Apr 18, 2016 - Influence of the macropore on transport (Lamy et al, 2009) ... The DPM models are often used in literature to model heterogeneous flow.
Preferential water and solute fluxes in a model macropored porous medium as a function of flow rate Batany Stéphane PhD student at IFSTTAR Nantes With authors: Pierre-Emmanuel Peyneau, Laurent Lassabatère, Béatrice Béchet, Pamela Faure and Patrick Dangla.
April 18th 2016
Pollutant transport in urban soil Context • •
Infiltration obeys the Darcy-Buckingham law for homogeneous porous media. Presence of heterogeneity generates preferential flows in saturated conditions.
Blue dye propagation in sand soil (Allaire et al, 2000)
Pollutant transport in urban soil Context • •
Infiltration obeys the Darcy-Buckingham law for homogeneous porous media. Presence of heterogeneity generates preferential flows in saturated conditions. •
Blue dye propagation in sand soil (Allaire et al, 2000)
The most frequently used models for flow and transport in heterogeneous porous media are dual-porosity (or dual-permeability) models (DPM).
Influence of the macropore on transport (Lamy et al, 2009)
Breakthrough curves • Breakthrough curves (BTCs) of solute in macropored sand soil with DPM model.
• The fitting parameters are the size of the enhanced flow SPF/MF and the ratio of permeability between macropore and porous matrix Rmacro/matrix. Fitting of tracer elution through a macropored column and a DPM approach. Best fitting with a SPF/MF = 50,5 % of column section (1% macropore area ratio), and a permeability ratio of Rmacro/matrix = 4. (Lamy et al, 2009) .
Scientific goal
• The DPM models are often used in literature to model heterogeneous flow and transport in macroporous media. • The assumption of a Darcian flow in the preferential flow for macropore is not physically based. • The high flow caused by the macropore doesn’t have to be restricted to the macropore boundaries. We need to know the main process of flow and transport in the macropore and the surrounding matrix at a local scale.
Materials and methods Column design
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The porous media is composed of spherical glass beads with an uniform distribution of grain size (diameter : 425 − 800 μm). The beads are glued together by araldite.
Model soil column (a), and glass beads composing the soil (b).
Materials and methods Column design
•
The porous media is composed of spherical glass beads with an uniform distribution of grain size (diameter : 425 − 800 μm). The beads are glued together by araldite.
Model soil column (a), and glass beads composing the soil (b).
Diagram of tracer injection into columns under saturated conditions.
Materials and methods Column design
•
The porous media is composed of spherical glass beads with an uniform distribution of grain size (diameter : 425 − 800 μm). The beads are glued together by araldite.
Normalisation of breakthrough curves Model soil column (a), and glass beads composing the soil (b).
Diagram of tracer injection into columns under saturated conditions.
Materials and methods
- Magnetic resonance imaging
Transport imaging
• We use non reactive gadolinium chelate (Gd+DTPA) for the MRI imaging of solute transport. Double spin echo sequence. • We make an temporal evolutive cartography of the solute concentration.
MRI facility for column imaging (Navier-IFSTTAR Champ-sur-Marne, France).
Gadolinium propagation in control column.
Materials and methods
-Flow simulation
Flow simulation by lattice-Boltzmann method •
Lattice boltzmann method allows to obtain velocity field at pore scale in a media at complex geometry.
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We use the BGK scheme with a unique relaxation time and half bounceback boundary conditions for grain and sample boundaries, under saturated condition.
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For a same flow rate we study the influence of the macropore size and the grain size distribution on velocity field.
Synthetic velocity field. Digital porous media.
Results :
-BTCs curves
Breakthrough curves of KBr elution (without dead volume) for both control and macropored column.
Normalised BTCs of KBr elution for different flow rates in the control (C) column.
Results :
-BTCs curves
Breakthrough curves of KBr elution (without dead volume) for both control and macropored column.
Normalised BTCs of KBr elution for different flow rates in the control (C) column.
Normalised BTCs of KBr elution for different flow rates in the macropored (M) column.
Results :
-Bimodal breakthrough
We make the assumption that the macropore is a pipe inserted in a porous media and lateraly closed pipe of same dimension. Experimental BTCs of a plastic pipe.
Normalised BTCs of KBr elution for different flow rates in a pipe of same dimension than the macropore.
Normalised BTCs of KBr elution for different flow rates in the macropored (M) column.
Results :
-Bimodal breakthrough
We make the assumption that the macropore is a pipe inserted in a porous media and lateraly closed pipe of same dimension. Experimental BTCs of a plastic pipe.
Normalised BTCs of KBr elution for different flow rates in the macropored (M) column.
Normalised BTCs of KBr elution for different flow rates in a pipe of same dimension than the macropore.
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High flow rates (~ 5 mL/min) : convective effects dominate -> macropore acts like a pipe.
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Low flow rates (~ 0,1 mL/min): diffusion dominates -> lateral diffusion in the porous matrix, effect of double permeability.
Results :
-Transport imaging
High flow rate
Control column q = 1 mL.min-1
Results :
-Transport imaging
High flow rate
Macropored column q = 1 mL.min-1
Results :
-Transport imaging
Low flow rate Macropored column q = 0,1 mL.min-1
Results :
-Transport imaging
Low flow rate Macropored column q = 0,1 mL.min-1
Diffusion coefficient of Gd-DTPA 5 times smaller than Br.
Results : Simulation of fluid flow with lattice-Botzmann method
Velocity
We study the influence of the macropore size and the grain size distribution on velocity field in the macroporous media.
Mean velocity from the center of the porous media for different macropore size.
-Modeling
Results :
-Modeling
Simulation of fluid flow with lattice-Botzmann method
Velocity
We study the influence of the macropore size and the grain size distribution on velocity field in the macroporous media.
Mean velocity from the center of the porous media for different macropore size.
Enhanced velocity zone in the porous matrix for different macropore size and different grain size distribution.
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Enhanced velocity zone width depends on grain size.
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Poiseuille-like flow in the macropore.
Conclusions
• For high flow rate, macropore acts like a pipe and the preferential flow in it, is the pathway of the solute. • With a lower flow rate an exchange with the surrounding porous media occurs. So bimodality for highly diffusive solute is primarily induces by the diffusive process • First LBM simulations show that at a same flow rate, the grain size distribution is decisive for the surrounding preferential flow in porous matrix, not the macropore size, and the flow in the macropore can be approximated with a Poiseuille flow. Final aim: establish a macropore law for transport in macroporous media, by the association of poiseuille-like flow in the macropore, Darcian flow in porous media, and determination of the transition zone in the vicinity of the macropore.
Thanks for your attention
