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 ki=0.046 W/m K and thickness Li=50 mm and steel panels, each of thermal conductivity kp= 60 W/m K and thickness Lp= 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 hi= ho=5 W/m2K. (Problem 3.5) 2.Consider a plane composite wall that is composed of two materials of thermal conductivities kA= 0.1 W/m K and kB= 0.04 W/m K and thicknesses LA=10 mm and LB=20 mm. The contact resistance at the interface between the two materials is known to be 0.30 m2K/W. Material A adjoins a fluid at 200ºC for which h =10 W/m2K, and material B adjoins a fluid at 40ºC for which h =20 W/m2K. (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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