Dropwise condensation mechanisms when varying vapor velocity

The promotion of dropwise condensation (DWC) has been identified as an effective strategy to significantly improve the heat transfer coefficient (HTC) as compared to filmwise condensation (FWC). Understanding the mechanisms governing dropwise condensation on modified wettability surfaces is crucial for a wide range of energy applications. In the literature, most of the experimental data are collected during DWC with quiescent vapor. On the other hand, in industrial applications, the vapor to be condensed can have a non-negligible velocity which is expected to affect the droplet population on the condensing surface and the heat transfer. However, measurements of heat transfer coefficient and droplet population with flowing steam are rare and the effect of vapor velocity on the drop-size distribution, which is a key parameter in DWC modeling, needs to be investigated. In the present work, the effect of steam velocity during DWC is experimentally studied on two different specimens: a sol–gel coated aluminum sample and a reduced graphene oxide coated copper sample. HTC, droplet departing radius and drop-size distribution measurements are performed at constant saturation temperature and heat flux, while varying the inlet vapor velocity in the range between 3 and 15.5 m s-1. Due to the increase of the vapor drag force on the droplets, a reduction of the droplet departing radius is observed along with an increase of the condensation HTC. The vapor flow is found to affect the droplet population and, in particular for the largest droplets radii, the drop-size distribution function is lowered when increasing vapor velocity. The experimental data are used to assess a model for the estimation of the droplet departing radius in presence of vapor velocity previously proposed by the present authors. As a second step, the equation for the calculation of the droplet departing radius is coupled with available models for droplet population and heat transfer through a single droplet to model the whole DWC process. The proposed calculation method is able to predict the effect of vapor velocity on the DWC heat transfer coefficient.