Consider a 20 m long section of a cylindrical thin-wall duct with a diameter of 20 cm. hot air enters this duct at 70 c with an average velocity of 5 m/s. the outer surface of the duct is exposed to air at t? =15 c. assuming that: ? the duct surface is at an average temperature of 40 c throughout ? entrance region is very short compared with the length of the duct ? radiation from the outer surface is negligible find: a. the exit temperature of the air from this section of the duct. b. the heat transfer coefficient between the duct and air surrounding the duct.