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The results of experimental study of heat transfer and crisis phenomena at pool boiling of liquid nitrogen with different heating conditions on the surface with capillary-porous coating are presented. Porous coatings with different thicknesses (400 and 1390 μm), morphology and high porosity (up to 80%) were obtained using the new plasma spraying technique. It was shown that at steady-state heat release the heat transfer at boiling essentially depends on the thickness and morphology of the coating. The maximum enhancement (∼300%) compared to the smooth heater was detected to coated heater with a thickness of 1390 μm at low heat fluxes. The mechanism of heat transfer enhancement at pool boiling by using the capillary-porous coatings was proposed. Heat transfer hysteresis was detected for the heater with the coating of 400 μm. For the smooth heaters and the heater with 1390 μm coating, the heat transfer coefficients almost coincide with an increase in the heat flux and with its decrease. Data on the effect of coatings with different thicknesses on the critical heat flux (CHF) at boiling under steady-state heat release are presented. It is shown that for the smooth heaters the value of critical heat flux at rapid heating decreases in comparison with steady-state heat release. Capillary-porous coatings have a significant influence on development of the transition process and crisis phenomena at stepwise heat release. There is degeneration of boiling crisis development at rapid heating on the coated surfaces at the heat fluxes below the value of the CHF at steady state heat release. Fast transition to film boiling at stepwise heat release on the coated heaters with different thicknesses is observed at the heat fluxes 2 times higher than the critical heat fluxes, obtained at steady state heating. [ABSTRACT FROM AUTHOR]

In this study, a working coil coupled with magnetic flux concentrators for barrel induction heating in an injection molding machine was investigated. Induction heating process simulations were developed to evaluate the effects of magnetic flux concentrators to the temperature distribution at surface inside a barrel of an injection molding machine. Different pitches of magnetic flux concentrators were applied to study the uniform heating capability of induction heating system with heating coil coupled with magnetic flux concentrators via commercial software, ANSYS. Models of working coils coupled with magnetic flux concentrators were experimented to verify the simulation results. The results from simulation and experiment agreed well. Simulation results showed that, changing diameters of a barrel or varying operation frequency of induction power supply had no effect on uniform temperature distribution at the inside surface of the barrel. [ABSTRACT FROM AUTHOR]

Abstract: The dynamics of boiling succeeding spontaneous nucleation on a small film heater immersed in ethyl alcohol are investigated at heating rates ranging from 107 K/s to approximately 109 K/s, under which spontaneous nucleation is dominant for the inception of boiling. Immediately after the concurrent generation of a large number of fine bubbles, a vapor film that covers the entire surface is formed by coalescence and rapidly expands to a single bubble. As the heating rate is increased, the coalesced bubble flattens and only a thin vapor film grows before cavitation collapse. Similar behaviors are also observed for water. Based on the observed results, a theoretical model of the dynamic bubble growth due to the self-evaporation of the superheated liquid layer, which develops before boiling incipience, is presented. The calculated results are compared with the observed results. [Copyright &y& Elsevier]

Abstract: Rapid heating of a liquid by a thin film heater surface may initiate homogeneous nucleation of vapor in the liquid in contact with the surface. In such circumstances, nucleation is generally expected to be most likely in the hottest liquid closest to the surface. It is known, however, that in many cases the liquid molecules closest to the surface will experience long-range attractive forces to molecules in the solid, with the result that the equation of state for the liquid near the surface will differ from that for the bulk liquid. In the investigation summarized here, a statistical thermodynamics analysis was used to derive a modified version of the Redlich–Kwong fluid property model that accounts for attractive forces between the solid surface molecules and liquid molecules in the near-wall region. In this model, the wall–fluid attractive forces are quantified in terms of Hamaker constants. This feature makes it possible to assess the effect of wall–fluid force interactions on the spinodal conditions for a variety of fluid and surface material combinations. The variation of pressure near the wall predicted by this model agrees well with the predictions of a hydrostatic model and molecular dynamics simulations. The results of the thermodynamic model analysis indicate that force interactions are important for a wide variety of fluids only within a few nanometers of the solid surface. The model specifically predicts that these forces increase the spinodal temperature in the near-surface region. The implications of this increase for nucleation near a solid surface during rapid heating are explored for circumstances similar to those in inkjet printer heads. The results of the analysis imply that during rapid transient heating, wall effects can result in homogeneous nucleation occurring first at a location slightly away from the solid surface. The results further suggest that on rough surfaces, wall effects may play a role in making some cavities preferred sites for homogeneous nucleation. [Copyright &y& Elsevier]