Simulation of single bubble dynamic process in pool boiling process under microgravity based on phase field method

We use the phase field method to track the gas-liquid interface based on the gas-liquid two-phase flow in the pool boiling process,and study the bubble nucleation,growth,deformation,departure and other dynamic behaviors on the heating surface under microgravity.By simulating the correlation between liquid undercooling and bubble dynamics,we find that the bubble growth time increases with the increase of liquid undercooling,but the effect of liquid undercooling on bubble height is not significant.Meanwhile,the gas-liquid-solid three-phase contact angle and the gravity level will also have an effect on the bubble growth time and bubble height.With the increase of the contact angle,the bubble growth time and bubble height when the bubble departs also increase.While the effect of gravity level is on the contrary,the smaller the gravity level is,the larger the bubble height and bubble growth time when the bubble separates.

Comparisons of Structured Surface Floors for Pool Boiling Enhancement at Low Heat Fluxes: Hands-On Learning Setup for Heat Transfer Classroom

Various enhanced surfaces have been proposed over the years to improve boiling heat transfer. This paper introduces an experimental setup designed for boiling demonstration in the graduate-level Heat Transfer course. The pool boiling performance of water under atmospheric pressure of 1.025 bar is investigated by using several structured surfaces at heat fluxes of 28 and 35 kW/m2. Surfaces with holes, rectangular grooves, and mushroom fins are manufactured by an NC-controlled vertical milling machine. The heat flux versus excess temperature graph is plotted by using thermocouple measurements of water and base temperatures of the boiling vessel. The separation, rise, and growth of individual vapor bubbles from the surface during boiling were recorded with a digital camera. The results for the plain surface are compared to the Rohsenow correlation. The enhancement of heat transfer coefficient (h) ranged between 15% - 44.5% for all structured surfaces. The highest heat transfer coefficient enhancement is observed between 41% - 56.5% for holed surface-3 (405 holes) compared to the plain surface. The excess temperature dropped around 29% - 34% for holed surface-3 (405 holes) compared to the plain surface. The heat transfer coefficient increases as the spacing between channels or holes decreases. While the bubbles on holed and mushroomed surfaces were spherical, the bubbles on the flat and grooved surfaces were observed as formless. The suggested economical test design could be appropriate to keep students focused and participating in the classroom.

Pool boiling performance of porous surface tubes under vacuum conditions

Two types of tube bundles are designed,which are,respectively,composed of six tubes arranged in the boiling chamber.The nucleate pool boiling performance of smooth tube bundles and sintered porous surface tube bundles with deionized water as a medium are experimentally studied at atmospheric and sub-atmospheric pressures,respectively.The experimental results indicate that the boiling heat transfer coefficients of the two types of tube bundles increase with the increase in pressure under vacuum conditions as they behave under ordinary pressure.As the pressure varies from 10 to 100 kPa,it also can be seen that the heat transfer coefficient of the sintered porous surface tube is increased by 0.2 to 4 times compared with the smooth one under the same operating parameters.In addition,the experimental data show that a definite bundle effect exists in both sintered porous surface tubes and smooth tubes under vacuum conditions.

Heat Transfer in Nucleate Pool Boiling of Binary and Ternary Refrigerant Mixtures

Heat transfer coefficients in nucleate pool boiling were measured on a horizontal copper surface for refrigerants, HFC-134a, HFC-32, and HFC-125, their binary and ternary mixtures under saturated conditions at 0.9MPa. Compared to pure components, both binary and ternary mixtures showed lower heat transfer coefficients.This deterioration was more pronounced as heat flux was increased. Experimental data were compared with some empirical and semi-empirical correlations available in literature. For binary mixture, the accuracy of the correlations varied considerably with mixtures and the heat flux. Experimental data for HFC-32/134a/125 were also compared with available correlated equation obtained by Thome. For ternary mixture, the boiling range of binary mixture composed by the pure fluids with the lowest and the medium boiling points, and their concentration difference had important effects on boiling heat transfer coefficients.

Visualization of Bubble Dynamics for Pool Boiling of Binary Refrigerant Mixture R11-R113

A digital photographic study of pool boiling with binary mixture Rll（CC13）-Rll3（CCl3CF3） was performed on a horizontal transparent heater at pressure of 0.1MPa. A high speed digital camera was applied to record the bubble behaviors in boiling process. Strong effects of composition on bubble departure diameter, deparatre time, nucleation density were observed, which was attributed to the nature of the activation of the boiling surface and mass diffusion effects. The bubble departure diameter, departure period and nucleation density as functions of composition for binary mixtures R 11-R 113 were presented respectively. From the video images, it can be concluded that evaporation of microlayer is very important to the growth of bubble. It is also observed that there is not any liquid recruited into the microlayer below the bubble.

