Thermodynamics

Chapter 7ENTROPYIn Chap. 6, we introduced the second law of thermodynamicsand applied it to cycles and cyclic devices. In thischapter, we apply the second law to processes. The firstlaw of thermodynamics deals with the property energy andthe conservation of it. The second law leads to the definitionof a new property called entropy. Entropy is a somewhatabstract property, and it is difficult to give a physical descriptionof it without considering the microscopic state of the system.Entropy is best understood and appreciated by studyingits uses in commonly encountered engineering processes,and this is what we intend to do.This chapter starts with a discussion of the Clausiusinequality, which forms the basis for the definition of entropy,and continues with the increase of entropy principle. Unlikeenergy, entropy is a nonconserved property, and there is nosuch thing as conservation of entropy. Next, the entropychanges that take place during processes for pure substances,incompressible substances, and ideal gases are discussed,and a special class of idealized processes, calledisentropic processes, is examined. Then, the reversiblesteady-flow work and the isentropic efficiencies of variousengineering devices such as turbines and compressors areconsidered. Finally, entropy balance is introduced andapplied to various systems.ObjectivesThe objectives of Chapter 7 are to:• Apply the second law of thermodynamics to processes.• Define a new property called entropy to quantify thesecond-law effects.• Establish the increase of entropy principle.• Calculate the entropy changes that take place duringprocesses for pure substances, incompressible substances,and ideal gases.• Examine a special class of idealized processes, calledisentropic processes, and develop the property relations forthese processes.• Derive the reversible steady-flow work relations.• Develop the isentropic efficiencies for various steady-flowdevices.• Introduce and apply the entropy balance to varioussystems.| 331