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______________________________________________________________________________________________________ CRYSPOM VI - Hamburg, Germany – September 19-21, 2018.
Evaporation of a saline solution in porous media: an overview
M. Prat
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Institut de Mécanique des Fluides de Toulouse (IMFT), Université de Toulouse, CNRS – Toulouse, France
Keywords : Evaporation, saline solution, crystallization, salt crust
During evaporation of a saline evaporation in a porous medium, the interactions between the ion transport, evaporation, crystallization phenomena and possibly the coupling with mechanical effects are challenging in terms of predicting saline water evaporation from porous media as well as regarding the mechanical consequences of the salt precipitation. The variety of associated problems can be illustrated through a series of recent or on-going studies developed in our group as listed hereafter.
Efflorescence morphology
As discussed for instance in [1] or [2], the morphology of the efflorescence can be quite diverse depending on the conditions (e.g., mean pore size of the porous medium, initial evaporation rate, temperature). Understanding the detail of the transition between the various patterns as the pore size or the evaporation conditions are varied is an example of open problem.
Locus of first crystals at the surface
Various factors have an impact on the locus of the first crystals at the evaporative surface of a porous medium (at the periphery or about elsewhere, in a low permeability region or a high permeability region, etc.). Among the factors that have been clearly identified, one can mention the distribution of the evaporation flux at the surface [3], and the permeability and porosity heterogeneities in the upper region of the medium [4].
The crust formation and its impact on evaporation
As reported in [1] or [2], the formation of a salt crust can lead to a severe decrease in the evaporation rate, by a factor of 10 or more compared to the initial drying rate. Understanding the exact mechanisms leading to this very low evaporation rate is currently a hot topic in this research area.
The surface heave phenomenon and the crystallization pressure
The formation of the crust within a granular material can lead to the displacement of its upper surface, a phenomenon referred to as the surface heave phenomenon [5]. This phenomenon has to do with the concept of crystallisation pressure, e.g. [6] and references therein, which permits to determine the stress induced by the growing crystal on the grain layer located above the internal crust.
References
[1] Veran-Tissoires, S., Prat, M., J. of Fluid Mechanics, vol. 749, pp.701-749 (2014)
[2] Eloukabi, H., Sghaier, N., Ben Nasrallah, S., Prat,M., Int. J. of Heat and Mass Tr., 56, 80–93, (2013) [3] Veran-Tissoires, S., Marcoux, M., Prat, M., Phys. Rev. Letters 108, 054502. (2012)
[4] Diouf, B., Geoffroy, S., Abou Chakra, A., Prat, M., Eur. Phys. J. Appl. Phys., 81 (1), 11102 (2018) [5] Diouf, B., Ph.D Thesis, University of Toulouse (2018)
