Thermodynamics HW Solutions 36

Thermodynamics HW Solutions 36 - Chapter 1 Basics of Heat...

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Chapter 1 Basics of Heat Transfer 1-77 An electric resistance heating element is immersed in water initially at 20°C. The time it will take for this heater to raise the water temperature to 80°C as well as the convection heat transfer coefficients at the beginning and at the end of the heating process are to be determined. Assumptions 1 Steady operating conditions exist and thus the rate of heat loss from the wire equals the rate of heat generation in the wire as a result of resistance heating. 2 Thermal properties of water are constant. 3 Heat losses from the water in the tank are negligible. Properties The specific heat of water at room temperature is C = 4.18 kJ/kg ⋅° C (Table A-2). Analysis When steady operating conditions are reached, we have . This is also equal to the rate of heat gain by water. Noting that this is the only mechanism of energy transfer, the time it takes to raise the water temperature from 20 ° C to 80 ° C is determined to be && QE == generated 800 W 120 ° C water 800 W h 5.225 = = ° ° =
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This note was uploaded on 01/14/2012 for the course PHY 4803 taught by Professor Dr.danielarenas during the Fall '10 term at UNF.

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