SIMULTANEOUS MEASUREMENT OF HEAT FLUX AND DROPLET POPULATION DURING DROPWISE CONDENSATION FROM HUMID AIR FLOWING ON A VERTICAL SURFACE

A new experimental apparatus for the investigation of dropwise condensation from humid air flowing over a vertical aluminum surface at controlled velocity is presented. Differently from other works on this subject, here we measure simultaneously, during condensation of flowing moist air, the total heat flux (by a heat flux sensor), the latent heat flux (by weighing the mass of condensate), the small and the large droplet population. Experimental tests were performed on two aluminum specimens that display different wettability: a mirror-polished surface (56° advancing contact angle, 46° contact angle hysteresis) considered as a baseline, and a coated surface functionalized by using a sol–gel coating (87° advancing contact angle, 15° contact angle hysteresis). The effects of the wall subcooling, moisture content, relative humidity and air velocity on the heat and mass transfer during dropwise condensation (DWC) are investigated. Enhanced condensation is observed on the coated surface but, with the increase of relative humidity and wall subcooling, the advantage of using the coated surface diminishes. Time-lapse videos of the condensation process, featuring droplets detection down to three microns, are used to investigate droplet population (both large and small droplets) and nucleation sites density. Usually, DWC models assume tentative values of the nucleation site density and tentative trends of the droplet population. In this paper we measure and discuss such parameters. The determination of the nucleation site density is crucial because it affects the droplet interactions and the drop size distribution, determining the overall heat transfer. The measurement of the nucleation site density is currently rare in the literature, especially during condensation of flowing humid air. Here the nucleation site density is determined with a double approach and it is found to vary between 3.3 × 108 sites/m2 and 6.1 × 108 sites/m2 in the investigated range of experimental conditions. The experimental droplet population is also compared against the predicted one, finding some disagreement which should be properly addressed for the development of improved DWC models.