X.F. Peng

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.

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.

A SIMPLIFIED MODEL FOR MEASURING THERMAL PROPERTIES OF DEEP GROUND SOIL

A simplified heat transfer model is proposed for conveniently determining the thermal properties of deep ground soil at worksites and other associated engineering applications. The model does not need exact information of the U-tube location, spacing between up and down tubes, the properties of the tube and the backfill, and other parameters in the borehole as usual cases and greatly eliminates the errors caused by measuring the parameters mentioned. A practical site test was conducted to obtain heat flux imposed on the buried loop, water flow rate, and temperatures at the outlet and inlet of the loop by an in-situ test apparatus. The simplified model, together with optimal estimation algorithm, was used to determine the thermal conductivity of the deep ground soil in the worksite.

Effects of water vapor condensation on the convection heat transfer of wet flue gas in a vertical tube

The annular thin film condensation of water vapor in wet flue gas flowing through a vertical tube was studied theoretically and experimentally. Especially discussions were conducted on the effects induced by condensation of small amount of water vapor (10–20% fraction) on convection heat transfer in a vertical tube. The convection heat transfer was enhanced by the condensation of the condensable gas (water vapor) existing in the wet flue gas. The experiments also indicate that the wall temperature was an important factor affecting the condensing rate and the fog formation in the wet flue gas. The fog formation in the wet flue gas has a significant influence on the condensing rate and associated heat transfer. The proportion of the sensible and latent heat contribution to total heat transfer would change with the Re number. When the wall temperature is much lower than the saturation temperature of the inlet wet flue gas, the effect of the superheat could be neglected. But when the fog forms in the bulk flow, the temperature and concentration profile would be altered.

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...

Multi-Phase Flow and Drying Characteristics in a Semi-circular Impinging Stream Dryer

A physical model was proposed to describe granular material drying in a semi-circular impinging stream dryer, and the multi-phase flow characteristics as well as the heat and mass transfer were numerically investigated. Specially, the influence of various factors (inlet air temperature, mass flow-rate ratio, initial moisture content etc.) on drying process was inspected. The results indicate that constant drying rate period does not exist in a semi-circular impinging stream dryer. Appropriate curvature radius, flow-rate ratio, air velocity and higher inlet air temperature should be chosen for improving the drying performance, and decreasing the energy consumption and operation cost. The numerical predictions were compared with the available experimental results, and they are in quite good agreement with each other.