[en] An examination of the behaviour of an initially subcooled water drop placed on a smooth quartz surface at 700 ⁰C led to the following observation. The water boiled for about 100 ms, then nucleation ceased and the drop became static. The rate of heat transfer from the water surface therefore decreased and the water consequently acquired a considerable superheat, temperatures of up to 200 ⁰C being measured at the wetted surface. When a further 100 ms nucleation recommenced the drop departed from the surface in an explosive manner. Evidence is provided that propagation of this explosive nucleation involves the impact on the water surface of droplets created during the rupture of an initiating vapour bubble. It is argued that this observation lends support to the hypothesis of secondary nucleation for nucleate boiling of highly superheated liquids. (author)

[en] Thermal bubble formation is a fundamental process in nucleate boiling heat transfer and in many microelectromechanical thermal systems. Here, we report an experimental study of the dynamic and thermal behavior of bubbles generated at a single site, that is, a microcavity filled with alumina particles. The thermal process associated with the bubble departure from the isolated cavity, in particular, was shown to be different from that of macroscale boiling. The bubble departure diameter remains constant in a low superheat (or Jakob number) regime which is solely determined by the balance of interfacial tension and buoyancy. In addition, the bubble departure frequency increases along the bubble size as the substrate temperature rises. The further-increased frequency of bubbles emerging from the cavity causes multiple bubbles to coalesce before the preceding bubble completely detaches from the substrate, thus, leading to the decrease of apparent departure frequency with the increase of substrate temperature

[en] The mechanisms of nucleation have been analysed. Starting from the assumption that the activation of micro-cavities in the wall surfaces is the most probable nucleation mechanism in practical flashing system, the authors study in detail the nucleation in a micro-cavity. A three step nucleation criterion is proposed, namely: trapping cavity, activable cavity and active cavity. Then, a new nucleation model is presented. The output of the model is the prediction of the bubble departure frequency versus the thermodynamic state of the liquid and the geometry of the cavity. The model can also predict the nucleation site density if the nature of the wall and the surface roughness are know. The prediction have been successfully compared with some preliminary experimental results. By combining the present model with Jones'theory, the flashing inception is correctly predicted. The use of this nucleation model for the complete modelling of a flashing non-equilibrium flow is in progress

No abstract available

[en] It is shown how to apply homogeneous nucleation theory to predict the limits of superheat of multicomponent liquid mixtures. Measurements of such limits for n-pentane-n-hexane, n-hexane-cyclohexane, benzene-cyclohexane, and benzene-n-hexane mixtures as a function of composition are presented and compared with theory

[en] The boiling occurrence plays an important role in the power reactors energy transfer. But still, there is not a final theory on the boiling mechanisms. This paper presents a critical analysis of the most important nucleated boiling models that appear in literature. The conflicting points are identified and experiments are proposed to clear them up. Some of these experiments have been performed at the Thermohydraulics laboratory (Bariloche Atomic Center). (Author)

[en] A number of measurements of the conditions required to nucleate boiling and cavitation in stagnant water are described, and used to test theories of nucleation by surface cavities. For given conditions prior to nucleation the concept of a constant cavity radius appears valid. Two different responses to deactivation of cavities by applying pressure beforehand are seen, neither as pronounced as the simply theory predicts. (author)

[en] The current stage of nucleation theory is based on the vapor trapped theory, which states that micro scale cavities or defects on a commercial heating surface trapped vapor and then act as a nucleus. The criteria for trapping vapor on a heating surface was suggested by Bankoff. For trapped vapor, a heating surface should have micron scale surface cavities. Based on that, previous researchers predicted the nucleation sites of the trapped vapors in boiling. Recently, however, bubble nucleation on a smooth heating surface free from trapped vapor was reported by several researchers. Benjamin and Balakrishnan observed the activated nucleation site densities on heating surfaces, even if when the roughness of the heating surface is less than a certain nanoscale. Theofanous conducted pool boiling experiment on a heating surface with nanoscale roughness. And they suggested the possibility of bubble nucleation on a nanoscale roughness surface. These results are not explained by the vapor trapped theory. In this study, we conducted experimental approach for the bubble nucleation on a smooth heating surface free from trapped vapor and discussed how bubble could be nucleated on a smooth surface. To understand bubble nucleation on a smooth heating surface free from trapped vapors, ONB experiments were conducted with different wetting surfaces. In this study, it is confirmed that the bubble nucleation on a smooth surface occurs, although the heating surface does not trap any vapors on micro-cavities or defects. Furthermore, ONB depended on the contact angle of the heating surface; higher contact angle surface required lower superheat for nucleation. We discussed the bubble nucleation on a smooth heating surface and the dependency of ONB on surface wettability with the potential gradient nucleation model

[en] The effect of subcooling on bubble waiting time and growth time for water boilng on a copper surface was examined in conjunction with measurements obtained over a range of subcooling from 0 to 15⁰C and three different levels of heat flux 166, 228, and 291 kW/m². The growth-time data was successfully correlated with a model that combined the bubble growth theory of Mikic Rohsenow, and Griffith with the bubble departure diameter relationship of Staniszewski, thereby establishing confidence in the measuring procedure. The waiting time data agreed with the predictions of the Han and Griffith waiting time theory at lower levels of subcooling but then showed a behavior contrary to that predicted for higher levels of subcooling

[en] This paper presents experimental results for saturated nucleated boiling of n-Pentane on a heating surface facing upward, at atmospheric pressure, for different degrees of confinement, s = 0.2, 0.5, 0.7 and s = 13 mm, corresponding to Bond numbers 0.13, 0.32, 0.45 and 8.35. Comparative studies with results from literature, together with analysis of the experimental data allowing the improvement of the experimental apparatus. The results show the enhancement of boiling heat transfer with a decreasing distance s between the heating surface and an unheated surface. The experimental heat transfer coefficients for unconfined boiling, s=13 mm, are compared with three empirical correlations. (author)