cen84959_ch04

# cen84959_ch04 - Chapter 4 ENERGY ANALYSIS OF CLOSED SYSTEMS...

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Chapter 4 ENERGY ANALYSIS OF CLOSED SYSTEMS | 165 I n Chap. 2, we considered various forms of energy and energy transfer, and we developed a general relation for the conservation of energy principle or energy balance. Then in Chap. 3, we learned how to determine the thermody- namics properties of substances. In this chapter, we apply the energy balance relation to systems that do not involve any mass flow across their boundaries; that is, closed systems. We start this chapter with a discussion of the moving boundary work or P d V work commonly encountered in recip- rocating devices such as automotive engines and compres- sors. We continue by applying the general energy balance relation, which is simply expressed as E in E out E system , to systems that involve pure substance. Then we define specific heats , obtain relations for the internal energy and enthalpy of ideal gases in terms of specific heats and temperature changes, and perform energy balances on various systems that involve ideal gases. We repeat this for systems that involve solids and liquids, which are approximated as incom- pressible substances . Objectives The objectives of Chapter 4 are to: Examine the moving boundary work or P d V work commonly encountered in reciprocating devices such as automotive engines and compressors. Identify the first law of thermodynamics as simply a statement of the conservation of energy principle for closed (fixed mass) systems. Develop the general energy balance applied to closed systems. Define the specific heat at constant volume and the specific heat at constant pressure. Relate the specific heats to the calculation of the changes in internal energy and enthalpy of ideal gases. Describe incompressible substances and determine the changes in their internal energy and enthalpy. Solve energy balance problems for closed (fixed mass) systems that involve heat and work interactions for general pure substances, ideal gases, and incompressible substances.

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4–1 MOVING BOUNDARY WORK One form of mechanical work frequently encountered in practice is associ- ated with the expansion or compression of a gas in a piston–cylinder device. During this process, part of the boundary (the inner face of the piston) moves back and forth. Therefore, the expansion and compression work is often called moving boundary work , or simply boundary work (Fig. 4–1). Some call it the P d V work for reasons explained later. Moving boundary work is the primary form of work involved in automobile engines . During their expansion, the combustion gases force the piston to move, which in turn forces the crankshaft to rotate. The moving boundary work associated with real engines or compressors cannot be determined exactly from a thermodynamic analysis alone because the piston usually moves at very high speeds, making it difficult for the gas inside to maintain equilibrium. Then the states through which the system passes during the process cannot be specified, and no process path can be drawn. Work, being a path function, cannot be determined analytically with-
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