Evaporation characteristics and salt deposition dynamics from a homogeneous porous medium consisting of mono-disperse glass beads under controlled IR heating from above

Abstract

The present study experimentally investigated the evaporation-precipitation dynamics at sub-millimetric to millimetric scales in 1 M NaCl salt-DI water solution-based homogeneous porous media consisting of nearly mono-disperse glass beads (ranging from 0.10 to 2.50 mm) under controlled infrared heating from above, mimicking realistic field scenarios. Three diagnostic tools were employed simultaneously: mass loss measurements using a precision weighing balance, surface temperature measurements using an IR camera, and optical imaging (at times with fluorescein dye) for visualization purposes. In all the cases, salt precipitates around $\sim 0.97$ of saturation. Experimental results show a strong interdependence between evaporation and salt deposition, significantly influenced by the particle sizes. For 0.70–0.85 mm case, evaporation characteristics with and without salt were found to be similar. For 0.40–0.60 mm and 0.70–0.85 mm cases, a constant evaporation rate persisted for $\sim$4–5 h at $\sim$1,000W/m² of incident heat flux, deviating from the regular ‘No Salt’ nature under external heating. In finer particles (0.10–0.30 mm), rapid salt deposition led to a steep fall in evaporation rate, while in medium and coarser sizes, it initially increased linearly, then exponentially during Stage 1 due to emerging precipitation sites within interconnected voids forming distinct wet patch, visible in IR imaging. Despite variations, $\sim$80 % of the exposed surface was covered during Stage 1 (except the 2.00–2.50 mm case due to weak capillary forces) in all the cases. Unlike pure water cases, the evaporative capillary length shows a non-linear trend with particle size with maximum value appearing for 0.70–0.85 mm case, interestingly similar to the value for the ‘No Salt’ case.