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# Sphw7a - HOMEWORK SET 7a 8.105 A piston/cylinder has...

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HOMEWORK SET 7a 8.105 A piston/cylinder has ammonia at 2000 kPa, 80 o C with a volume of 0.1 m 3 . The piston is loaded with a linear spring and outside ambient is at 20 o C, shown in Fig. P8.71. The ammonia now cools down to 20 o C at which point it has a quality of 10%. Find the work, the heat transfer and the total entropy generation in the process. CV Ammonia out to the ambient, both 1 Q 2 and 1 W 2 Energy Eq.5.11: m(u 2 u 1 ) = 1 Q 2 1 W 2 Entropy Eq.8.37: m(s 2 - s 1 ) = 1 Q 2 / T ambient + 1 S 2 gen Process: P = A + BV => 1 W 2 = P dV = ½ m(P 1 + P 2 ) (v 2 - v 1 ) State 1: Table B.2.2 v 1 = 0.07595 m 3 /kg, u 1 = 1421.6 kJ/kg, s 1 = 5.0707 kJ/kg K m = V 1 /v 1 = 0.1/0.07595 = 1.31665 kg State 2: Table B.2.1 v 2 = 0.001638 + 0.1 × 0.14758 = 0.016396 m 3 /kg u 2 = 272.89 + 0.1 × 1059.3 =378.82 kJ/kg s 2 = 1.0408 + 0.1 × 4.0452 = 1.44532 kJ/kg K 1 W 2 = ½ m(P 1 + P 2 )( v 2 - v 1 ) = ½ × 1.31665 (2000 + 857.5)( 0.016396 – 0.07595) = - 112 kJ 1 Q 2 = m(u 2 u 1 ) + 1 W 2 = 1.31665 (378.82 – 1421.6) –112 = - 1484.98 kJ 1 S 2 gen = m(s 2 s 1 ) – ( 1 Q 2 / T amb ) = 1.31665 (1.44532 – 5.0707) – –1484.98 293.15 = – 4.77336 + 5.0656 = 0.292 kJ/K P v 1 2 2 1 T s P 2

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