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ps1 - ME 322 PS#1 21.02.2008 1 The walls of a refrigerator...

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ME 322 PS #1 21.02.2008 1. The walls of a refrigerator are typically constructed by sandwiching a layer of insulation between sheet metal panels. Consider a wall made from fiberglass insulation of thermal conductivity k i = 0.046 W/m K and thickness L i = 50 mm and steel panels, each of thermal conductivity k p = 60 W/m K and thickness L p = 3 mm. If the wall separates refrigerated air at T ,i = 4ºC from ambient air at T ,o = 25ºC, what is the heat gain per unit surface area? Coefficients associated with natural convection at the inner and outer surfaces may be approximated as h i = h o = 5 W/m 2 K. (Problem 3.5) 2. Consider a plane composite wall that is composed of two materials of thermal conductivities k A = 0.1 W/m K and k B = 0.04 W/m K and thicknesses L A = 10 mm and L B = 20 mm. The contact resistance at the interface between the two materials is known to be 0.30 m 2 K/W. Material A adjoins a fluid at 200ºC for which h = 10 W/m 2 K, and material B adjoins a fluid at 40ºC for which h = 20 W/m 2 K. (Problem 3.22) (1) What is the rate of heat transfer through a wall that is 2 m high by 2.5 m wide?
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