Engineering Thermodynamics with Worked Examples

The following sections are included:

• Preface
• Contents

In this chapter we shall introduce some of the important concepts and definitions that provide the basic framework for the study of engineering thermodynamics. The logical development and generalization of the first and second laws of thermodynamics in later chapters of the book rely on these concepts and definitions.

In many engineering systems, energy transfers are carried out using a working fluid. In the case of thermal power plants and refrigeration systems the working fluid undergoes changes in phase while it moves from one component to the next. Gas turbine plants on the other hand use working fluids that remain in the gaseous phase during its passage through the system.

In this chapter we shall discuss the different types of work interactions that a thermodynamic system might experience. Although work is a familiar concept in mechanics, it needs to be generalized in a manner that fits the broad framework of thermodynamic systems and processes. We will consider work interactions in mechanical, elastic, electrical and magnetic systems. Heat interaction will be defined by using the concepts of temperature and equilibrium.

In the preceding chapter we categorized all energy interactions between a system and its surroundings either as work or heat. This was a useful starting-point for the introduction of the First Law of Thermodynamics, which is essentially the law of conservation of energy formulated in a unified manner to include all energy forms. The first law relates work and heat interactions that occur at the boundary of a system, to the energy possessed or stored by a system, which we shall call the internal energy of the system. The emphasis in this chapter will be the application of the first law to a closed system undergoing steady processes. In the next chapter we shall consider open systems and transient or unsteady processes.

In the last chapter we applied the first law of thermodynamics to closed systems where there was no mass exchange between the system and the surroundings. However, many engineering systems, especially those involving energy conversion devices, consist of components where working fluids flow in and out of them. In the present chapter we propose to extend the formulation of the first law to such open systems, which are also called control volumes.

In the last two chapters of this book we applied the first law of thermodynamics to closed and open systems considering both quasistatic and non-quasi-static processes. A question that arises naturally is whether all processes that satisfy the first law could be carried out in practice. In order to explore this question we need to introduce the second law of thermodynamics which teaches us that there are additional conditions for the feasibility of a process. In particular, the irreversibility of natural processes is an important consequence of the second law.

In the preceding chapter we obtained a number of important results by applying the second law to cyclic processes associated with heat engines and reversed heat engines, operating with one and two thermal reservoirs. The concept of a reversible process, though an idealization of real processes, provided a means to find the upper limit of the efficiency of heat engines operating with two heat reservoirs. The maximum efficiency, called the Carnot-cycle efficiency, is a unique function of the two reservoir temperatures. This lead to the development of the thermodynamic temperature scale. Finally, we generalized these results for cyclic systems experiencing heat interactions with any number of thermal reservoirs.

The second law and its important consequences were introduced in chapters 6 and 7 where the emphasis was on applications to closed systems. In this chapter we shall apply the entropy balance equation to open systems or control volumes with the aim of developing the tools for availability or exergy analysis of such systems. More recently, this type of performance evaluation of engineering systems has been called second law analysis, although it is the combination of the first and second laws that forms the basis of exergy analysis and the second law efficiency.

In this chapter we shall consider the analysis of vapor power cycles that convert the stored energy of fuels to mechanical work. The main distinguishing feature of these power cycles is that the working fluid undergoes phase change during the operation of the cycle. Steam power plants, which use water as the working fluid, generate much of the electrical power in the world. Although the design and construction of such power plants involve many engineering considerations, our focus in this chapter will be the thermodynamic aspects that impact on the efficiency of the plant.

In the last chapter we analyzed vapor power cycles with working fluids that undergo phase change during operation. However, many common prime movers like petrol engines, diesel engines, and gas turbines use gases as working fluids and their operating cycles are known as gas power cycles. In this chapter we shall analyze gas power cycles considering their performance mainly under ideal conditions.

The main function of refrigeration systems or reversed heat engines is to transfer heat continuously from a low temperature region to a high temperature region using energy from an external source. In the more common applications of refrigeration, like food preservation and air conditioning, the aim of the refrigeration system is to maintain the temperature of the cold space below the local ambient temperature. The same system, however, may be used to heat the high temperature region by using the heat extracted from the cold region. It is then called a heat pump.

In chapter 2 we studied the properties of a pure substance which is homogeneous and of fixed chemical composition. In the absence of chemical reactions, a homogeneous mixture of pure substances may also be treated as a pure substance. For example, in our work thus far we have considered air, which is a mixture of several gases, as a pure substance. The properties of a mixture of pure substances could be determined by knowing the values of the corresponding properties of the various constituents of the mixture.

In chapter 12 we considered the properties of mixtures of gases and vapors, that are pure substances. We shall now extend this study to mixtures that undergo chemical reactions where the chemical constituents of the system change during the process. The main focus of this chapter will be on combustion processes. These processes are used to provide high temperature thermal energy sources for numerous engineering applications. We have already discussed some of these applications in earlier chapters of this book.

In this chapter we shall develop some general relationships between the thermodynamic properties that were presented in earlier chapters. In chapter 1, pressure, volume and density were introduced as basic properties of a system, and the zeroth law was used to define temperature and clarify its measurement. The property called the internal energy followed from the first law in chapter 4. It was used to derived the properties, internal energy, enthalpy, and the specific heat capacity. The second law resulted in the property called entropy. From it we derived two additional properties, the Helmholtz function and the Gibbs function, which were related to the maximum work output of a system. We also used thermodynamic property tables [3] on a number of occasions to solve numerical problems.

The following sections are included:

• Background
• Boltzmann Equation
• Model for Thermal Entropy
• Model for Configurational Entropy
• Summary
• Worked Examples
• Problems
• References

The following sections is included:

• Index