Thermodynamics HW Solutions 189

Thermodynamics HW Solutions 189 - Chapter 15 Steady Heat...

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Chapter 15 Steady Heat Conduction 3-56E A wall is to be constructed using solid bricks or identical size bricks with 9 square air holes. There is a 0.5 in thick sheetrock layer between two adjacent bricks on all four sides, and on both sides of the wall. The rates of heat transfer through the wall constructed of solid bricks and of bricks with air holes are to be determined. Assumptions 1 Heat transfer is steady since there is no indication of change with time. 2 Heat transfer through the wall is one-dimensional. 3 Thermal conductivities are constant. 4 Heat transfer coefficients account for the radiation heat transfer. Properties The thermal conductivities are given to be k = 0.40 Btu/h ft °F for bricks, k = 0.015 Btu/h ft °F for air, and k = 0.10 Btu/h ft °F for sheetrock. Analysis ( a ) The representative surface area is . The thermal resistance network and the individual thermal resistances if the wall is constructed of solid bricks are A == (./ ) . 7 5 12 7 5 12 0 3906 ft 2 F/Btu h 57 . 308 ft )] 12 / 5 . 0 ( ) 12 / 7 ( F)[ Btu/h.ft. 10 . 0 ( ft 12 / 9 F/Btu h 288 ft )] 12 / 5 . 0 ( ) 12 / 5 . 7 ( F)[
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