Surface and gas-phase resistances to the evaporation of droplets.

Attempt has been made to asses quantitatively the magnitudes of both the gas-phase resistance and the surface resistance to the evaporation of droplets and to determine the transition point at which the evaporation changes from a diffusion-controlled to a surface-controlled process. This transition point is strongly dependent upon the ambient pressure and the droplet size. It is found that at atmospheric pressure the particle radius at which the transition takes place is in the range of 1 - 10mu for water, 0.1mu for n-dibutylphthalate, 0.1 - 1.0mu for mercury, and 0.001mu for liquid helium. Furthermore, we have found that the surface resistance and the gas-phase resistance vary with the size of the droplet in opposite direction, leading to the existence of a maximum rate of evaporation at a certain value of the droplet radius, this radius being dependent upon the characteristics of the liquid concerned. This conclusion may be useful in the search for an optimum and most efficient method of combustion of fuel sprays in automotive engines, gas turbines, and oil-burning power plants.