Civil Engineering Reference
In-Depth Information
k
intrinsic permeability
k
relative
relative permeability
l
viscosity
5.2.8 Moisture transport; Capillary flow
Capillary flow is induced by capillary attraction force that governs the action of a
liquid against solid surfaces in small and confined areas. Darcy's law also
describes the transport of liquid due to pressure. However, capillary attraction
creates a negative pressure; therefore, the mass flux of liquid is written as
k
liquid
l
liquid
J
liquid
pressure
¼
q
liquid
rð
P
p
c
Þ
ð
16
:
12
Þ
where
p
c
capillary pressure
The capillary pressure is defined as the pressure difference between the liquid
and the gaseous phase.
5.2.9 Moisture Transport: Swelling
Vegetable fibres absorb large amounts of moisture which may lead to swelling.
Studies have shown that the amount of water retained by swollen fibres varies from 6
to 100 % of dry weight of fibres and swelling in water could reach 22 % of the initial
fibre volume (Rowell et al.
1998
). Swelling changes the structure and molecular
arrangement of the fibres, such as the capillary pore, the porosity and the perme-
ability (Chatterjee and Gupta
2002
). Swelling is an important issue in modelling
flow transport in vegetable fibres. Swelling of vegetable fibres is a time-dependent
process and a function of micro- and macro-structure of the porous fibres.
There are two commonly used models for simulating swelling fibres. One is
based on the Lucas-Washburn approach assuming that the porous medium is a
bundle of aligned capillary tubes of the same radii (Greenkorn
1983
). Another one
is based on the modified Darcy's law and continuity equation that include the
swelling effects in the model (Masoodi et al.
2010
). A time-dependent function for
permeability is adopted. The mass flux takes the form of
pressure
¼
S
o
e
r
J
waters
ð
16
:
13
Þ
ot
where S presents sink source which is assumed to be linearly proportional to the
water rate.
