Civil Engineering Reference
In-Depth Information
relative humidity and the temperature difference. Moisture in the cellulose
increased slightly more than that in the fibreglass under the same conditions.
Hokoi and Kumaran (
1993
) studied heat and moisture transport through glass fibre
insulation experimentally and analytically. They reported that for a medium
density glass fibre insulation (about 50 kg/m3), the value of the local moisture
content at the fully saturated moisture state may reach 20 kg/kg of dry insulation
material. Ogniewicz and Tien (
1981
) got a semi-analytical solution for solving a
quasi-steady one-dimensional forced convective problem with convective
boundary conditions. They found that the rate of condensation, though relatively
small, increased with external humidity, temperature levels and overall tempera-
ture differences. Motakef and El-Masri (
1986
) demonstrated the existence of a wet
region sandwiched between two dry zones in a porous slab. Chen et al. (
1997a
) and
(
b
) investigated a transient two-dimensional forced convective case with a warm
plate at the top and a cold plate experimentally and numerically. The total mass of
moisture and frost accumulation on the insulation slab and cold surface was
0.0156 kg was found for one case which was 50 % of the total vapour that flowed
through the system in a 3.5-h test. A 20 % decrease in the effective thermal
conductivity was predicted to cause temperature, moisture and heat flux variations
over 3-5 h of up to 1.5 K, 0.5 and 11 %, respectively.
Vafai and Sarkar (
1986
) conducted two-dimensional simultaneous heat and
mass transfer in porous insulation. Gas pressure was assumed to be constant. They
demonstrated that condensation occurred at any point in the insulation where the
water vapour concentration became greater than the saturation concentration
corresponding to the temperature at the point. Vafai and Whitaker (
1986
) studied
condensation effects in fibrous insulation slab and concluded that the condensation
process was significantly affected by the thermophysical properties of the insu-
lation, the infiltration velocity and the humidity levels. Condensation was a serious
problem for large Peclet numbers. Tao et al. (
1991a
,
b
) developed frost model for
studying the motion of the frozen boundary which started at a cold plate (sink) at
subzero temperature a fibrous insulation slab.
In the recent literature, Fan et al. (2004) developed a coupled heat and moisture
transfer model with phase change and mobile condensates in fibrous insulation. A
super-saturation state in condensing region, the dynamic moisture absorption of
fibrous materials and the movement of liquid condensates were considered. Foss
et al. (
2003
) presented a simultaneous heat and mass transfer model through a porous
material for investigating transient moisture sorption by paper sheets from humid
air. The model showed nonlinearities of the moisture content isotherm and heat of
sorption. Charoenvai et al. (
2005
) reported the result on heat and moisture transport
in durian (Durio zibethinus)-fibre-based lightweight construction materials com-
posed of cement, sand and waste fibre from durian peel. The results showed that the
weekly mean water content on the surface of material was quite low. The effect of
moisture on the apparent thermal performance of the composite was higher as water
absorbed in the pore structure contributed to higher thermal conductivity than the air
it replaced. However, the mean value of thermal conductivity in material is still
rather low as the mean value of water content in material is low. An anisotropic 3D
