If the system boundary is taken to be the outer

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Unformatted text preview: determine whether a process is internally reversible. A process is internally reversible if no entropy is generated within the system boundaries. Therefore, the heat transfer process discussed in Example 7–21 is internally reversible if the inner surface of the wall is taken as the system boundary, and thus the system excludes the container wall. If the system boundary is taken to be the outer surface of the container wall, then the process is no longer internally reversible since the wall, which is the site of entropy generation, is now part of the system. For thin walls, it is very tempting to ignore the mass of the wall and to regard the wall as the boundary between the system and the surroundings. This seemingly harmless choice hides the site of the entropy generation from view and is a source of confusion. The temperature in this case drops suddenly from Tsys to Tsurr at the boundary surface, and confusion arises as to which temperature to use in the relation Q/T for entropy transfer at the boundary. Note that if the system and the surrounding air are not isothermal as a result of insufficient mixing, then part of the entropy generation will occur in both the system and the surrounding air in the vicinity of the wall, as shown in Fig. 7–70c. SYSTEM SURROUNDING Tsys Boundary Wall Tsys Tsurr Heat transfer Entropy transfer Q Tsurr Q Q Sgen Q Tsys (a) The wall is ignored Q Tsurr Wall Tsys Q Tsys Q Tsurr Q Loca...
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