a) A black body having surface area 0.035 m is enclosed in an enclosure bounded by 50 mm thick wall and having surface area of 0.6 m. The inner surface of the enclosure is maintained at 235C while the outer surface is at 33C. It can be assumed that the inner and outer surface areas of the enclosure are equal and at steady state conditions. Determine the following: The net radiant energy emitted by the black body. (3 marks) (ii) The radiant energy conducted through the enclosure walls. (iii) The temperature of the black body. (3 marks) (4 marks) Data: Stefan-Boltzmann constant = 5.67 x 10* W/(m K") Thermal conductivity of enclosure wall = 0.9 W/(m.K) Ambient air temperature = 33C. b) A thermos flask is made up of two concentric cylinders of diameter 75 mm and 88 mm and 250 mm length. The space between the two cylinders is evacuated to reduce the heat loss. The outer surface of inner cylinder and inner surface of outer cylinder are silver polished. The contents of the thermos flask are at 96C and the ambient air temperature is 27C. Determine the following: The rate of heat loss of the flask. (7 marks) (11) If the space between the two cylinders is replaced by cork, how much thickness of cork will be required to have the same heat loss as is produced by vacuum. Data: Stefan-Boltzmann constant o = 5.67 x 10* W/(m* K*) (3 marks) Thermal conductivity of cork = 0.042 W/(m.K) Emissivity of silver polished surfaces = 0.025.