Question

a. Discuss why car inside temperature is high when you park the car outside in a sunny day even under the shade and the relation with glass transmissivity (Greenhouse effect). [5 marks]

b. A double walled spherical tank is used to store iced water at O'C. The tank internal diameter is 2 m and wall thickness is 0.5 cm. The inner and outer walls are separated with the 2 cm thick space. The air between the two walls of the tank is evacuated in order to minimize the heat transfer. The surfaces surrounding the evacuated space are polished so that each surface has an emissivity, e = 0.15. The temperature of the outer wall of the tank is measured to be 20"C. Assume the inner wall of the steel tank is at 0"C and conduction resistances of the walls are negligible. Determine:

The rate of heat transfer to the iced water írÍ the tank [5 marks]

ii. The amount of ice at 0"C that melts during a 24 h period. [5 marks]

c. A thermocouple is used to measure the temperature of hot air flowing in a duct, whose walls are maintained at 000 K shows a temperature reading of 700 K. lf the emissívity of the thermocouple junction ís 0.6 and the convection heat transfer coefficient is 100 Wlm2.K, determine the actual temperature of air. [5 marks]

Answer 1

(a)The heating of the car relates to the same phenomenon as that occurs in the heating of the Earth which is mainly to the greenhouse effect--- the phenomenon in which the sunlight passes through the glass having very energetic photons whose equilibrium temperature is upto 5500 degrees, but the surfaces in the interior re-emits that light as photons whose inner temperature is same as that of the interior temperature. As a result the interior will have a higher temperature, in which the outgoing thermal power becomes equal to the incoming non-thermal solar radiation.