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SP18-20 - ME300 Spring 2011 Special Problems SP18 A...

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ME300 Spring 2011 - Special Problems SP18 A piston-cylinder arrangement, as shown below, contains methane. Weight is slowly added to the piston, so that the methane is compressed isothermally during the process. The initial molar specific volume is 1 v =2.2522 m 3 /kmol and the temperature is T 1 =382 K. The gas is compressed isothermally to a final molar specific volume of 2 v =0.1284 m 3 /kmol. Do not use the ideal gas equation. Use the van der Waal’s equation of state to analyze the process. (a) Find the initial and final pressures, p 1 and p 2 [bar]. (b) Find the entropy change 2 1 s s [kJ/kmol K] for the process. Answers: (a) p 2 =232 bar, (b) -27.02 kJ/kmol K Hint: See Table A-24 for appropriate constants, and start part (b) with equation 11.48, page 590. SP19 An individual having a mass of 81.647 kg is ice skating on blades which have a total area in contact with the ice of 20.645 mm 2 . The temperature of the ice is -2 ºC. Will the ice melt under the blades? (Answers: T 2 = 270.3 K) Hints: 1) Look at figure 3.1b on page 83, and the solid liquid line.
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