Question

Atmospheric air enters a thin walled (D = 5 mm) long tube (L = 2 m) at an inlet temperature of 100°C. An electric heater, that draws constant power, is

wrapped around the tube heating the surface of the air in the tube. Assume all the heat from the heater goes into heating the air in the tube. If the mass flow rate of air is 135 x 106 kg/s and the tube surface temperature at the exit of the tube is 160°C, determine the heat rate entering the tube. For air at this temperature: µ = 230.1 x 107 N.s/m?, Pr = 0.690 and k =0.0338W/m-K and c, = 1014 J/kg-K.

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