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.
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(a) |
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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.
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Calculate: |
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(i) |
the total emissive power in kW/m2; |
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(ii) |
the total amount of radiation emitted by the ball in 3 minutes. |
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(b) |
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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: |
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(i) |
write the expression of the energy balance at the outer surface; |
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(ii) |
write the expression of conduction heat flux; |
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(iii) |
calculate the convection heat flux; |
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(iv) |
calculate the radiation heat flux; |
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(v) |
calculate the effective thermal conductivity of the wall. |
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