The growth of a vapor bubble in a superheated liquid is controlled by three factors: the inertia of the liquid, the surface tension, and the vapor pressure. As the bubble grows, evaporation takes place at the bubble boundary, and the temperature and vapor pressure in the bubble are thereby decreased. The heat inflow requirement of evaporation, however, depends on the rate of bubble growth, so that the dynamic problem is linked with a heat diffusion problem. Since the heat diffusion problem has been solved, a quantitative formulation of the dynamic problem can be given. A solution for the radius of the vapor bubble as a function of time is obtained which is valid for sufficiently large radius. This asymptotic solution covers the range of physical interest since the radius at which it becomes valid is near the lower limit of experimental observation. It shows the strong effect of heat diffusion on the rate of bubble growth. Comparison of the predicted radius‐time behavior is made with experimental observations in superheated water, and very good agreement is found.

The growth of vapor bubbles in superheated liquids. report no. 26-6

A variety of industrial applications such as power generation, water distillation, and high-density cooling rely on heat transfer processes involving boiling. Enhancements to the boiling process can improve the energy efficiency and performance across multiple industries. Highly wetting textured surfaces have shown promise in boiling applications since capillary wicking increases the maximum heat flux that can be dissipated. Conversely, highly nonwetting textured (superhydrophobic) surfaces have been largely dismissed for these applications as they have been shown to promote formation of an insulating vapor film that greatly diminishes heat transfer efficiency. The current Letter shows that boiling from a superhydrophobic surface in an initial Wenzel state, in which the surface texture is infiltrated with liquid, results in remarkably low surface superheat with nucleate boiling sustained up to a critical heat flux typical of hydrophilic wetting surfaces, and thus upends this conventional wisdom. Two distinct boiling behaviors are demonstrated on both micro- and nanostructured superhydrophobic surfaces based on the initial wetting state. For an initial surface condition in which vapor occupies the interstices of the surface texture (Cassie-Baxter state), premature film boiling occurs, as has been commonly observed in the literature. However, if the surface texture is infiltrated with liquid (Wenzel state) prior to boiling, drastically improved thermal performance is observed; in this wetting state, the three-phase contact line is pinned during vapor bubble growth, which prevents the development of a vapor film over the surface and maintains efficient nucleate boiling behavior.

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Enabling Highly Effective Boiling from Superhydrophobic Surfaces.

Effect of diameter of metal nanowires on pool boiling heat transfer with FC-72

Pool boiling heat transfer enhancement with electrowetting

Simulations of saturated boiling heat transfer on bio-inspired two-phase heat sinks by a phase-change lattice Boltzmann method

This study focuses on the detailed description of the heat transfer and bubble dynamics processes occurring for the boiling of water on surfaces with extreme wetting regimes, namely hydrophilicity and superhydrophobicity. The wettability is changed by modifying the surface chemistry and without significant variations in the mean surface roughness. Under these conditions and for the range studied here the effect of the extreme wetting regimes was dominant, thus the influence of surface topography was not addressed. A particular trend is observed for the boiling curve obtained with the superhydrophobic surfaces, as the heat flux increases almost linearly with the superheat, although with a much lower slope than the hydrophilic surfaces. This occurs due to the formation of a large stable vapour film over the entire surface just at around 1 K superheat, because of the almost immediate coalescence of the bubbles generated on the surface. This behaviour agrees with the so-called “quasi-Leidenfrost” regime recently reported in the literature and with a theoretical prediction of the heat flux that is presented in this study. Furthermore, a comprehensive analysis of bubble dynamics, useful for comparison with numerical simulations is given. Such analysis is based on the temporal evolution of the bubble diameter together with bubble contact angle and with the velocity of the contact line. The results suggest that the existing models and correlations can predict the trends of the bubble growth using a modified contact angle value, called the bubble contact angle (or its supplemental value), for the hydrophilic surfaces, even if they cannot accurately predict bubble sizes. Approximating the modified contact angle with the quasi-static contact angle, obtained during surface characterization is practical for a qualitative evaluation, but the results obtained here do not support for its use when estimating the bubble departure diameter. On superhydrophobic surfaces, the effect of the vapour film must be considered, since although this is not the starting point of the boiling process, it represents the actual working conditions when using this kind of surfaces.

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Patrícia Valente

Papers

Effect of extreme wetting scenarios on pool boiling