An un-insulated air conditioning duct of rectangular cross section 1 m × 0.5 m, 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°C-30°C, data are as follows: thermal conductivity =0.025 W/m.K; viscosity = 18$\mu$ Pa.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 metre length of the duct is