Notes_10_Control_Volume_Mass_Conservation0

Notes_10_Control_Volume_Mass_Conservation0 - Meeting...

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1 Meeting 10--Section 5-1
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Announcements Exam 1 Monday June 27th. 10 AM. Zach 104B. Chapters 1 – 4. Same format as previous semester exams posted on e-learning. Make sure to work the current HW 10. 2
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What we covered Monday Examples of First Law analysis for closed systems. Specific Heats. 3
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Example: 4-73 Argon is compressed in a polytropic process with n = 1.2 from 120 kPa and 10 ºC to 800 kPa in a piston-cylinder device. Determine the work produced and heat transferred during this compression process, in kJ/kg. 4
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5 Argon is compressed in a polytropic process. The work done and the heat transfer are to be determined. Assumptions 1 Argon is an ideal gas since it is at a high temperature and low pressure relative to its critical point values of 151 K and 4.86 MPa. 2 The kinetic and potential energy changes are negligible, 0 pe ke 2245 2245 . Properties The properties of argon are R = 0.2081kJ/kg K and c v = 0.3122 kJ/kg K (Table A-2 a ). Analysis We take argon as the system. This is a closed system since no mass crosses the boundaries of the system. The energy balance for this system can be expressed as ) ( 1 2 out , in energies etc. potential, kinetic, internal, in Change system mass and work, heat, by nsfer energy tra Net out in T T mc U W Q E E E b - = = - = - v        Using the boundary work relation for the polytropic process of an ideal gas gives kJ/kg 109.5 1 120 800 1.2 - 1 K) 283 )( K kJ/kg 2081 . 0 ( 1 1 2 . 1 / 2 . 0 / ) 1 ( 1 2 1 out , - = - = - - = - n n b P P n RT w Thus, kJ/kg 109.5 = in , b w The temperature at the final state is K .2 388 kPa 120 kPa 800 K) 283 ( 2 . 1 / 2 . 0 1 ( 1 2 1 2 = = = )/n n- P P T T From the energy balance equation, kJ/kg 6 . 76 K ) 283 K)(388.2 kJ/kg 3122 . 0 ( kJ/kg 5 . 109 ) ( 1 2 out , in - = - + - = - + = T T c w q b v Thus, kJ/kg 76.6 = out q Q Argon 120 kPa 10°C P v n = constant
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Teamplay: Problem 4-81 Air is contained in a cylinder device fitted with a piston-cylinder. The piston initially rests on a set of stops, and a pressure of 300 kPa is required to move the piston. Initially, the air is at 100 kPa and 27 ºC and occupies a volume of 0.4 m3. Determine the amount of heat transferred to the air, in kJ, while increasing the temperature to 1200 K. Assume air has constant specific heats evaluated at 300 K.
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