The spectral absorptivity of a large diffuse surface is α λ = 0.9 for λ < 1 μ m and α λ = 0.3 for λ ≥ 1 μ m . The bottom of the surface is well insulated, while the top may be exposed to one of two different conditions. Case ( a ) Case ( b ) (a) In case (a) the surface is exposed to the sun, which provides an irradiation of G S = 1200 W/m 2 , and to an airflow for which T ∞ = 300 K . If the surface temperature is T s = 320 K , what is the convection coefficient associated with the airflow? (b) In case (b) the surface is shielded from the sun by a large plate and an airflow is maintained between the plate and the surface. The plate is diffuse and gray with an emissivity of ε p = 0.8 . If T ∞ = 300 K and the convection coefficient is equivalent to the result obtained in pan (a), what is the plate temperature T p that is needed to maintain the surface at T s = 320 K ?
The spectral absorptivity of a large diffuse surface is α λ = 0.9 for λ < 1 μ m and α λ = 0.3 for λ ≥ 1 μ m . The bottom of the surface is well insulated, while the top may be exposed to one of two different conditions. Case ( a ) Case ( b ) (a) In case (a) the surface is exposed to the sun, which provides an irradiation of G S = 1200 W/m 2 , and to an airflow for which T ∞ = 300 K . If the surface temperature is T s = 320 K , what is the convection coefficient associated with the airflow? (b) In case (b) the surface is shielded from the sun by a large plate and an airflow is maintained between the plate and the surface. The plate is diffuse and gray with an emissivity of ε p = 0.8 . If T ∞ = 300 K and the convection coefficient is equivalent to the result obtained in pan (a), what is the plate temperature T p that is needed to maintain the surface at T s = 320 K ?
Solution Summary: The diagram for the surface exposed to Sun is shown in Figure 1 Figure 2 The expression for heat transfer coefficient is given by, h=alpha _sG
The spectral absorptivity of a large diffuse surface is
α
λ
=
0.9
for
λ
<
1
μ
m
and
α
λ
=
0.3
for
λ
≥
1
μ
m
. The bottom of the surface is well insulated, while the top may be exposed to one of two different conditions.
Case (a)
Case (b)
(a) In case (a) the surface is exposed to the sun, which provides an irradiation of
G
S
=
1200
W/m
2
, and to an airflow for which
T
∞
=
300
K
. If the surface temperature is
T
s
=
320
K
, what is the convection coefficient associated with the airflow? (b) In case (b) the surface is shielded from the sun by a large plate and an airflow is maintained between the plate and the surface. The plate is diffuse and gray with an emissivity of
ε
p
=
0.8
. If
T
∞
=
300
K
and the convection coefficient is equivalent to the result obtained in pan (a), what is the plate temperature Tpthat is needed to maintain the surface at
T
s
=
320
K
?
An opaque surface with the prescribed spectral, hemispherical reflectivity distribution is subject
to the spectral irradiation shown. Find:
1.0
600
0.5
0
05
10
15
λ (μm)
G₂(W/m²•μm)
300
0
0
5
10
15 20
20
λ (μm)
(1) Find the spectral absorptivity distribution (αλ).
(2) Determine the total irradiation G on the surface
(3) Determine the radiant flux that is absorbed by the surface.
(4) The total absorptivity a.
as fast as.
Spectral hemispherical reflectivity distribution of an opaque surface is shown below. Surface is subjected to the spectral irradiation as shown.
1.0
400
a 0.4
200
5
10 15 20
a (um)
5
10
15
2 (um)
Calculate the total irradiation on the surface in W/m2
Calculate the irradiation absorbed by the surface in W/m2
3250 || 3750 || 4250|| 5000 || 5750|| 6250|| 7500
1050 || 1200
1400|| 1600|| 1750| 1850 || 2000
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