On the Self Diffusion of Water Vapour

Calculated values of the coefficient of self diffusion of steam, based on a theoretical model and covering the pressure range 0.07–27 bar, are presented. By considering the equilibrium vapour pressure surrounding a large droplet placed in its own vapour, its temperature, and hence the temperature field, have been estimated. From the heat transfer rate, the rate of mass transfer and consequently the coefficient of self diffusion have been deduced. The calculations are repeated over a large range of conditions and the results correlated.

Application to the nett mass transfer rate is next considered. By drawing comparisons with the case of a droplet in the free molecule regime and other transport processes, an expression covering the whole Knudsen number range is developed.