Transport phenomena and dynamics of an evaporating water meniscus at low heat fluxes

Abstract

It is known that evaporation efficiency of novel solar thermal vapor generation/desalination systems can potentially increase by incorporating interfacial heat localization techniques utilizing porous wicks. The ensuing efficiency is a strong function of the geometric and thermal properties of the wicks and the ambient operating parameters. We investigate the role of these critical process parameters on the evaporation dynamics of pure and saline water from a single-layered wire mesh operating at low heat fluxes (1 kW/m2, 5 kW/m2and 10 kW/m2), which are typical for devices working on solar energy. Evaporation rates of water from stainless steel and nylon meshes are determined using non-invasive infra-red thermography and digital microscopy. A finite element based computational model is developed to understand local transient evaporation phenomenon at the microscale. Arbitrary Lagrangian-Eulerian (ALE) formulation is used to solve the coupled transport equations on a deforming computational grid, incorporating the Fick's law of diffusion at the evaporating water-air interface. With increasing heat fluxes, there is a deviation from purely diffusion-based transport. Higher evaporation rates are observed in meshes with smaller wire diameter and higher thermal diffusivity. In addition, the decisive role of local thin capillary films is exemplified. Simulations also indicate the diminutive role of thermocapillary convection during evaporation at such low input heat fluxes. Finally, we also investigate the effect of mesh size on the nature of salt crystallization. The evolution of salt concentration gradient at the microscale is computed that complements the experimental observations. Optimal exploitation of heat localization technique for enhanced evaporation necessitates understanding of microscale mesh level local transport effects.