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Advanced drying theory of capillary porous media from high-performance-computing pore network simulations – DRYCAP

Theory of drying in porous media from pore network simulations

Improvement of drying theory of porous media from simulations on pore network. Consideration of the local non-equilibrium, the distinction between liquid percolating and non-percolating phases, coupling with external transfers.

Improvement of continuum models describing the drying of porous media.

Evaporation in a porous medium is a subject of much study because it is a key phenomenon in soil science and in the drying stages in many industrial processes. Despite its practical importance and the large number of studies devoted to this topic, the prediction of evaporation and therefore drying of a porous medium remains an open subject. In fact, there is no truly predictive model yet. This problem is generally addressed within the framework of continuum approach to porous media. In this context, the major objective of the project has been to improve the continuum models. Three main avenues for improvement were followed. The first concerns the questioning of the local equilibrium hypothesis, which consists in supposing that on the scale of a small volume of porous medium, called representative elementary volume, the liquid phase and its vapor are in thermodynamic equilibrium. The second track concerns the modeling of the liquid phase. We have explored the approach according to which the liquid phase is separated into the percolating liquid phase and the non-percolating liquid phase. The third concerns the coupling with external transfers which was developed using the concept of macroscopic interfacial resistance.

To improve the continuum models, we proceeded by comparison with numerical simulations of drying directly at the pore scale. These simulations were developed using a so-called pore network model in which the pore space is represented by a network of sites (corresponding to local enlargements of the pore space) connected by links (corresponding to constrictions of the pore space). The fields obtained in the simulations are then spatially averaged in order to produce macroscopic quantities that can be compared to those predicted by continuum models. One of the advantages of this approach is to access quantities that are generally impossible to obtain experimentally (such as, for example, the spatial distribution of the vapor pressure at the surface of the porous medium or the solute concentration in the percolating liquid phase and non-percolating liquid phase). This allows the fine characterization of macroscopic parameters such as for example the interfacial and external resistance which play a key role for the prediction of the evaporation flux.

The drying continuum models have been improved in three ways:
-by developing so-called local non-equilibrium (NLE) models.
-by building a continuum model of drying making the explicit distinction between percolating liquid phase and non-percolating liquid phase. We have shown that this new approach is particularly well suited to situations where drying is accompanied by the transport of ions or particles present in the liquid phase.
-by developing coupling with external transfers using the concept of macroscopic interfacial resistance.

The models developed offer significantly improved prediction capabilities, especially during drying with transport of ions, solute or particles in the liquid phase. This situation is encountered in many problems (alteration of building materials due to crystallization of a salt, manufacturing processes where drying leads to deposition of particles or polymer on the solid matrix, etc.). However, these improved models have only been validated for certain conditions. They should therefore be extended to cover a wider range of drying conditions.

The work carried out within the framework of this project led to eight articles (5 published, 3 submitted) in reference journals, 3 communications in international conferences and 1 communication in a national conference. They led to two doctoral theses (1 in Germany, 1 in France).


Drying of porous media is central to many environmental and engineering applications. In this context, this project aims at performing a major breakthrough in the modelling of the drying process in capillary porous media. The work will be based on a combination of state of the art pore network modelling, pore network simulations and new experiments.

Two and three equations continuum models will be developed taking into account the non-local equilibrium condition of the vapour and from the distinction between the percolating and non-percolating liquid clusters. The secondary capillary structures corresponding to the liquid trapped in various geometrical singularities of the pore space will be characterized experimentally and from numerical simulations and will be taken into account as a distinct and specific phase in the continuum models.

The pore network models will be developed so as to perform high performance computing (HPC) simulations, which is necessary to meet the length scale separation constraints allowing the computation of continuum model parameters from pore network simulations.

Experiments of drying with a dissolved salt will be performed in order to obtain additional validation of the pore network and continuum models developed in the project, noting that situations where a dissolved species is present in the liquid are of paramount importance in many applications. In the present project, the formation and distribution of salt crystallisation spots will be used as key validation factors of the models and as physical signatures of the drying process, especially as regards the impact of the second capillary structures developing during the drying process.

Project coordinator

Monsieur Marc PRAT (Institut de Mécanique des Fluides de Toulouse)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

OvGU,IVT/TVT Institut für Verfahrenstechnik
IMFT Institut de Mécanique des Fluides de Toulouse

Help of the ANR 154,540 euros
Beginning and duration of the scientific project: April 2017 - 36 Months

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