10th International Conference on Boiling and Condensation Heat Transfer 12th-15th March 2018 in Nagasaki, Japan www.icbcht2018.org DROPLET SIZE DISTRIBUTIONS ON VERTICAL AND HORIZONTAL SUPERHYDROPHOBIC SURFACES DURING JUMPING-DROPLET CONDENSATION Patrick Birbarah 1 , Chengpu Li 1 and Nenad Miljkovic 1,2,* 1 Department of Mechanical Science and Engineering, University of Illinois, Urbana, Illinois 61801, USA 2 International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Moto- oka, Nishi-ku, Fukuoka 819-0395, Japan ABSTRACT Water vapor condensation governs the efficiency of a number of important industrial processes. Jumping-droplet condensation of water has been shown to have a 10X heat transfer enhancement compared to state-of-the-art filmwise condensation due to the removal of condensate at much smaller length scales (~1 μm) than what is capable with gravitational shedding (~1 mm). In order to model heat transfer performance during jumping-droplet condensation, individual droplet heat transfer models and droplet size distributions are needed. Although heat transfer through a condensate droplet is relatively well understood, jumping-droplet size distributions are lacking. In this study, we develop a full numerical simulation of jumping droplet condensation on vertically and horizontally oriented superhydrophobic surfaces. We start by simulating hydrophobic surfaces with a contact angle of 95° in order to compare our results with the well-known distribution for hemispherical droplets undergoing dropwise condensation [1]. Figure 1 shows time lapse images of the simulated dropwise condensation on a hydrophobic vertical surface with a random nucleation site density of 10 4 sites/cm 2 . Figure 1 : Droplet distribution on a vertical surface (1cm x 1cm) with a contact angle of 95°. The simulation screenshots are taken (a) 20 ms, (b) 80 ms, (c) 100 ms and (d) 500 ms after the onset of condensation (heterogeneous nucleation). Droplets nucleate in a spatially random fashion with a nucleation site density of 10 4 sites/cm 2 . Droplets are considered to nucleate with a nucleation radius of 10 nm and shed from the surface once they reach radii of 500 μm. In order to model the individual droplet growth, we utilize the a recent droplet grow model [2], with a surface temperature of 15°C and water saturation temperature of 24°C, neglecting the effects of roughness features for hydrophobic surfaces. The correlations for the droplet growth rate were fitted in a form of d/d = / where is the droplet radius, is time, and = 0.015 and = 0.3 are the fitted parameters. The initial nucleation radius was assumed to be 10 nm in accordance with heterogeneous nucleation theory on hydrophobic substrates [3]. The droplets are assumed to fall due to gravitational force after reaching a radius of 0.5 mm. The growth and coalescence are achieved sequentially through the algorithm, with the falling droplets merging with stationary