A plane wall connected in series and parallel combination having wall of surface area 1 m2 has three layers of thickness 0.4 m, 0.3 m, 0.1 m and has thermal conductivities 0.8, 0.2, 0.3, and 0.2 W/m°C, respectively. The middle wall area is bisected by the line as shown in the figure. There is no generation of thermal energy within the wall. If the inner and outer temperatures of the composite wall are 1540°C and 440°C, respectively. the rate of heat transfer through this wall is:
1
500 W
2
400 W
3
900 W
4
1000 W