Insulation Materials Made with Vegetable Fibres - Nearly Zero Energy Building Refurbishment - page 429

5.2.3 Heat Transport: Convection
By this mode, heat is transferred via the movement of fluid. It can be classified
according to its nature as natural convection and forced convection. Both forced
and natural types of convection may exits together. Natural convection results
from thermal buoyancy effects. Forced convection is typically driven by wind
pressure and HVAC equipment. The Rayleigh number is a dimensionless number
which is associated with buoyancy-driven airflow and can be regarded as a
measure of the driving forces of natural convection. The Nusselt number is another
dimensionless number which is associated with forced convection and a function
of wind velocity and the shape of the fibrous insulation. In a fibrous material,
convection exists due to the air movement in the pores between the fibres. But due
to the small dimensions of the pores, convection is often neglected without losing
in accuracy (Karamanos et al. 2008 ; Milandri et al. 2002 ).
5.2.4 Heat Transport: Radiation
Radiation is the transfer of energy by electromagnetic wave motion. It travels at
the speed of light and does not require a medium to carry it. The radiative heat flux
depends mainly on the absolute temperature of the emitting body based on the
Stefan-Boltzmann law (Holman 1997 ). For general porous media, the calculation
of radiative heat transfer commonly requires solving the radiative transfer equation
(Holman 1997 ; Larkin and Churchill 1959 ; Siegel et al. 1992 ; Batycky and
Brenner 1997 ; Webb 1994 Klemens and Kim 1985 ). But for fibrous materials, the
prediction of radiative heat transfer means not only solving of radiative transfer
equation but also requiring of the knowledge of the radiative properties of the
concerned material due to its absorbing and scattering capabilities. Hence, the
transport process of radiation in the fibrous insulation is complicated because of
both its complex morphology and the inherent complexities associated with the
transport mechanism itself. Such complexity of the heat transfer makes the anal-
ysis and the design of insulation quite difficult.
There has been extensive research on this topic in fibrous materials theoreti-
cally, experimentally and empirically (Tien 1988 ; Hendricks et al. 1994 ; Singh and
Kaviany 1992 ; Lee and Cunnington 2000 ; Lee 1991 ). The refractive index, the
scattering and absorption coefficients are critical in determining the radiative
properties. Viskanta and Menguc ( 1989 ) and Baillis and Sacadura ( 2000 ) rigor-
ously reviewed methodology applied to the calculation of radiative transfer and
properties in dispersed media. Two basic methods are principally used: theoretical
model and identification approaches. Assuming that the media are composed of
spherical particles and starting from the properties of the basic components such as
the optical indices, the theoretical model, based on the Mie's theory, describes the
interaction of an electromagnetic wave to predict the radiative properties of a
fibrous material (Baillis and Sacadura, 2000 ). These models can provide the
understanding of the physical side of the material. However, with the increasing