Chapter 8 Internal Forced Convection8-67Air enters the underwater section of a duct. The outlet temperature of the air and the fan power needed to overcome the flow resistance are to be determined. Assumptions1Steady flow conditions exist. 2The inner surfaces of the duct are smooth. 3The thermal resistance of the duct is negligible. 4Air is an ideal gas with constant properties. 5 The pressure of air is 1 atm. PropertiesWe assume the bulk mean temperature for air to be 20°C since the mean temperature of air at the inlet will drop somewhat as a result of heat loss through the duct whose surface is at a lower temperature. The properties of air at 1 atm and this temperature are (Table A-15) 7309.0PrCJ/kg.1007/sm10516.1CW/m.02514.0kg/m204.125-3=°=×=°==pCkυρWater 25°C 3 m/s River water 15°C L= 15 mD = 20 cm Mineral deposit0.15 mm AnalysisThe Reynolds number is 42510959.3/sm10516.1m)m/s)(0.2(3Re×=×=υ=−hmDVwhich is greater than 4000. Therefore, the flow is turbulent and the entry lengths in this case are roughly
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