Thermodynamics HW Solutions 11

Thermodynamics HW Solutions 11 - Chapter 1 Basics of Heat...

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Chapter 1 Basics of Heat Transfer 1-30 A room is heated by the radiator, and the warm air is distributed by a fan. Heat is lost from the room. The time it takes for the air temperature to rise to 20°C is to be determined. Assumptions 1 Air is an ideal gas since it is at a high temperature and low pressure relative to its critical point values of -141 ° C and 3.77 MPa. 2 The kinetic and potential energy changes are negligible, . 3 Constant specific heats at room temperature can be used for air, C ΔΔ ke pe ≅≅ 0 p = 1.007 and C v = 0.720 kJ/kg·K. This assumption results in negligible error in heating and air-conditioning applications. 4 The local atmospheric pressure is 100 kPa. Properties The gas constant of air is R = 0.287 kPa.m 3 /kg.K (Table A-1). Also, C p = 1.007 kJ/kg·K for air at room temperature (Table A-15). Analysis We take the air in the room as the system. This is a closed system since no mass crosses the system boundary during the process. We observe
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