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.

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

Theoretical Aspects of Nucleate Pool Boiling with Dielectric Liquids

Direct cooling with inert,dielectric liquids may well become the technique of choice for the thermal manage- ment of future electronic systems.Due to the efficiency of phase-change processes and the simplicity of natural circulation,nucleate pool boiling is of great interest for this application.This paper examines the characteristics of vapor bubbles and nucleate pool boiling of the dielectric liquids.The results provide a theoretical foundation for understanding and interpreting the often complex empirical results reported in the literature.

An Improved Treatment on the Apparent Contact Angle of a Single-Bubble in Consideration of Microlayer for Simulations of Nucleate Pool Boiling

Numerical simulation of single-bubble growth behavior during nucleate pool boiling was developed based on the volume of fluid method considering the thin liquid layer under the bubble(microlayer).However,the experimental values of apparent contact angle(the small region connecting the microlayer and bulk liquid)are crucial for the simulations.Reliance on experimental results limited the further application of such numerical method.In this study,a new method calculating the force balance,used to determine the interface shape near the apparent contact angle,was proposed instead of using the experimental values of the apparent contact angle.As a result,the good agreement was shown between the simulation results obtained based on the new and previous numerical methods.The simulation results were also in consistent with the experimental results.It can be concluded that the single-bubble behavior,including the heat transfer characteristics,during nucleate pool boiling can be simulated based on the proposed method.

Lattice Boltzmann Simulation of Nucleate Pool Boiling in Saturated Liquid

A thermal lattice Boltzmann method(LBM)for two-phase fluid flows in nucleate pool boiling process is proposed.In the present method,a new function for heat transfer is introduced to the isothermal LBM for two-phase immiscible fluids with large density differences.The calculated temperature is substituted into the pressure tensor,which is used for the calculation of an order parameter representing two phases so that bubbles can be formed by nucleate boiling.By using this method,twodimensional simulations of nucleate pool boiling by a heat source on a solid wall are carried out with the boundary condition for a constant heat flux.The flow characteristics and temperature distribution in the nucleate pool boiling process are obtained.It is seen that a bubble nucleation is formed at first and then the bubble grows and leaves the wall,finally going up with deformation by the buoyant effect.In addition,the effects of the gravity and the surface wettability on the bubble diameter at departure are numerically investigated.The calculated results are in qualitative agreement with other theoretical predictions with available experimental data.

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.

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.

Nucleate Pool Boiling of Pure Liquids and Binary Mixtures：PartI－Analytical Model for Boiling Heat Transfer of Pure Liquids on Smooth Tubes

A mechanism is proposed for nucleate pool boiling heat transfer along with a general model for both pure liquids and binary mixtures. A combined physical model of bubble growth is also proposed along with a corresponding bubble growth model for pure liquids on smooth tubes. Using the general model and the bubble growth model for pure liquids, an analytical model for nucleate pool boiling heat transfer of pure liquids on smooth tubes is developed.

Investigation of Enhanced Boiling Heat Transfer from Porous Surfaces

Experimental investigations of boiling heat transfer from porous suffaces at atmospheric pressure were performed. The porous surfaces are plain tubes covered with metal screens, V-shaped groove tubes covered with screens, plain tubes sintered with screens, and V-shaped groove tubes sintered with screens.The experimental results show that siatering metal screens around spiral V-shaped groove tubes can greatly improve the boiling heat transfer. The boiling hysteresis was observed in the experiment. This paper discusses the mechanism of the boiling heat transfer horn those kinds of porous surfaces stated above.