1. A tube bank uses an aligned arrangement of 10-mm- diameter tubes with ST = SL = 20 mm. There are 10 rows of tubes with 50 tubes in each row. Consider an application for which cold water flows through the tubes, maintaining the outer surface temperature at 27°C, while flue gases at 427°C and a velocity of 5 m/s are in cross flow over the tubes. The properties of the flue gas may be approximated as those of atmospheric air at 427°C. What is the total rate of heat transfer per unit length of the tubes in the bank?
2. An air duct heater consists of an aligned array of elec- trical heating elements in which the longitudinal and transverse pitches are SL = ST = 24 mm. There are 3 rows of elements in the flow direction (NL = 3) and 4 elements per row (NT = 4). Atmospheric air with an upstream velocity of 12 m/s and a temperature of 25°C moves in cross flow over the elements, which have a diameter of 12 mm, a length of 250 mm, and are main- tained at a surface temperature of 350°C.
(a) Determine the total heat transfer to the air and the temperature of the air leaving the duct heater.
(b) Determine the pressure drop across the element bank and the fan power requirement.
(c) Compare the average convection coefficient obtained in your analysis with the value for an isolated (single) element. Explain the difference between the results.
(d) What effect would increasing the longitudinal and transverse pitches to 30 mm have on the exit tem- perature of the air, the total heat rate, and the pres- sure drop?