tutorial6 - Indian Institute of Technology Kharagpur...

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1 Indian Institute of Technology Kharagpur Department of Mechanical Engineering Thermodynamics ME22002 Tutorial 6 (Exergy) 1. A piston cylinder device initially contains 2 L of air at 100 kPa and 25°C. Air is now compressed to a final state of 600 kPa and 150°C. The useful work input is 1.2 kJ. Assuming the surroundings are at 100 kPa and 25°C, determine (a) the exergy of the air at the initial and the final states, (b) the minimum work that must be supplied to accomplish this compression process, and (c) the second-law efficiency of this process. Answers: (a) 0, 0.171 kJ, (b) 0.171 kJ, (c) 14.3 percent 2. An iron block of unknown mass at 85°C is dropped into an insulated tank that contains 100 L of water at 20°C. At the same time, a paddle wheel driven by a 200- W motor is activated to stir the water. It is observed that thermal equilibrium is established after 20 min with a final temperature of 24°C. Assuming the surroundings to be at 20°C, determine (a) the mass of the iron block and (b) the exergy destroyed during this process. Answers: (a) 52.0 kg, (b) 375 kJ 3. Refrigerant-134a at 1 MPa and 100°C is throttled to a pressure of 0.8 MPa. Determine the

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