The porosity, tortuosity, permeability and heat exchange characteristics from binary
packings, containing mixtures of small d and large D spherical particles, are analysed in
the present work. Binary packing porosity (ε), tortuosity (τ), permeability and heat
exchange performance are dependent on the volume fraction of large particles, xD,
present in the mixtures, as well as on the particle size ratio, δ = d/D. In the region of
minimum porosity from the binary mixtures (containing spheres with diameter d and D),
heat exchange performance and permeability from binary packing are higher than that of
the packing containing the small particles d alone (mono-size packing). The δ region
where the permeability of binary packing is higher than the permeability of mono-size
packing of particles d is located in the range 0.1 ≤ δ < 1.0. An increase in permeability
by a factor of two is achieved for particle size ratios between 0.3 and 0.5. Tortuosity can
be modelled by the simple function τ = 1/ε α and it is shown that, in the region of minimum porosity, α varies between 0.5 (mono-size packing) and 0.4 (binary packing
with δ close to 0.03). Due to the tortuosity increase, binary mixtures give rise to
Kozeny´s coefficients substantially higher than five.
Using the commercial finite element software package POLYFLOW® it was possible
to confirm the heat exchange enhancement referred above. The obtained improvement on
the thermal performance is related to the increase of effective thermal conductivity in the
binary packing and to the increase in transversal thermal conductivity due to the porosity
decrease and tortuosity increase. For non-Newtonian fluids from the power-law type, τ decreases with the decrease of the flow index behaviour
