Thermodynamics HW Solutions 298

Thermodynamics HW Solutions 298 - Chapter 4 Transient Heat...

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Chapter 4 Transient Heat Conduction 4-20 An iron whose base plate is made of an aluminum alloy is turned on. The time for the plate temperature to reach 140 ° C and whether it is realistic to assume the plate temperature to be uniform at all times are to be determined. Assumptions 1 85 percent of the heat generated in the resistance wires is transferred to the plate. 2 The thermal properties of the plate are constant. 3 The heat transfer coefficient is constant and uniform over the entire surface. Properties The density, specific heat, and thermal diffusivity of the aluminum alloy plate are given to be ρ = 2770 kg/m 3 , C p = 875 kJ/kg. ° C, and α = 7.3 × 10 -5 m 2 /s. The thermal conductivity of the plate can be determined from α = k /( ρ C p )= 177 W/m. ° C (or it can be read from Table A-3). Analysis The mass of the iron's base plate is Air 22 ° C IRON 1000 W mV L A == = = ρρ () ( . ) ( . ) . 2770 0 005 0 03 0 4155 kg / m m m kg 32 Noting that only 85 percent of the heat generated is transferred to the
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