HW_16_Solution

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Solutions of homework assignment 16 1. (a) The ideal process for all three devices is the reversible adiabatic (i.e., isentropic) process. The adiabatic efficiencies of these devices are defined as energy kinetic exit c insentropi energy kinetic exit actual and , input work actual input work c insentropi , output work c insentropi output work actual N C T (b) Yes. Because the entropy of the fluid must increase during an actual adiabatic process as a result of irreversibilities. Therefore, the actual exit state has to be on the right hand side of the isentropic exit state 2. A steam power plant operates between the pressure limits of 5 MPa and 20 kPa. The ratio of the turbine work to the pump work is to be determined. Assumptions 1 Liquid water is an incompressible substance. 2 Kinetic and potential energy changes are negligible. 3 The process is reversible. 4 The pump and the turbine are adiabatic. Properties The specific volume of saturated liquid water at 20 kPa is v 1 = f @ 20 kPa = 0.001017 m 3 /kg (Table A 5). Analysis Both the compression and expansion processes are reversible and adiabatic, and thus isentropic, s 1 = s 2 and s 3 = s 4 . Then the properties of the steam are 4 @ 20 kPa 4 4 @ 20 kPa 3 3 34 2608.9 kJ/kg 20 kPa 7.9073 kJ/kg K . 5 MPa 4318.9 kJ/kg g g hh P ss sat vapor P h  Also, 1 = f @ 20 kPa = 0.001017 m 3 /kg.

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