B.X. Wang

Boiling nucleation during liquid flow in microchannels

Abstract The boiling of liquids in microchannels/microstructures is currently of great interest due to its very unusual phenomena and its many potential applications in a wide variety of advanced technologies. The thermodynamic aspects of phase transformations of liquids in microchannels was analyzed to further understand the boiling characteristics and to determine the conditions under which a portion of such liquids is likely to undergo phase change. A nondimensional parameter and related criteria, that determine the phase transition in microchannels, were derived theoretically. The size of the microchannels results in dramatically high heat fluxes and superheats for liquid nucleation when the microchannel is sufficiently small. The effect that the liquid thermophysical properties have on the nucleation is also described by the analysis.

Effects of surface wettability on nucleate pool boiling heat transfer for surfactant solutions

Abstract Experiments for pool boiling of deionised water and acetone with different surfactant, 95% sodium dodecyl sulfate (SDS), Triton X-100 and octadecylamine, have been conducted under atmospheric pressure to investigate the effect of surface wettability. The boiling curves for different concentrations of surfactant solution on both smooth and roughened surfaces were obtained. The results show that the addition of surfactant can enhance the water boiling heat transfer, and the enhancement is more obvious for SDS solution; but has little influence on the acetone boiling curve. While the roughened surface enhanced the heat transfer for Triton X-100 solution, it also decreased the heat transfer coefficient for SDS solution. All these can be explained by including the changing of surface wettability, which has been neglected for a long time and should be an important parameter influencing boiling heat transfer. By incorporating such effects, the modified Mikic–Rohsenow pool boiling model, we proposed, can predict these experimental data well.

Numerical modeling and experimental investigation on the characteristics of molten pool during laser processing

The article illustrates melting processes in fixed meshes, based on mixture continuity equations, momentum equations and energy equations for both liquid and solid phases by introducing scalar of liquid fraction and applying source-based method. The model was applied to laser processing of AISI 304 stainless steel. The effects of heat conduction, Marangoni flow and thermal buoyancy on melting process and shape of molten pool were analyzed and physical fundament of laser processing were evidenced. Compared to the experimental outcome, the proposed model and numerical techniques are validated.

Nucleation site interaction during boiling

Abstract Classical analyses of boiling nucleation employ a linear approach with investigation focusing on a single site and the heat transfer obtained from the active site density assuming uniform wall superheat. This paper uses synergetic concepts and the self-organizing effect to analyze the interactions among active sites or bubbles induced by non-uniform temperature distribution, which depend mainly on boiling wall properties and thickness. Boiling experiments on the surface of different material plate walls with different thicknesses were conducted to reveal the nucleation site interaction. The results illustrate the experimental observations available in the literature and may provide a more reasonable mechanistic description of nucleation in boiling systems.

FLOW BOILING THROUGH V-SHAPE MICROCHANNELS

An experimental facility was set up to study the flow and boiling heat transfer characteristics of water and methanol flowing through V-shape microchannels. The microchannels have hydraulic diameters ranging from 0.2 to 0.6 mm and V-shape groove angles 0 of 30 to 60°. Both the heat transfer and the pressure drop were affected by the thermofluid and geometric parameters such as liquid flow velocity, subcooling, and the hydraulic diameter and groove angle of the microchannels. The experiments indicted that there exists both an optimum hydraulic diameter and an optimum groove angle. The visualization experiments showed that, if there was a good seal between the glass cover and the microchanneled test plate, no bubbles were observed in the microchannels for flow boiling with heat fluxes as high as of the order of 106 W/m2, at which fully nucleate boiling with a large number of bubbles would be expected in conventional situations. Fluctuating liquid flow was induced in the microchannels when many bubbles forme...

