Two concentric spheres of diameters D1 = 15 cm and D2 = 25 cm are separated by air at 1 atm pressure. The surface temperatures of the two spheres enclosing the air are T1 = 350 K and T2 = 275 K, respectively, and their emissivities are 0.75. Determine the rate of heat transfer from the inner sphere to the outer sphere by (a) natural convection and (b) radiation.

2_5190874259647370... A cylindrical shape is with diameter of 1.2m and length of 1.2m. The top surface has an emissivity of 0.7 and temperature 500 K. The bottom surface has an emissivity of o.5 and temperature 650 K. The side surface has an emissivity of 0.4. Determine the shape factors for all surfaces. E 0.0 L=650 K II >

2. Consider steady heat transfer between two large parallel plates at constant temperatures of T1 = 295 K and T2 = 155 K that are L = 2 cm apart. Assuming the surfaces to be black, determine the rate of heat transfer between the plates per unit surface area assuming the gap between the plates is (a) filled with atmospheric air,(k = 0.01979 W/m- o C) (b) evacuated,(c) filled with superinsulation having an apparent thermal conductivity of 0.00015 W/m · °C.

This experiment is conducted to determine the emissivity of a certain material. A long cylindrical rod of diameter D₁ = 0.01 m is coated with this new material and is placed in an evacuated long cylindrical enclosure of diameter D₂ = 0.1 m and emissivity 2 = 0.95, which is cooled externally and maintained at a temperature of 200 K at all times. The rod is heated by passing the electric current through it. When steady operating conditions are reached, it is observed that the rod is dissipating electric power at a rate of 16 W per unit of its length, and its surface temperature is 600 K. Based on these measurements, determine the emissivity of the coating on the rod. The emissivity of the coating on the rod is 0.1165