When a drop breaks free from a liquid film or feeding orifice and falls through an atmosphere of lower temperature it experiences a transient heat and mass transfer process involving acceleration, the development of hydrodynamic, thermal, and concentration boundary layers in the gas, oscillation of the drop shape, and the development of internal circulation within the drop. This problem, which is of importance in evaporative cooling systems, has been studied experimentally for water drops 3–6 mm in diameter falling through air. Study of a simplified set of governing equations indicates that similitude does not exist in this problem. However, it has been found that for this size range an approximate procedure based on the assumption of negligible internal thermal resistance and an empirical transient correction factor applied to the Ranz-Marshall [1] correlation could describe the data very well.

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