8-134 8-181 Air is compressed in a piston-cylinder device. It is to be determined if this process is possible. Assumptions 1 Changes in the kinetic and potential energies are negligible. 4 Air is an ideal gas with constant specific heats. 3 The compression process is reversible. Properties The properties of air at room temperature are R = 0.287 kPam3/kgK, cp = 1.005 kJ/kgK (Table A-2a). Analysis We take the contents of the cylinder as the system. This is a closed system since no mass enters or leaves. The energy balance for this stationary closed system can be expressed as E E in out Net energy transfer by heat, work, and mass 'E system Change in internal, kinetic, potential, etc. energies Wb,in Qout 'U Wb,in Qout mc p (T2 T1 ) Wb,in Qout 0 Qout m(u 2 u1 ) (since T2 Air 100 kPa 27°C T1 ) Wb,in Heat The work input for this isothermal, reversible process is win RT ln P2 P1 q out win 78.89 kJ/kg (0.287 kJ/kg K)(300 K)ln 250 kPa 100 kPa 78.89 kJ/kg That is, The entropy change of air during this isothermal process is 's air c p ln T2 P R ln 2 T1 P1 R ln P2 P1 (0.287 kJ/kg K)ln 250 kPa 100 kPa 0.2630 kJ/kg K The entropy change of the reservoir is 's R qR TR 78.89 kJ/kg 300 K 0.2630 kJ/kg K Note that the sign of heat transfer is taken with respect to the reservoir. The total entropy change (i.e., entropy generation) is the sum of the entropy changes of air and the reservoir: 's total 's air 's R 0.2630 0.2630 0 kJ/kg K Not only this process is possible but also completely reversible. PROPRIETARY MATERIAL. © 2008 The McGraw-Hill Companies, Inc. Limited distribution permitted only to teachers and educators for course preparation. If you are a student using this Manual, you are using it without permission.

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# 8-134 constant specific heats. c