RADIATION
1) The temperature of a black surface 0.2 m2 in area is 540°C. Calculate:
i) The total rate of energy emission. ii) The intensity of normal radiation.
iii) The wavelength of maximum monochromatic emissive power.
2) Two concentric spheres of 20m and 30m are used to store liquid nitrogen at 120 K in a room at
300 K. The space between the spheres is made perfect vacuum. The surfaces are highly polished to
attain an emissivity of 0.04. Calculate the rate of evaporation of liquid nitrogen per hour if the latent
heat of nitrogen is 1255 kJ/kg.
3) Determine the radiation heat loss per meter length of 8 cm diameter steel pipe at 3000C, if
i) Enclosed in a 16 cm diameter red brick conduit at a temperature of 270C
II) Located in a large room with walls at a temperature of 270C. Take emissivity of steel pipe = 0.79
and emissivity of brick conduit = 0.93.
4) Determine radiation heat loss from each meter of 20 cm diameter heating pipe when it is placed
centrally in a brick duct of square section 30cm side. Temperature of pipe surface = 200ºC
Temperature of brick duct =20ºC. Ɛpipe=0.8 and Ɛbrick duct = 0.9.
5) To reduce the heat loss to surrounding by radiation, a furnace is surrounded by a steel screen. If
the temperature of external surface of the brickwork is 107ºC & that of screen is 57ºC. Estimate the
loss of heat per m² of the furnace area in Watts. Assume emissivity of brickwork to be 0.85 & that of
screen as 0.75. Also assume that annular space between brickwork & screen is negligible compared
to furnace dimensions
6) Calculate the net radiant heat exchange per unit area for two large parallel planes at
temperatures of 427°C and 27°C respectively. Take Ɛ for hot and cold planes to be 0.9 and 0.6
respectively. If a polished aluminium shield is placed between them, find the percentage reduction in
the heat transfer, given Ɛ for shield = 0.04.
7) Two large parallel plates with emissivities 0.5 and 0.8 are maintained at 809 K and 600 K
respectively. A radiation shield having an emissivity of 0.1 on one side and 0.05 on the other side is
placed in between. Calculate the heat transfer per unit area with and without the radiation shield.
8) Two large parallel plates with Ɛ = 0.5 each are maintained at different temperatures and are
exchanging heat only by radiation. Two equally large radiation shields with surface emissivity 0.05 are
introduced in parallel to the plates. Find the percentage reduction in net radiation heat transfer.
9) Two large parallel planes with emissivity 0.6 are at 900 K and 300 K. A radiation shield with one
side polished and having emissivity of 0.05 and the other side unpolished with emissivity of 0.4 is
proposed to be used between them. Which side of the shield should face the hotter plane, if the
temperature of the shield is to be kept minimum? Justify your answer. Also calculate the rate of heat
transfer and minimum temperature of the shield.
10) An enclosure measures 1.5 m x 1.7 m with a height of 2 m. The wall and ceiling are maintained at
2500C, and the floor at 1300C. The walls and ceiling have an emissivity of 0.82 and the floor 0.7. Find
the net radiation to the floor.
