Te 15 C Uninsulated Top 3 Mm Thick Insulated Sides And Bottom 0 4 M 15 M Warm Air Is Flowing Through An Exposed Rectan 1 (68.09 KiB) Viewed 18 times
TE=15°C Uninsulated top, 3 mm thick Insulated sides and bottom 0.4 m 15 m Warm air is flowing through an exposed, rectangular PVC duct (k=0.2 W/m K) that is 0.4 m wide x 0.2 m tall. The PVC wall are 3 mm thick. The duct is well insulated on three sides, but the insulation has been removed on the top. The duct passes through a 15-m-long section of basement, which is at a temperature of 15°C and which exhibits h=20 W/m2 K. The convection coefficient in the interior of the duct is 50 W/m2 K. Take the specific heat of air as 1.0 kJ/kg K. Initially, it is determined that the moving air inside the duct is at a uniform temperature of approximately 28°C and heat is lost only through the top of the duct.
Question 6 5 pts Determine the thermal resistance due to conduction through the top wall of the duct (in K/W). Question 7 5 pts Determine the thermal resistance between the internal flow in the duct and the inner wall due to convection in K/W). Question 8 5 pts Determine the thermal resistance between the outer wall of the exposed duct section and the external flow due to convection (in K/W).
Question 9 10 pts Determine the rate of heat loss (in W) through the top of the duct. Again, assume a uniform temperature of the interior air of 28°C. Neglect radiation losses. Question 10 10 pts Upon closer inspection, an engineer notices that the temperature of the air drops from 30°C to 26°C as it passes through the 15 m section. Assuming that the mass flow rate through the duct is measured to be 0.23 kg/s, determine the heat loss (in W) through the duct. (Hint: you should use a different approach than in the previous parts of this problem).
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