An un-insulated air conditioning duct of rectangular cross-section 1m×0.5m, carrying air at 20 C with a velocity of 10 m/s, is exposed to an ambient of 30 C. Neglect the effect of duct construction material. For air in the range of 20−30 C, data are as follows: thermal conductivity =0.025 W/m.K; viscosity =18 μPa.s; Prandtl number =0.73; density =1.2 kg/m3.. The laminar flow Nusselt number is 3.4 for constant wall temperature conditions and, for turbulent flow, Nu=0.023*Re^0.8*Pr^0.33. The heat transfer per meter length of the duct, in watts, is _______ A. 3.8 B. 5.3 C. 89 D. 769
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