Two-phase annular flow deposition was studied. Experiments were performed to determine where small water droplets in an air stream in a round tube would reach the wall. Results indicated that, for fully accelerated droplets whose diameters were within a factor of two of 150 microns, the number reaching the wall was characteristic of exponential decay with distance downstream. Further, the mean free path to the wall, measured axially, could be taken proportional to droplet diameter. With the assumption that dispersed liquid flow rate in annular two-phase flow regimes consists chiefly of droplets traveling at or near gas velocity, and with arbitrary choices of droplet, diameter spectrum and magnitude of entrainment rate, it was possible to derive analytical expressions for mass transfer coefficient, deposition flow rate, dispersed liquid flow rate, mean diameter and spectrum as they all changed downstream. Some experimental measurements by others were successfully reproduced by these expressions. An important result was that droplet size spectrum “hardening” (preferential depletion of small sizes) operates to decrease the deposition rate downstream, especially if there is no longer any entrainment.

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