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cen84959_ch07

# cen84959_ch07 - Chapter 7 ENTROPY n Chap 6 we introduced...

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Chapter 7 ENTROPY | 337 I n Chap. 6, we introduced the second law of thermody- namics and applied it to cycles and cyclic devices. In this chapter, we apply the second law to processes. The first law of thermodynamics deals with the property energy and the conservation of it. The second law leads to the definition of a new property called entropy . Entropy is a somewhat abstract property, and it is difficult to give a physical descrip- tion of it without considering the microscopic state of the sys- tem. Entropy is best understood and appreciated by studying its uses in commonly encountered engineering processes, and this is what we intend to do. This chapter starts with a discussion of the Clausius inequality, which forms the basis for the definition of entropy, and continues with the increase of entropy principle. Unlike energy, entropy is a nonconserved property, and there is no such thing as conservation of entropy . Next, the entropy changes that take place during processes for pure sub- stances, incompressible substances, and ideal gases are dis- cussed, and a special class of idealized processes, called isentropic processes , is examined. Then, the reversible steady-flow work and the isentropic efficiencies of various engineering devices such as turbines and compressors are considered. Finally, entropy balance is introduced and applied to various systems. Objectives The objectives of Chapter 7 are to: Apply the second law of thermodynamics to processes. Define a new property called entropy to quantify the second-law effects. Establish the increase of entropy principle . Calculate the entropy changes that take place during processes for pure substances, incompressible substances, and ideal gases. Examine a special class of idealized processes, called isentropic processes , and develop the property relations for these processes. Derive the reversible steady-flow work relations. Develop the isentropic efficiencies for various steady-flow devices. Introduce and apply the entropy balance to various systems.

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7–1 ENTROPY The second law of thermodynamics often leads to expressions that involve inequalities. An irreversible (i.e., actual) heat engine, for example, is less efficient than a reversible one operating between the same two thermal energy reservoirs. Likewise, an irreversible refrigerator or a heat pump has a lower coefficient of performance (COP) than a reversible one operating between the same temperature limits. Another important inequality that has major consequences in thermodynamics is the Clausius inequality . It was first stated by the German physicist R. J. E. Clausius (1822–1888), one of the founders of thermodynamics, and is expressed as That is, the cyclic integral of d Q/T is always less than or equal to zero . This inequality is valid for all cycles, reversible or irreversible. The symbol (inte- gral symbol with a circle in the middle) is used to indicate that the integration is to be performed over the entire cycle. Any heat transfer to or from a system can be considered to consist of differential amounts of heat transfer. Then the
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