Porous Media

Porous media are more common than we think. Sponges, rocks and plant stems constitute some of the countless examples. Even the (human) brain can be treated as a porous medium! What’s particular about these kinds of media is that, as with turbulent flows, there is a scale hierarchy, but not of the same kind. In fact, porous media can be described at different scales, from the smallest to the largest and everything in between, and we can observe different behaviors at each one.

 

A lot of engineers try to model flows through porous media (that is, they compute numerical simulations) through averaging the equations that describe the flow in a representative volume that is neither too large not too small, just big enough to have a (kind of) accurate description of the medium. This is of fundamental importance because it helps us model what to expect from a particular system relatively inexpensively. Part of my job in this project was to study porous media in three different scales: macro (big or global), micro (the smallest, where we can see inside the pores) and meso (a scale in between). It is in the latter that the averaging is done for the numerical simulations. This is done because the equations that describe the system are unclosed, which means that we need more information in order to be able to fully model the physical system, and closure models are typically based in a kind of average.

 

A particular case of porous media are fixed beds of spherical particles. These are basically packed spheres that do not move, even if there is a fluid flowing in between the pores. They can be used as nuclear reactors, catalyzers and even thermal energy storage units, so their study has a strong industrial motivation. Not only that, but they are a relatively simple porous medium that can be used as a toy model for other more “complicated” media (whatever “complicated” means).

 

Fixed beds play a big role in a kind of renewable energy source -a topic that I hold close to my heart- which is AA-CAES, whose acronym stands for Advanced Adiabatic Compressed Air Energy Storage. Basically, this technology aims to store energy in rocks, and a thermal energy storage unit is needed. It is based in the compression and expansion of cold and hot air respectively, so that there is a flow through the porous medium (you can learn more about it here). 

 

I designed and conducted two different experimental campaigns to study fixed beds at the macro and micro scales and computed simulations using the open-source CFD (computational fluid dynamics) code OpenFOAM. We did this in order to have a full scale description of the hydrodynamics in fixed beds and to provide information on the mesoscale.

 

In my opinion, the most surprising result that I got from this work is that we can have a macroscopically laminar (i.e. not turbulent) flow but when we look at the smallest scales, we have a flow that is virtually turbulent! We found striking similarities between the flow at the porescale and a typical turbulent homogenous isotropic flow.

 

My PhD dissertation was based on this, so feel free to ask for the full copy of the thesis!