Biomedical Engineering Reference
In-Depth Information
These components are given by the following empirical relations,
which are used to calculate the directional values of thermal conductivities
(all thermal conductivities are in w/m K):
10
2
3
T
10
2
6
T
2
K
w
52
0
:
487
5
:
887
7
:
39
1
3
3
10
2
3
10
2
4
T
10
2
7
T
2
K
g
52
7
:
494
1
:
709
2
:
377
3
1
3
3
10
2
10
T
3
10
2
14
T
4
10
2
17
T
5
2
:
202
9
:
463
1
:
581
1
3
3
1
3
K
s
5
0
:
52
in perpendicular direction
(3.11)
T
3
d
pore
K
rad
5
5
:
33e
rad
σ
(3.12)
σ
where e
rad
is the emissivity in the pores having diameter d
pore
,
is the
Stefan
Boltzmann constant, and T is the temperature in K. The contribution
of gas radiation in the pores, K
rad
, to conductivity is important only at high
temperatures.
Figure 3.13
shows the variation in the thermal conductivity of wood
against its dry density. The straight line represents the thermal conductivity
parallel to the fibers. The curved line gives the thermal conductivity across
the fibers. The straight line is calculated from
Eq. (3.10)
. Table C.10 that
lists thermal conductivity of some wood, shows higher conductivity for hard-
wood, which also has higher density.
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
200
400
600
800
1000
1200
1400
Dry density (kg/m
3
)
Along grain
Across grain
FIGURE 3.13
The thermal conductivity of biomass along the grain (straight line) and across
the grain (curved line) increases with the dry density of the biomass. The plot is for dry wood.
Source: Data from Thuman and Leckner (2002).
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