Experimental analysis of drop-size density distribution during dropwise condensation of steam

Dropwise condensation (DWC) has attracted research interest since its first description in 1930 due to the high heat transfer coefficients that can be obtained by promoting this mechanism as compared to the case of filmwise condensation. The modelling of dropwise condensation heat transfer is complex since it must account for droplets nucleation, growth and removal: researchers proposed different methods and the first semi-empirical model was presented in 1966 by Le Fevre and Rose. Their model predicts the total heat transfer across a surface during DWC by estimating the drop-size density distribution and the heat transfer through a single droplet. The goal of the present work is to investigate the large drop-size density distribution during dropwise condensation considering different operating conditions (heat flux and surface coating). In this work, a new illumination system (torus-shaped LED) has been designed and built to allow droplets detection with diameters from millimetres down to few microns. The torus-shaped LED is used as light source for a high-speed camera coupled with microscope lens. The recorded images have been analysed by a homemade software in MATLAB code, and the measured droplet population has been compared with the theoretical formula proposed by Le Fevre and Rose [1]. More than six million droplets have been detected and an excellent agreement with the Le Fevre and Rose theory has been found.