Investigation on drop-size density distribution during dropwise condensation

Dropwise condensation (DWC) is a very effective heat transfer process since it can realize heat transfer coefficients one order of magnitude higher compared to filmwise condensation mode of saturated steam. Fluid properties, surface wettability, gravity and vapour shear stress play a crucial role in this type of phase-change phenomenon. The prediction of the dropwise condensation heat transfer coefficient is not an easy task, due to the different mechanisms involved such as droplet nucleation, coalescence and shedding. Researchers proposed different methods for DWC modelling and the first semi-empirical model was presented in 1966 by Le Fevre and Rose (3rd Int. Heat Transfer Conference). 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. Considering the DWC models recently proposed, even if improved methods for the determination of the heat transfer through a single drop have been introduced, the drop-size density distribution by Le Fevre and Rose is still adopted. Depending on droplet size and growth mechanism, the droplet population can be divided in small droplet and large droplet population. The aim of the present work is to investigate the large drop-size density distribution during saturated steam condensation considering different surface treatments and varying the heat flux. DWC takes place over a vertical metallic surface and a thermosyphon loop is used to provide the condensing vapour. To be able to detect droplets down to a diameter of few microns, a new torusshaped LED has been designed and built at the Department of Industrial Engineering of the University of Padova. 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. More than six million of droplets have been detected and an excellent agreement with the Le Fevre and Rose theory has been found.