2.2. Surface evaporation can only take place from superheated liquid in a surface layer. The evaporation
mechanism, whereby the heat is absorbed by the latent heat of liquid molecules leaving the surface, is contained in a
surface layer a few mm, only, in thickness. The heat energy is transmitted by micro-convection cells and then by
thermal conduction across a 1 – ȝPWKLFNODHUWRDWKLQVXUIDFHODHUFRQWUROOLQJWKHIOX[RIPROHFXOHVHYDSRUDWLQJ
from liquid to vapour phase. The details of this mechanism are not fully understood because the molecular
evaporation flux is not uniform spatially across the surface, and varies rapidly with time with observed variations in
magnitude up to 10% of the average boil-off rate.
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