Effect of Drop Shape on Heat Transfer During Dropwise Condensation Underneath Inclined Surfaces

Abstract

The instantaneous and time-averaged heat transfer coefficient and wall shear stress in dropwise condensation depend on several parameters such as the physical and chemical texture of the substrate, its inclination, and interfacial properties. These factors affect the shape, size, and drop distribution of the condensing drops. On an inclined surface, contact angle varies over the three-phase contact line and the drops get deformed. The commonly used "two-circle model" approximates drop shape as a part of the sphere with a circular footprint. In the present work, this approximation is relaxed and the shape of the drop is obtained by solving the three-dimensional force equilibrium equation. With the shape prescribed, the critical size of drop at instability is determined. Drop-level flow and heat transfer rates associated with fluid motion within have been determined numerically by solving the 3D Navier-Stokes and energy equations in the true drop geometry with applicable boundary conditions. The dropwise condensation model proceeds from the thermodynamically stable liquid droplets, to growth by direct condensation and coalescence, and drop instability, followed by fresh nucleation. Numerical data obtained from the simulation show that wall shear stress and heat transfer coefficient are sensitive to the prescription of the drop shape. The approach proposed in the present study shows a longer time for the drop to become unstable and a larger critical drop size, both factors lowering the estimates of average wall shear stress and heat flux, in comparison to the two-circle approximation.

Calculation of the heat flux near the liquid–gas–solid contact line

“…Insufficient understanding of transfer processes in the zone of three-phase contact line holds back the studies of several phenomena, such as boiling, cavitation, rivulet flows, destruction of liquid films, spreading and evaporation of droplets, droplet condensation, etc. [1][2][3][4]. The thickness of liquid in this zone with the length of about several microns, often called the microregion, decreases from 1 − 3 μm to 10 − 20 nm (the adsorbed film).…”

Visualization study on droplet dynamic characteristics in dropwise condensation on inclined super-hydrophobic tube

“…Caruso等人 [14] 研究了混合蒸汽的冷凝传热性能, 考 虑不同的入口雷诺数和膜厚, 当倾角45°时结果发生 了明显变化. Sikarwar等人 [15] 分析滴状冷凝的液滴尺 寸与倾斜角度的关系, 发现液滴平均尺寸随冷凝表 面与水平面夹角的增加而减小. Chung等人 [16] 在倾斜 平板上下面进行纯蒸汽的滴状冷凝实验, 结果显示, 与水平面夹角大有利于传热, 且上表面传热量高于 下表面.…”

Mechanisms of dropwise condensation on aluminum coated surfaces

“…DWC is a quasi-cyclic process with characteristic timescales, fraction of the surface covered by droplets, and drop-size distribution [2]. DWC begins at the molecular scale with droplets nucleation in preferred sites.…”