Evaporation From Layered Porous Medium in the Presence of Infrared Heating

Abstract

We report an experimental study of evaporation of two configurations of layered porous media that are heated from above by infrared (IR) radiation having ~1,000 W/m2 intensity. We used nearly monodisperse glass beads in each of the layers. The two configurations are (1) coarse and fine beads, stacked side by side and (2) fine beads over coarse beads (FoC). The IR heater mimics the natural evaporation process in soils, and the aim is to study, using three diagnostic tools, how the layering affects the evaporation process, and compare it with the homogeneous porous medium case. For each experiment we use three diagnostics simultaneously, evaporation rate measurement using a precision balance, surface temperature imaging using an IR camera, and visualization of the evaporation process with fluorescein dye. The constant (evaporation) rate period (CRP) regime, found in homogeneous porous media, is drastically changed for the two layered configurations. We show new results for side-by-side configuration. In the FoC configuration we show that the near-surface water content in the finer particles does not change in CRP and a true constant evaporation rate is possible. The average water depth at end of CRP in the porous medium, for a wide range of diameter ratios for the FoC configuration are compared with the predictions from theoretical relations proposed by Shokri et al. (2010, https://doi.org/10.1029/2009JB006743) and Assouline et al. (2014, https://doi.org/10.1002/2013WR014489). Using a simple surface energy budget, we show that knowing the surface temperature, the evaporation rate can be estimated with reasonable accuracy (±5%) during drying of a porous medium.