Experimental study on the dryout heat flux of falling liquid film

Abstract A combined analytical and experimental investigation was carried out for the critical heat flux leading to dryout of falling liquid films. A modified model for predicting the dryout heat flux was correlated with the recent experimental results, with an average relative deviation of 12.7%, which is significantly less than that of previously reported ones. This improvement is believed to be the result of including the capillarity-induced interfacial evaporation [B.X. Wang, J.T. Zhang, X.F. Peng, Effects of interfacial evaporation on flow and heat transfer of thin falling liquid film, Science in China Ser. E, 1999, submitted] and the modification of streamwise thermocapillarity. The interfacial heat flux is showed to be important for heat transfer and for predicting dryout heat flux of falling liquid films. The new model indicates that, the bulk outlet temperature of falling film is required for accurate prediction of the critical heat flux leading to dryout. The results, compared with the current experimental results, demonstrate that the new model provides better predicted results in general.

Effects of initial molten pool and Marangoni flow on solid melting

Abstract This study investigated the role of initial molten pool on the numerical simulation to melting process. The critical time for heat conduction mechanism to control the formation of initial molten pool is estimated first. A realistic formulation to the initial molten pool is then proposed. The linear stability characteristics of the initial molten pool when subject to Marangoni flow induced by the perturbations of surface liquid temperature were also analyzed. For initial pool having a melting time less than the critical time, the subsequent numerical results are independent of the assumed initial pool. An arbitrarily assumed initial pool could yield erroneous numerical results for melting. Detailed numerical investigations on the shape evolutions of molten pool during initial stage and the long-term stage were conducted. Effects of the heat transfer intensity and the Marangoni number on the melting process were also investigated.

Falling liquid film thickness measurement by an optical-electronic method

An optical-electronic method was developed for measurement of falling liquid film thicknesses. The method is based on the postulate that a sheet light beam passing tangentially through a vertical liquid film on a cylinder will be blocked by the falling liquid film. Hence, when the beam is much wider than the film, the output of a photodiode probe which is located on the opposite side of the cylinder from the light source will be reduced by an amount proportional to the film thickness. The shadow of the transient film shape will also appear on a screen behind the falling film. Therefore, the variation of the amplitude of the film waves can be measured from the output of the photodiode and the average film thickness can be measured from pictures of the shadow using computer aided image analysis. The average film thickness measured agrees well with commonly accepted empirical equations. The maximum relative deviation between the experimental and calculated results is 18.5% with a standard deviation of 4.34×1...

Effect of capillarity at liquid–vapor interface on phase change without surfactant

Abstract Thermodynamic analysis has been employed in this paper to study the effect of capillarity on the interfacial phase change of liquid without surfactant, and a modified expression of phase change rate of liquid without surfactant has been developed. The new expression demonstrates that the capillarity of the concave liquid surface will promote the interfacial vaporization if sufficient heat is exerted on liquid, and the capillarity of the convex liquid surface will be helpful for increasing the condensation rate from vapor to liquid. The analyses of this paper showed that the newly developed expression is in accordance with practical observations reported in the literature.

Study on the interfacial evaporation of aqueous solution of SDS surfactant self-assembly monolayer

Abstract Researches in chemical, biological and medical engineering consider that the self-assembly monolayer formed by SDS surfactants being to retard interfacial evaporation of aqueous solution. In heat transfer, the application of SDS surfactant in water is considered to enhance the boiling heat transfer. However, to our knowledge, the mechanism of interfacial evaporation in boiling heat transfer with the application of SDS surfactants has still not been clarified. The authors tried to give a new insight into this mechanism of the interfacial evaporation through the self-assembly monolayer of SDS surfactants by developing a model, which indicates that the accessible area and the hydrophobic interaction retard interfacial evaporation, while the decrease in the incipient temperature of phase change by SDS surfactants promotes interfacial evaporation. Thus, the effect of SDS surfactants in aqueous solution on interfacial evaporation is dual: as the concentration of SDS surfactant additives is lower than an optimal value and the interfacial superheat is not extremely high, the application of SDS surfactant promotes the interfacial evaporation; otherwise, retards the interfacial evaporation.