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.