Critical evaluation of enhancement factors for vapor transport through unsaturated porous media

Publikationstyp
Journal Article
Date Issued
2009-10-01
Author(s)
Shokri, Nima  
Lehmann, Peter  
Or, Dani  
TORE-URI
http://hdl.handle.net/11420/10698
Journal
Water resources research  
Volume
45
Issue
10
Article Number
W10433
Citation
Water Resources Research 45 (10): W10433 - (2009-10-01)
Publisher DOI
10.1029/2009WR007769
Scopus ID
2-s2.0-72149116176
Previous experiments have demonstrated "enhanced" water vapor fluxes from partially saturated porous media relative to fluxes predicted based on Fick's law. This led to various mechanistic and phenomenological enhancement factors to reconcile discrepancies between experiments and predictions. In the present study, analyses of abrupt transitions from liquid-to diffusion-controlled mass transport during evaporation from porous media is used to offer new insights regarding key assumptions and adequacy of experimental setups used to develop vapor transport enhancement factors. We reevaluate these concepts considering the role of capillary-induced liquid flow through the unsaturated zone in supplying the total fluxes. The extent and existence of hydraulic connections through the unsaturated zone has been predicted theoretically and demonstrated experimentally using high-resolution X-ray tomography of sand columns during evaporation. Extending these results to the late stages of evaporation from porous media shows simultaneous capillary induced liquid flow and vapor diffusion supplying the total vapor fluxes as validated by laboratory evaporation experiments. Additionally, analysis of evaporation from porous media provides well defined conditions for testing vapor transport based solely on Fick's law. New insights regarding transport mechanisms controlling evaporation rates from porous media emphasize the extended role of capillary flows beyond previously assumed models and may reconcile observations with prediction without invoking unobservable local thermal gradients and similar enhancement factors. We illustrate that considering the coupling between capillary flow and vapor diffusion predicted by Fick's law of diffusion without any enhancement factors is the essential key to estimate the total vapor flux.