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Thermodynamics HW Solutions 688

# Thermodynamics HW Solutions 688 - Chapter 8 Internal Forced...

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Chapter 8 Internal Forced Convection 8-67 Air 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. Assumptions 1 Steady flow conditions exist. 2 The inner surfaces of the duct are smooth. 3 The thermal resistance of the duct is negligible. 4 Air is an ideal gas with constant properties. 5 The pressure of air is 1 atm. Properties We 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 . 0 Pr C J/kg. 1007 /s m 10 516 . 1 C W/m. 02514 . 0 kg/m 204 . 1 2 5 - 3 = ° = × = ° = = p C k υ ρ Water 25 ° C 3 m/s River water 15 ° C L = 15 m D = 20 cm Mineral deposit 0.15 mm Analysis The Reynolds number is 4 2 5 10 959 . 3 /s m 10 516 . 1 m) m/s)(0.2 (3 Re × = × = υ = h m D V which is greater than 4000. Therefore, the flow is turbulent and the entry lengths in this case are roughly
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