The moment method proposed by Lees (1959) is applied to the problem of vapour condensation on and evaporation from spherical liquid droplets when the droplet is not in equilibrium with its surrounding. Using a four-moment solution, an analytical expression is derived for the mass flux to or from the droplet surface when the droplet is surrounded by a pure vapour. By neglecting changes in temperature, an analytical solution is also obtained for the mass flux when the droplet is immersed in a vapour-gas mixture. The results of both of these analyses are applicable in the range from λ/R → 0 to λ/R ∞, where λ is the mean free path and R the droplet radius, and it is shown that in the limits the results reduce to the appropriate free molecule and continuum expressions.
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