Subcooled pool boiling heat transfer in fractal nanofluids:A novel analytical model

A novel analytical model to determine the heat flux of subcooled pool boiling in fractal nanofluids is developed. The model considers the fractal character of nanofluids in terms of the fractal dimension of nanoparticles and the fractal dimen- sion of active cavities on the heated surfaces; it also takes into account the effect of the Brownian motion of nanoparticles, which has no empirical constant but has parameters with physical meanings. The proposed model is expressed as a function of the subcooling of fluids and the wall superheat. The fractal analytical model is verified by a reasonable agreement with the experimental data and the results obtained from existing models.

Pool Boiling Heat Transfer in Dilute Water/Triethyleneglycol Solutions

Boiling of water/triethyleneglycol(TEG)binary solution has a wide-ranging application in the gas processing engineering.Design,operation and optimization of the involved boilers require accurate prediction of boiling heat transfer coefficient between surface and solution.In this investigation,nucleate pool boiling heat transfer coefficient has been experimentally measured on a horizontal rod heater in water/TEG binary solutions in a wide range of concentrations and heat fluxes under ambient condition.The present experimental data are correlated using major existing correlations.In addition a correlation is presented for prediction of pool boiling heat transfer for the system in which the vapour pressure of one component is negligible.This model is based on the mass transfer rate equation for prediction of the concentration at the bubble vapor/liquid interface.Based on this prediction,the temperature of the interface and accordingly,the boiling heat transfer coefficient could be straightforwardly calculated from the known concentration at the interface.It is shown that this simple model has sufficient accuracy and is acceptable below the medium concentrations of TEG when the vapor equilibrium concentration of TEG is almost zero.The presented model excludes any tuning parameter and requires very few physical properties to apply.

Enhanced Pool Boiling Heat Transfer on Copper Foam Welded Surfaces

Enhanced pool boiling heat transfer of the porous structure is critical to the thermal management technology.In this paper,pool boiling heat transfer experiments are performed on copper foam welded surfaces in de-ionized water to investigate the effects of basic parameters of copper foam on heat transfer enhancement.Boiling phenomenon is observed to facilitate the understanding of enhancement mechanism.The results show that copper foam welded surfaces can significantly enhance the pool boiling heat transfer performance,reduce the boiling incipience temperature by 7-9℃,and reach two times heat transfer coefficient compared with smooth plain surfaces due to numerous nucleation sites,extended surface areas,and enhanced turbulent effect.Pore density and thickness of foam have two side effects on heat transfer.

Investigations on pool boiling critical heat flux, transient characteristics and bonding strength of heater wire with aqua based reduced graphene oxide nanofluids

In the present work, the pool boiling critical heat flux, transient heat transfer characteristics, and bonding strength of thin Ni-Cr wire with aqua based reduced graphene oxide(r GO) nanofluids are experimentally studied. Results indicate:(i) the critical heat flux(CHF) of 0.01, 0.05, 0.1, 0.2, and 0.3 g·L^(-1) concentrations of r GO-water nanofluids varies from 1.42 to 2.40 MW·m^(-2);(ii) the CHF remains same for the tested samples during transient heat transfer studies and(iii) a constant value of CHF upto 10 tests when the nanocoated Ni-Cr wire is tested with DI water and deterioration occurs beyond this which implies a chance of peel off of r GO layer below the critical coating thickness.

Examination of two-phase behaviors in porous media during pool boiling

The enhancement of pool boiling heat transfer using porous media has been extensively studied.Although the two-phase distribution and evolution in porous media are crucial to the heat transfer performance,including the critical heat flux(CHF)and heat transfer coefficient(HTC),direct observation of the two-phase flow inside the media is limited owing to the blockage of the direct view from the porous structures.In this study,pool boiling visualization experiments were conducted on porous samples with different throat widths in deionized water.The results showed that the HTC increased with the throat width.Additionally,the growth-contraction cycle of the vapor region and the formation and drying of the wall liquid film inside the porous media were investigated.The vapor region,including the maximum and minimum areas in the boiling cycle,was quantitatively described.Furthermore,the relationship between the minimum gas-phase area and HTC peak was identified.A one-dimensional transient model was developed considering solid skeleton heat conduction,liquid film evaporation,and vapor region growth to quantitatively study the influence of heat flux on the internal two-phase flow.The model successfully captured the maximum gas-phase area,duration of boiling cycles,and HTC trends at specific heat fluxes.The results of this quantitative study provide insights into the internal two-phase distribution and evolution induced by pool boiling.

