Thermodynamics HW Solutions 279

Thermodynamics HW Solutions 279 - 3 2 1 total 2 2 2 conv,2...

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Chapter 3 Steady Heat Conduction 3-168 A wall constructed of three layers is considered. The rate of hat transfer through the wall and temperature drops across the plaster, brick, covering, and surface-ambient air are to be determined. Assumptions 1 Heat transfer is steady. 2 Heat transfer is one-dimensional. 3 Thermal conductivities are constant. 4 Heat transfer by radiation is accounted for in the heat transfer coefficient. Properties The thermal conductivities of the plaster, brick, and covering are given to be k = 0.72 W/m °C, k = 0.36 W/m °C, k = 1.40 W/m °C, respectively. Analysis The surface area of the wall and the individual resistances are 2 m 8 . 16 m) 8 . 2 ( m) 6 ( = × = A C/W 02253 . 0 00350 . 0 00085 . 0 01653 . 0 00165 . 0 C/W 00350 . 0 ) m 8 . 16 ( C) . W/m 17 ( 1 1 C/W 00085 . 0 ) m 8 . 16 ( C) W/m. 4 . 1 ( m 02 . 0 C/W 01653 . 0 ) m 8 . 16 ( C) W/m. 72 . 0 ( m 20 . 0 C/W 00165 . 0 ) m 8 . 16 ( C) W/m. 36 . 0 ( m 01 . 0 conv,2
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Unformatted text preview: 3 2 1 total 2 2 2 conv,2 o 2 3 3 covering 3 2 2 2 brick 2 2 1 1 plaster 1 ° = + + + = + + + = ° = ° = = = ° = ° = = = ° = ° = = = ° = ° = = = R R R R R A h R R A k L R R A k L R R A k L R R T 1 T ∞ 2 R 1 R 2 R 3 R o The steady rate of heat transfer through the wall then becomes W 665.8 = ° ° − = − = ∞ C/W 02253 . C ) 8 23 ( total 2 1 R T T Q & The temperature drops are C 2.3 C 0.6 C 11.0 C 1.1 ° = ° = = Δ ° = ° = = Δ ° = ° = = Δ ° = ° = = Δ ) C/W 00350 . )( W 8 . 665 ( ) C/W 00085 . )( W 8 . 665 ( ) C/W 01653 . )( W 8 . 665 ( ) C/W 00165 . )( W 8 . 665 ( conv conv covering covering brick brick plaster plaster R Q T R Q T R Q T R Q T & & & & 3-122...
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This note was uploaded on 01/19/2012 for the course PHY 4803 taught by Professor Dr.danielarenas during the Fall '10 term at UNF.

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