Thermophysical
methods and properties/ Hőfizikai
módszerek és anyagjellemzők
Modelling
mass flow properties of porous
media
/ Porózus anyagokban lévő
tömegáramlás modellezése
A.F.Miguel,
A.M.Silva Univ. of Évora,
Centre of Geophysics, Évora,
Portugal
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| Modelling
fluid flow in porous media is of
growing interest in a wide range
of technical and environmental
domains. However, determination
of fluid flow properties of
porous media has been a long-standing
problem. This is due to the fact
that the fluid properties are
characteristics for each
particular medium. Moisture flow
properties of porous media are
usually determined based on
experiments [1-3]. Though
much important work has been
done, some of these techniques
are still very demanding in terms
of time and not easy to execute.
The aim of this study is to
present a coherent analytical
modelling of mass flow properties
that is physically meaningful and
useful in practice. This is
achieved within the framework of
thermodynamics. Philip and
De Vries [4] proposed
for moist porous media the
following relationship
u=Dqgrad(q)+DTgrad(T)
where Dq
represents the
conductivity under moisture
content gradients (grad
q); DT
the conductivity
under temperature gradients
(grad T); and u the moisture
velocity.
Formulating the
problem in the framework of
thermodynamics one can determine
both coefficients Dq
andDT
Dq=0.14Psat,oqsat-1
(ksat/m)(q/qsat)1.86[(Tc-T)/Tc]2
DT=1.5Psat,oTc-1(ksat/m)(q/qsat)2.86[(Tc-T)/Tc]2
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A sensitivity study
is performed in order to
investigate the influence of the
involved quantities, mainly
moisture content and temperature
on Dq
and
DT. For Psat,o=740
Pa, ksat/m=10-6
m2/(Pa s) and qsat,0=0.45 the
following results are obtained.
Figure 1:
Effect of
moisture content on
Dq
for
different temperatures
Figure 2:
Effect of temperature on
DT
for
different moisture contents
Both coefficients Dq
and
DT are
sensitive to the change of
temperature and moisture content
of the medium. The increase of Dq
with the
temperature is most remarkable in
the range of moisture content
larger than 0.2, being negligible
for q<0.1. The
coefficient DT
increases as
moisture content and the
temperature increase. The curves
represented in these figures can
be compared with experimental
data of van der Kooi [5] and
Crausse [6], obtained
for cellular concrete and for
sand, respectively. This
comparison shows that the
tendency of our curves is similar
to the curves
presented by the referred authors.
REFERENCES
[1]
J. Bear, 1972. Dynamics
of Fluids in Porous Media.
Dover, New York
[2] M.
Sahimi, 1995. Flow and
Transport in Porous Media.
VCH, Weinheim
[3] A. F.
Miguel, 2000.
Contribution to flow
characterization through
porous media. Int. J.
Heat Mass Transf., 43,
2267-2272
[4] D. A.
de Vries, 1987. The
Theory of Heat and
Moisture Transfer in
Porous Media Revisited.
Int. J. Heat Mass
Transfer, 30, 1343-1350
[5] J.
van der Kooi, 1971.Moisture
Transport in Cellular
Concrete Roofs. PhD
thesis, Technische
Hogeschool te Eindhoven,
The Netherlands
[6] P.
Crausse, 1983. Etude
Fondamental des
Transferts Couplés de
Chaleur et d' humidité
en Milieux non Saturés.
PhD thesis. Inst. Nac.
Polyt. de Toulouse,
Toulouse
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