Heat Transfer of Boiling R134a and R142b on a Twisted Tube with Machine Processed Porous Surface

The objective of this work was to investigate nucleate pool boiling heat transfer performance and mechanism of R134a and R142b on a twisted tube with machine processed porous surface （T-MPPS tube） as well as to determine its potential application to flooded refrigerant evaporators. In the experimental range, the boiling heat transfer coefficients of R134a on a T-MPPS tube were 1.8-2.0 times larger than those of R134a on a plain tube. In addition, the developed experimental correlations verified that the predictions of the heat transfer coefficients of boiling R134a and R142bon a T-MPPS tube at the experimental conditions were considerably accurate.

Application of Superhydrophobic Surface on Boiling Heat Transfer Characteristics of Nanofluids

Boiling heat transfer is a mode using the phase change of working medium to strengthen the heat exchange due to its good heat exchange capability,and it is widely used in heat exchange engineering.Nanofluids have been used in the direction of enhanced heat transfer for their superior thermophysical property.The wetting,spreading and ripple phenomena of superhydrophobic surfaces widely exist in nature and daily life.It has great application value for engineering technology.In this article,the boiling heat exchange characteristics of nanofluids on superhydrophobic surface are numerically studied.It was found that with the increase of superheating degree,the steam volume ratio of unmodified heated surface increases to saturation,while the steam volume and evaporation ratio of modified superhydrophobic surface increase firstly and then decrease.At the same time,bubbles are generated and accumulated more fully on superhydrophobic surface.It was also found that nanofluids with low viscosity are more affected by superhydrophobic surface characteristics,and the increase is more significant with high superheating degree,and the superhydrophobic surface is beneficial to enhancing boiling heat exchange.Compared with the simulation results,it could be concluded that the boiling heat exchange performance of CuO-water nano-fluids on the modified superhydrophobic surface is better than that of CuO-ethylene glycol nanofluids under high superheating degree.

Numerical investigation of heat transfer performance and bubble behavior of nucleate pool boiling in a confined space under rolling conditions

Nucleate pool boiling process is widely used in heat exchangers because of its excellent heat transfer performance.With the gradual increase of applications,more and more equipments work in a non-static state,but there is little research under rolling conditions.Therefore,it is necessary to investigate the influence of rolling motion on the nucleate pool boiling process.In this study,a numerical investigation of the nucleate pool boiling process under static and rolling conditions is performed based on the volume-of-fluid(VOF)method.Physical fields and phase distribution under static state and rolling motion are compared to investigate the effect of rolling motion on the nucleate pool boiling process.The results show that rolling motion greatly influences the bubble behavior and void fraction owing to the differences between flow fields.The void fraction decreased by 11.84%,48.82%,and 56.87%as the maximum rolling angle increased from 15°to 45°,and by 11.84%,22.27%,and 21.81%as the rolling period increased from 1 s to 3 s.The void fraction decreased by 11.84%,48.82%,and 56.87%as the maximum rolling angle increased from 15°to 45°.The heat transfer coefficients of different cases are compared,and it is found that the effects of rolling motion on heat transfer coefficients can be ignored.

Investigations on Heat Transfer Enhancement in Pool Boiling with Water-CuO Nano-Fluids

The main focus of the present work is to investigate Critical Heat Flux (CHF) enhancement using CuO nanofluid relative to CHF of pure water. To estimate the effect of nanoparticles on the CHF, pool boiling CHF values were measured for various volume concentrations of CuO nanofluid and compared with pure water. CHF enhancement of 130% was recorded at 0.2 % by volume of CuO nano-fluids. Surface roughness of the heater surface exposed to three measured heating cycles indicated surface modifications at different volume concentrations of nanofluid. SEM image of the heater surface revealed porous layer build up, which is thought to be the reason for CHF enhancement.

Dynamics of Discrete Bubble in Nucleate Pool Boiling on Thin Wires in Microgravity

A space experiment on bubble behavior and heat transfer in subcooled pool boiling phenomenon has been performedutilizing the temperature-controlled pool boiling (TCPB) device both in normal gravity in the laboratoryand in microgravity aboard the 22^(nd) Chinese recoverable satellite. The fluid is degassed R113 at 0.1 MPa andsubcooled by 26℃ nominally. A thin platinum wire of 60 μm in diameter and 30 mm in length is simultaneouslyused as heater and thermometer. Only the dynamics of the vapor bubbles, particularly the lateral motion and thedeparture of discrete vapor bubbles in nucleate pool boiling are reported and analyzed in the present paper. It'sfound that these distinct behaviors can be explained by the Marangoni convection in the liquid surrounding vaporbubbles. The origin of the Marangoni effect is also discussed.

