A test is carried out to determine the overall heat transfer coefficient in a new automotive radiator that is a compact cross-flow water-to-air heat exchanger with both fluids (air and water) unmixed. The radiator has 40 tubes of internal diameter 5 mm and length 650 mm, in a closely spaced plate-finned matrix. Hot water enters the tubes at 90°C and a rate of 0·6 kg/s and leaves at 65°C. Air flows across the radiator through the interfin spaces and is heated from 20°C to 40°C.
(a)Determine the overall heat transfer coefficient, Ui, of this radiator based on the inner surface area of the tubes.20
(b)Explain why the required value of Uiis based on water side, rather than the air side.
(c)Discuss the changes that might occur in this value overthe life-time of the radiator, and the causes of them.
