Evaporation From Confined Porous Media Due to Controlled IR Heating From Above

Kumar, Navneet and Arakeri, Jaywant H (2018) Evaporation From Confined Porous Media Due to Controlled IR Heating From Above. In: TRANSPORT IN POROUS MEDIA, 125 (2). pp. 311-340.

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Abstract

We report an experimental study of evaporation due to controlled infrared (IR) heating from above from an initially saturated confined porous medium consisting of nearly `mono-disperse' particles which has been rarely used earlier. We have used three diagnostic tools simultaneously, evaporation rate measurements using a precision weighing balance, surface temperature measurements using IR imaging, and fluorescein dye mixed with water to visualize the drying front and the evaporation sites. IR images show that the first stage, so-called constant rate period (CRP), was maintained due to films of water reaching the top surface from the saturated region below. Gradually reducing evaporation rate in stage 1 is shown to be related to `shrinking evaporating patches' on the top surface, clearly revealed as lower-temperature regions in the IR images. End of CRP coincides with disappearance of the low-temperature patches. We give end of CRP in terms of the average depth (L-cap) of the liquid level from the top surface at that time. L-cap and duration of CRP are strong functions of the porous medium bead size, transition to stage 2 happening earlier for coarser spheres. The obtained L-cap values deviated from the predictions of Lehmann et al. (Phys Rev E 77(5):056309, 2008) which we show is due to a small range of pore sizes in the current experiments. For both water and highly volatile n-pentane, we show that L-cap normalized by a length scale derived from gravity-surface tension force balance goes like Bo(0.20), for Bo varying from 2.0E - 04 to 1.0E - 01; Bo is the Bond number. Fluorescein dye imaging shows a different view of the evaporation stages. During CRP, highly concentrated deposits of the fluorescein dye particles, orange in colour, are seen in the top few bead layers. These orange deposits represent the sites on the beads surfaces where the evaporation has taken place. Even with external heating, evaporation from such a porous medium is limited to a finite depth from the evaporating end, similar to the observation by Lehmann et al. (2008) for isothermal evaporation in Hele-Shaw cell.

Item Type:	Journal Article
Publication:	TRANSPORT IN POROUS MEDIA
Publisher:	SPRINGER
Additional Information:	Copy right for this article belong to SPRINGER
Keywords:	Evaporation; Porous media; Thermal imaging; Fluorescein dye deposits
Department/Centre:	Division of Mechanical Sciences > Mechanical Engineering
Date Deposited:	15 Oct 2018 14:20
Last Modified:	15 Oct 2018 14:20
URI:	http://eprints.iisc.ac.in/id/eprint/60870

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