8-15. O A power plant stack is 100 m high and has an inside diameter of 6 m. The wall is made of concrete and varies in thickness linearly from 50 cm at the bottom to 25 cm at the top. Exhaust gas enters the base of the stack at a rate of 100 kg/s and at a temperature of 600 K. Air at 300 K and 1 bar blows at 5 m/s across the stack. Determine the exit temperature of the stack gas.