The inner and outer surfaces of a 25-cm-thick wall in summer are at 27°C and 44°C, respectively. The outer surface of the wall exchanges heat by radiation with surrounding surfaces at 40°C and by convection with ambient air also at 40°C with a convection heat transfer coefficient of 8?W/m2•K. Solar radiation is incident on the surface at a rate of 150?W/m2. If both the emissivity and the solar absorptivity of the outer surface are 0.8, determine the effective thermal conductivity of the wall.
What will be an ideal response?
The outer surface of a wall is exposed to solar radiation. The effective thermal conductivity of the wall is to be determined.
Assumptions 1 Steady operating conditions exist. 2 The heat transfer coefficient is constant and uniform over the surface.
Properties Both the solar absorptivity and emissivity of the wall surface are given to be 0.8.
Analysis The heat transfer through the wall by conduction is equal to net heat transfer to the outer wall surface:
Solving for k gives
k = 0.961W/m•°C
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