Heat transfer by radiation occurs between two large parallel plates, which are maintained at temperatures T 1 and T 2 , with T 1 > T 2 . To reduce the rate of heat transfer between the plates, it is proposed that they be separated by a thin shield that has different emissivities on opposite surfaces. In particular, one surface has the emissivity ε s < 0.5 , while the opposite surface has an emissivity of 2 ε s . (a) How should the shield be oriented to provide the larger reduction in heat transfer between the plates? That is, should the surface of emissivity ε s or that of emissivity 2 ε s be oriented toward the plate at T 1 ? (b) What orientation will result in the larger value of the shield temperature T s ?
Heat transfer by radiation occurs between two large parallel plates, which are maintained at temperatures T 1 and T 2 , with T 1 > T 2 . To reduce the rate of heat transfer between the plates, it is proposed that they be separated by a thin shield that has different emissivities on opposite surfaces. In particular, one surface has the emissivity ε s < 0.5 , while the opposite surface has an emissivity of 2 ε s . (a) How should the shield be oriented to provide the larger reduction in heat transfer between the plates? That is, should the surface of emissivity ε s or that of emissivity 2 ε s be oriented toward the plate at T 1 ? (b) What orientation will result in the larger value of the shield temperature T s ?
Solution Summary: The author explains that the heat transfer rate in this case does not depend on orientation. The emissivity of opposite surface is 2epsilon .
Heat transfer by radiation occurs between two large parallel plates, which are maintained at temperatures T1and T2, with
T
1
>
T
2
. To reduce the rate of heat transfer between the plates, it is proposed that they be separated by a thin shield that has different emissivities on opposite surfaces. In particular, one surface has the emissivity
ε
s
<
0.5
, while the opposite surface has an emissivity of
2
ε
s
. (a) How should the shield be oriented to provide the larger reduction in heat transfer between the plates? That is, should the surface of emissivity
ε
s
or that of emissivity
2
ε
s
be oriented toward the plate at T1? (b) What orientation will result in the larger value of the shield temperature Ts?
Two surfaces make up an enclosure where surface 1 is flat and has area A1,temperature T1 and emissivity 1. Surface 2 is black and has temperature T2. Showthat the net power transfer rate (net heat flux) in W/m2 at surface 1 is given by
Two parallel plates 0.5 m by 1.0 m are spaced 150 mm apart. One plate is maintained at 1,000 C and the other at 500 C. The emissivities of the plates are 0.20 and 0.50, respectively. The plates exchange heat with each other and only the plate surfaces facing each other are to be considered in the analysis. Find the net heat transfer rate to each other. Neglect other modes of heat transfer (conduction and convection). View Factor is based on the case shown below:
2.
(a)
Consider a 25-cm-diameter spherical ball at 700 K suspended in air and assume the emissivity of the ball to be ε=0.95.
Calculate:
(i)
the total emissive power in kW/m2;
(ii)
the total amount of radiation emitted by the ball in 3 minutes.
(b)
The inner and outer surfaces of a 25-cm-thick wall are at 27 oC and 45 oC, respectively. The outer surface of the wall exchanges heat by radiation with surrounding surfaces at 40 oC, and convection with ambient air at 42 oC with convection heat transfer coefficient of 9.0 W/m2 K. Solar radiation incident on the surface is at a rate of 150 W/m2. If the emissivity and the solar absorptivity of the outer surface are 0.75 and 0.85, respectively:
(i)
write the expression of the energy balance at the outer surface;…
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