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Chapter 11
Fundamentals of Thermal Radiation
1142
The variation of reflectivity of a surface with
wavelength is given. The average reflectivity,
emissivity, and absorptivity of the surface are to be
determined for two source temperatures.
Analysis
The average reflectivity of this surface for
solar radiation (
= 5800 K) is determined to be
T
λμ
μ
λ
T
=→
=
(.
3
0 978746
m)(5800 K) = 17400 mK
f
ρ
λ
λ
,
μ
m
3
0.95
0.35
ρ
ρρ
λλ
()
(. )
) (. )
(
.
)
Tf
T
ff
=+
−
−
=
−−
∞
10
2
12
11
1
0 35 0 978746
0 95 1 0 978746
0.362
Noting that this is an opaque surface,
τ =
0
At
= 5800 K:
T
0.638
=
−
=
−
=
⎯→
⎯
=
+
362
.
0
1
1
1
ρα
Repeating calculations for radiation coming from surfaces at
= 300 K,
T
μ
λ
T
=⎯
→
⎯=
3
0 0001685
m)(300 K) = 900 mK
f
1
0.95
=
−
+
=
)
0001685
.
0
1
)(
95
.
0
(
)
0001685
.
0
)(
35
.
0
(
)
(
T
At
= 300 K:
T
0.05
=
−
=
−
=
⎯
=
+
95
.
0
1
1
1
and
ε
α
==
0.05
The temperature of the aluminum plate is close to room temperature, and thus emissivity of the plate will
be equal to its absorptivity at room temperature. That is,
εε
αα
room
s
005
0638
.
.
which makes it suitable as a solar collector. (
0
and
1
room
=
=
s
for an ideal solar collector)
1143
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This note was uploaded on 01/24/2012 for the course PHY 4803 taught by Professor Dr.danielarenas during the Fall '10 term at UNF.
 Fall '10
 Dr.DanielArenas
 Thermodynamics, Mass, Radiation

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