Effect of Surface Renewal on the Drop Size Distribution in Dropwise Condensation within a Hybrid Surface

Abstract

The description of liquid drop growth and drop distribution are two key models in evaluating the thermal performance of dropwise condensation (DWC) heat transfer. The drop size distribution describes the growth process of small drops by direct condensation and large drops by coalescence. The present work investigates the effect of surface renewal and coalescence intensities of DWC within a hybrid surface. Additionally, it examines the validity of the current empirical expression of the drop size distribution that is developed for DWC without considering surface renewal and coalescence intensities. The simulation work illustrates the drop growth process and surface renewal as drops depart and merge with neighboring film regions. The simulation results show that in hybrid DWC, the area fraction occupied by drops (f) lies between 0.28 to 0.296 for the ratio of maximum drop diameter to DWC region width (RD) from 0.125 to 1 and a total temperature drop (∆T) of 2, 4, and 8 ℃. Thus, the drop population is less sensitive to RD, and an average f of 0.288 is generalized. On the other hand, the surface renewal for DWC within the hybrid surface shows improvement for RD > 0.5 with the highest enhancement of 64 to 85% taking place at RD = 1, mainly due to the merging effect. In addition, results for drop size distribution profiles of DWC within the hybrid surface are characterized by a lower population of large drops and a higher population of small drops than full DWC. Additionally, the constant exponent (n) in the literature’s empirical expression is replaced by a polynomial series as a function of drop effective and maximum radii. The impact of surface renewal on coalescence intensity is presented in a relatively steeper slope on the logarithmic scale.