The wetted area fraction of dropwise condensation with inert gases and drop drainage

The main two differences between filmwise and dropwise condensation, viz. condensatetopology and heat transfer performance, and their relationship are investigated. A newmethod to determine the wetted area fraction is presented. It is based on local, instantaneoustemperatures measured with infrared thermography, and on validated averagingprocedures. With two independent methods, the wetted area fraction of a polymer platecondenser has been measured for gas mixtures of moist air with various concentrations ofethanol. A fraction of about 55% is found, with no systematic dependence on any inletparameter if the inlet mass fraction of steam exceeds 0.1. The drainage of the condensateoff the cooled plates, by gravity and shear, hardly or not affects the wetted area fraction.This result is interpreted with the aid of measurements of drop size distributions.