2 × 2) in ground gravity and microgravity were studied. It was found that the gravity scaling parameters developed under controlled wall temperature condition cannot be applicable to the controlled wall heat flux condition. Under controlled wall heat flux condition, the microgravity boiling heat transfer performance on chip S 1 × 1 is in surface tension dominated boiling (SDB) regime in the whole heat flux range. While for chip S 2 × 2, it is in SDB regime when heat flux is smaller than 6.44 W·cm−2, but enters into buoyancy dominated boiling (BDB) regime when heat flux is larger than 6.44 W·cm−2. The critical point of the two boiling mechanisms occurs when the threshold gravitational acceleration equals to the experimental microgravity. In BDB regime, the power law coefficient reflecting the gravity effect on nucleate boiling heat transfer performance (mBDB) for controlled wall temperature condition is larger than that for controlled wall heat flux condition, indicating a greater impact of gravity level under controlled wall temperature condition. Moreover, mBDB for controlled wall temperature condition is affected only by non-dimensional wall temperature T*. While for controlled wall heat flux condition, mBDB increases with increasing T* but decreases with increase in heater size, and heater size has a greater influence on mBDB. Based on the available data, the gravity scaling parameters for controlled wall heat flux boiling were developed, which can accurately predict the microgravity boiling heat transfer performance on two sized chips.

The effect of dynamic wetting behavior on boiling heat transfer mechanisms during bubble growth and departure

Decades of prior study has yet to fully disentangle the complex transport mechanisms that are attributed to highly effective heat transfer during boiling. Rational design of enhanced surfaces to maintain lower surface temperatures during boiling requires improved insight into the individual heat transfer processes and their dependence on surface characteristics. This study seeks to advance the understanding of the fundamental role that surface wettability plays in determining the relative contributions of different heat transfer mechanisms and on the overall heat transfer efficacy during bubble growth. Two-phase, diabatic simulations of single bubble growth considering interfacial phase change and a custom dynamic contact angle framework are employed to investigate how the distinct contact-line and bubble dynamics that are experienced on hygrophilic, hygrophobic, and ambiphilic surfaces impact heat transfer. The local surface temperature and heat flux profiles underneath the bubble are examined during the receding, pinning, and advancing stages of bubble growth to explore the dominant heat transfer modes at each stage. The results indicate that both hygrophilic and ambiphilic surfaces are promising candidates for the development of enhanced surfaces, but for different reasons related to microlayer heat transfer versus nucleation characteristics, respectively. Target ranges for the dynamic receding and advancing contact angles within each wettability regime are suggested to inform design of surfaces with tailored wettability that maximize performance. These findings indicate that the contact line dynamics play an important role in determining the heat transfer efficacy of a surface and provide a framework for the development of enhanced boiling surfaces.

Numerical and experimental study of pool boiling heat transfer mechanisms in V-shaped grooved porous metals

In this study, pool boiling heat transfer mechanisms in V-shaped grooved porous metals are studied using our porous metal model. The model eliminates the small Biot number restriction of linear temperature-distribution assumption and thus extends to large Biot number conditions, such as boiling heat transfer of porous metals relating multi-bubble agitation and coalescence. The effects of groove number, groove distance and groove width on pool boiling heat transfer are investigated using lattice Boltzmann method. The experimental study is also conducted to reveal bubble escaping paths. The results show that bubble departure frequency on the porous metal with double V-shaped grooves is higher than those on the porous metal with single V-shaped groove and the uniform porous metal. The experimental result indicates that the venting bubbles in lateral grooves are entrained by the central departure bubbles, which validate the numerical results. At the low heat fluxes, the wall superheat increases with increasing groove distance. The lotus-shaped bubbles form in the central porous metal. The bubble escaping resistance reduction leads to the heat transfer performance enhancement for the porous metal with the wide V-shaped grooves.