Effect of Pore Size on the Nucleate Pool Boiling of Structured Enhanced Tubes

In this study, pool boiling test results are provided for the structured enhanced tubes having pores with connecting gaps. The surface geometly of the present tube is similar to that of Turbo-B. Three tubes with different pore size (0.20 mm, 0.23 mm and 0.27 mm) were manufactured and tested using R-11, R-123 and R-134a. The pore size which yields the maximum heat transfer coefficient varied depending on the refrigerant. For R-134a, the maximum heat transfer coefficient was obtained for the tube having 0.27 nun pore size. For R-11 and R- 123, the optimum pore size was 0.23 mm. One novel feature of the present tubes is that their boiling curves do not show a cross-over characteristic, which existing pored tubes do. The connecting gaps of the present tube are believed to serve an additional route for the liquid supply and delay the dry-out of the tunnel. The present tubes yield the heat transfer coefficients approximately equal to those of the existing pored enhanced tubes. At the heat flux 40 kW/m2 and saturation temperature 4.4° C, the heat transfer coefficients of the present tubes are 6.5 times larger for R-11, 6.0 times larger for R-123 and 5.0 times larger for R-134a than that of the smooth tube

Numerical Investigation on Pool Boiling Mechanism of Hybrid Structures with Metal Foam and Square Column by LBM

In the present study, pool boiling heat transfer performance and bubble behaviors of hybrid structures with metal foam and square column are investigated by lattice Boltzmann method. By using the vapor-liquid phase change model of Gong-Cheng and Peng-Robinson equation of state, the effects of structural parameters, including metal foam thickness, porosity, column height and ratio of column width(W) to gap spacing(D) are investigated in details. The results show that hybrid structure performs better than pure columnar structure in pool boiling heat transfer. The hybrid structure accelerates bubble growth by fluid disturbance while metal skeletons prevent the bubble escaping. The optimum ratio of column width to gap spacing decreases with the increase of heat flux and HTC(heat transfer coefficient) can achieve an increase up to 25% when W/D change from 5/3 to 1/3. The increase of column height enhances heat transfer by expanding surface area and providing space for bubble motion. The metal foam thickness and porosity have a little influence on pool boiling heat transfer performance, but they have an important effect on bubble motion in the regime.

Nucleate Pool Boiling of Pure Liquids and Binary Mixtures: Part II-Analytical Model for Boiling Heat Transfer of Binary Mixtures on Smooth Tubes and Comparison of Analytical Models for both Pure Liquids and Binary Mixtures with Experimental Data

A combined physical model of bubble growth is proposed along with a corresponding bubble growth model for binary mixtures on smooth tubes. Using the general model of Wang et al.[1] and the bubble growth model for binary mixtures, an analytical model for nucleate pool boiling heat transfer of binary mixtures on smooth tubes is developed.In addition, nucleate pool boiling heat transfer of pure liquids and binary mixtures on a horizontal smooth tube was studied experimentally. The pure liquids and binary mixtures included water,methanol, ethanol, and their binary mixtures. The analytical models for both pure liquids and binary mixtures are in good agreement with the experimental data.

High-temperature acoustic properties of porous titanium fiber metal materials

The high-temperature acoustic absorption performance of porous titanium fiber material was investigated in terms of sample thickness, porosity, temperature, air-cavity thickness and double-layer structure arrangement. The effects on absorption coefficient were systematically assessed. The results show that the sound absorption performance is improved by increasing the sample porosity and/or thickness, and/or increasing the air-cavity thickness. Meanwhile, increasing the temperature gives better acoustic absorption performance in the low frequency range but also lowers the performance in the high frequency range, while double-layer structure enables better acoustic absorption performance.

Nucleate pool boiling heat transfer measurements of HFC23/CFC13 system

Nucleate pool boiling heat transfer data of the HFC23/CFC13 system have been systematically measured in a wide range of pressures and heat fluxes. The experimental results are also compared with the measured data of R508A and R508B. It is found that the heat transfer coefficient of R503 （HFC23/CFC13=0.511/0.489） is higher than that of R508A and R508B. Furthermore, the measured data were compared with the predicted results with three well-known correlations. Correlation by Fujita and Tsutsui can provide acceptable results. Most of the data fall within ±20% of this correlation.