CHAPTER7_SECTION6 - Chapter 7 Entropy Review Problems...

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Chapter 7 Entropy Review Problems (cont.) 7-154 The validity of the Clausius inequality is to be demonstrated using a reversible and an irreversible heat engine operating between the same temperature limits. Analysis Consider two heat engines, one reversible and one irreversible, both operating between a high- temperature reservoir at T H and a low-temperature reservoir at T L . Both heat engines receive the same amount of heat, Q H . The reversible heat engine rejects heat in the amount of Q L , and the irreversible one in the amount of Q L, irrev = Q L + Q diff , where Q diff is a positive quantity since the irreversible heat engine produces less work. Noting that Q H and Q L are transferred at constant temperatures of T H and T L , respectively, the cyclic integral of δ Q/T for the reversible and irreversible heat engine cycles become 0 1 1 = - = - = - = L L H H L L H H L L H H rev T Q T Q Q T Q T T Q T Q T Q since ( Q H /T H ) = ( Q L /T L ) for reversible cycles. Also, 0 , < - = - - = - = L diff L diff L L H H L irrev L H H irr T Q T Q T Q T Q T Q T Q T Q since Q diff is a positive quantity. Thus, 0 T Q . 7-126 T L T H Irrev HE Q H Q L , irrev · Q H · Q L · W net, irrev · W net, rev · Rev HE
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Chapter 7 Entropy 7-155 The inner and outer surfaces of a window glass are maintained at specified temperatures. The amount of heat transfer through the glass and the amount of entropy generation within the glass in 5 h are to be determined Assumptions 1 Steady operating conditions exist since the surface temperatures of the glass remain constant at the specified values. 2 Thermal properties of the glass are constant. Analysis The amount of heat transfer over a period of 5 h is kJ 57,600 = × = = ) s 3600 5 )( kJ/s 3.2 ( t Q Q cond We take the glass to be the system, which is a closed system. Under steady conditions, the rate form of the entropy balance for the glass simplifies to W/K 0.287 = = + - = + - = = + - glass gen, wall gen, glass gen, , , entropy of change of Rate 0 system generation entropy of Rate mass and heat by ansfer entropy tr net of Rate 0 K 276 W 3200 K 283 W 3200 0 0 S S S T Q T Q S S S S out b out in b in gen out in     Then the amount of entropy generation over a period of 5 h becomes J/K 5160 = × = = s) 3600 W/K)(5 (0.287 glass gen, glass gen, t S S 7-127 Glass 3 ° C 10 ° C
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Chapter 7 Entropy 7-156 Two rigid tanks that contain water at different states are connected by a valve. The valve is opened and steam flows from tank A to tank B until the pressure in tank A drops to a specified value. Tank B loses heat to the surroundings. The final temperature in each tank and the entropy generated during this process are to be determined. Assumptions
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This homework help was uploaded on 04/18/2008 for the course EML 3007 taught by Professor Chung during the Spring '08 term at University of Florida.

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CHAPTER7_SECTION6 - Chapter 7 Entropy Review Problems...

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