2. Two very long concentric cylinders of diameters D=0.35 m and D2=0.5 m are maintained at uniform temperatures of T1 = 950 K and T2 = 500 K and have emissivities ɛ1 = 1 and ɛ2 = 0.55, respectively. Determine the net rate of radiation heat transfer between the two cylinders per unit length of the cylinders. What happens if the properties are just reversed.

Could you give me some explanation, the correct answer is at the bottom of the question.

It's a complete question. Need help asap.

A composite wall is comprised of two large plates separated by sheets of refractory insulation. In the installation process, the sheets of thickness L = 50 mm and thermal conductivity k = 0.05 W/mK are separated at 1-m intervals by gaps of width w = 10 mm. The hot and cold plates have temperatures and emissivities of T1 = 400 deg C, emissivity1 = 0.85 and T2 = 35 deg C, emissivity2 = 0.5, respectively. Assume that the plates and insulation are diffuse-gray surfaces. %3D Determine the heat loss by radiation through the gap per unit length of the composite wall (normal to the page). Recognizing that the gaps are located on a 1-m spacing, determine what fraction of the total heat loss through the composite wall is due to transfer by radiation through the insulation gap. Hot side Gap w = 10 mm A. 47 W/m, 9.2% T1 = 400°C B. 47 W/m, 10.2% L = 50 mm C. 37 W/m, 10.2% D. 37 W/m, 9.2% T2 = 35°C Cold side 1 m Insulation, k = 0.05 W/m-K