Thermodynamics HW Solutions 19

Thermodynamics HW Solutions 19 - Chapter 1 Basics of Heat...

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Chapter 1 Basics of Heat Transfer 1-40 Air is moved through the resistance heaters in a 1200-W hair dryer by a fan. The volume flow rate of air at the inlet and the velocity of the air at the exit are 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. 4 The power consumed by the fan and the heat losses through the walls of the hair dryer are negligible. ΔΔ ke pe ≅≅ 0 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 ( a ) We take the hair dryer as the system. This is a control volume since mass crosses the system boundary during the process. We observe that this is a steady-flow process since there is no change with time at any point and thus Δ Δ mE CV CV and = = 00 & , and there is only one inlet and one exit and thus . The energy balance for this steady-flow system can be expressed in the rate form as
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