A roof-cooling system, which operates by maintaining a thin film of water on the roof surface, may be used to reduce air-conditioning costs or to maintain a cooler environment in nonconditioned buildings. To determine the effectiveness of such a system, con- sider a sheet metal roof for which the solar absorptivity α s is 0.50 and the hemispherical emissivity ◰ is 0.3. Representative conditions correspond to a sur- face convection coefficient h of 20 W/m 2 ⋅ K , a solar irradiation G s of 700 W/m 2 , a sky temperature of − 10 ∘ C , an atmospheric temperature of 30 ∘ C , and a relative humidity of 65%. The roof may be assumed to be well insulated from below. Determine the roof surface temperature without the water film. Assuming the film and roof surface temperatures to be equal, determine the surface temperature with thefilm. The solar absorptivity and the hemispherical emissivity of the film−surface combination are α s = 0.8 and ε = 0.9 , respectively.
A roof-cooling system, which operates by maintaining a thin film of water on the roof surface, may be used to reduce air-conditioning costs or to maintain a cooler environment in nonconditioned buildings. To determine the effectiveness of such a system, con- sider a sheet metal roof for which the solar absorptivity α s is 0.50 and the hemispherical emissivity ◰ is 0.3. Representative conditions correspond to a sur- face convection coefficient h of 20 W/m 2 ⋅ K , a solar irradiation G s of 700 W/m 2 , a sky temperature of − 10 ∘ C , an atmospheric temperature of 30 ∘ C , and a relative humidity of 65%. The roof may be assumed to be well insulated from below. Determine the roof surface temperature without the water film. Assuming the film and roof surface temperatures to be equal, determine the surface temperature with thefilm. The solar absorptivity and the hemispherical emissivity of the film−surface combination are α s = 0.8 and ε = 0.9 , respectively.
A roof-cooling system, which operates by maintaining a thin film of water on the roof surface, may be used to reduce air-conditioning costs or to maintain a cooler environment in nonconditioned buildings. To determine the effectiveness of such a system, con- sider a sheet metal roof for which the solar absorptivity
α
s
is 0.50 and the hemispherical emissivity ◰ is 0.3. Representative conditions correspond to a sur- face convection coefficient
h
of
20 W/m
2
⋅
K
, a solar irradiation
G
s
of
700 W/m
2
, a sky temperature of
−
10
∘
C
, an atmospheric temperature of
30
∘
C
, and a relative humidity of 65%. The roof may be assumed to be well insulated from below. Determine the roof surface temperature without the water film. Assuming the film and roof surface temperatures to be equal, determine the surface temperature with thefilm. The solar absorptivity and the hemispherical emissivity of the film−surface combination are
α
s
=
0.8
and
ε
=
0.9
, respectively.
An astronaut performing an extra-vehicular activity(space walk) shaded from the Sun is wearing a spacesuitthat can be approximated as perfectly white (e = 0) exceptfor a 5 cm × 8 cm patch in the form of the astronaut’snational flag. The patch has emissivity 0.300. The spacesuitunder the patch is 0.500 cm thick, with a thermalconductivity k = 0.0600 W/m °C , and its inner surface isat a temperature of 20.0 °C . What is the temperature of thepatch, and what is the rate of heat loss through it? Assumethe patch is so thin that its outer surface is at the sametemperature as the outer surface of the spacesuit under it.Also assume the temperature of outer space is 0 K. You willget an equation that is very hard to solve in closed form,so you can solve it numerically with a graphing calculator,with software, or even by trial and error with a calculator.
Heat transfer problem.The internal surface area is an enclosure is 50 meter square. The surface is black and maintained at constant temperature. A small opening in the enclosure has area 0.05 meter square. The radiant power emitted from the opening is 52W. (A) what’s the temperature of the interior enclosure wall. (B)if the interior surface is maintained in this temperature, but polished so that emissivity is 0.15, what will be the radiant power emitted in the opening.
A dryer is shaped like a long semicylindrical duct of diameter 1.5 m. The base of the dryer is occupied with water soaked materials to be dried, and maintained at a temperature of 370 K and emissivity of 0.5. The dome of the dryer is maintained at 1000 K with emissivity of 0.8. Determine the drying rate per unit length experienced by the wet materials.
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