Evaporating Liquid Films with Nucleate Boiling

Evaporating Liquid Films With Nucleate Boiling... 

The heat transfer mechanisms that are active in boiling in micro-channels can be summarized as follows (i) in bubbly flow, nucleate boiling and liquid convection would appear to be dominant, (ii) in slug flow, the thin film evaporation of the liquid film trapped between the bubble and the wall and convection to the liquid and vapor slugs between two successive bubbles are the most important heat transfer mechanisms, also in terms of their relative residence times, (iii) in annular flow, laminar or turbulent convective evaporation across the liquid film should be dominant, and (iv) in mist flow, vapor phase heat transfer with droplet impingement will be the primary mode of heat transfer. For those interested, a large number of two-phase videos for micro-channel flows from numerous laboratories can be seen in the e-book of Thome [22]. 

Experiments show that a drop of liquid evaporates rapidly when placed on a horizontal surface heated to a few degrees above the saturation temperature the evaporation is slow, however, if the surface is well above saturation. In the first case the liquid can wet the surface, creating nucleate boiling with bubble formation, liquid film evaporation, and high rates of heat transfer. In the second case the drop remains suspended on a poorly conducting vapor film, which prevents direct contact between the liquid and the hot surface. The latter is the Leidenfrost problem of film boiling and is our concern here. 

Wall superheat reductions up to a factor of 10 have been reported with some of these surfaces. It should be noted that the mechanism of vaporization for these surfaces is different from that for normal cavity boiling. Here the liquid flows via selected paths or channels to the interior, where thin-film evaporation occurs over a large surface area the vapor is then ejected through other paths by bubbling [38,39]. It should be emphasized that the performance of these special surfaces is quite sensitive to surface geometry and fluid condition. Additionally, very low temperature differences are involved hence, it is necessary to be especially careful, or at least consistent, in measuring wall temperatures and saturation temperatures (pressures). The first comprehensive comparison of the nucleate boiling performances... 

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