Chapter 1 Basics of Heat Transfer1-40Air 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. Assumptions1 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, . 3Constant 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. ΔΔkepe≅≅0Properties The gas constant of air is R = 0.287 kPa.m3/kg.K (Table A-1). Also, Cp= 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 volumesince 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 ΔΔmECVCVand ==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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