scroll identifier for mobile
main-content

Thermodynamics is a subject that all engineering students have to face and that most of them treat with great respect. This makes it all the more important to offer a good and easy-to-understand approach to the laws of energy conversion. This is what this textbook is intended to do: It covers the basics of classical technical thermodynamics as they are typically taught at universities: The first and second law of thermodynamics as well as equations of state are explained for idealized and real fluids which are subject to a phase change. Thermodynamic mixtures, e.g. humid air, are treated as well as chemical reactions. Components and thermodynamic cycle that convert energy are presented. The book attaches great importance to drawings and illustrations, which should make it easier to comprehend complex matter. Technical applications and apparatus are presented and explained. Numerous exercises and examples conclude the book and contribute to a better understanding of the theory.

### Chapter 1. Introduction

Abstract
Thermodynamics is one of the most difficult and probably even one of the most challenging disciplines in mechanical engineering. However, thermodynamics is necessary to understand the principles of energy conversion, e.g. when designing thermal machines such as gas turbines, internal combustion engines or even fuel cells in modern applications.
Achim Schmidt

### Chapter 2. Energy and Work

Abstract
It is well known from physics lessons that energy is conserved. The principle of energy conservation
Achim Schmidt

### Chapter 3. System and State

Abstract
In this chapter it is clarified what a thermodynamic system is and how its state can be described. Any system is separated from an environment by a system boundary as it is done in other technical disciplines, e.g. technical mechanics, as well. First, the permeability of the system boundary is categorised. However, after classifying the system and the system boundary, it is the next step to identify the internal state. This leads to so-called state values, e.g. pressure and temperature, that fix the state of a system. Our everyday experience shows, that the state can be varied by external impacts across the boundary.
Achim Schmidt

### Chapter 4. Thermodynamic Equilibrium

Abstract
For the further understanding of thermodynamics the principle of thermodynamic equilibrium is essential. Systems in thermodynamic equilibrium do not change their state without any impact from outside, i.e. they are perfectly in rest.
Achim Schmidt

### Chapter 5. Equations of State

Abstract
Chapter 3 has shown, how to quantify the internal state of a system. Basically, a distinction has been made between thermal and caloric state values. Since not all state values can be measured directly, equations to calculate them are required. The Paddle Wheel experiment for instance, see Sect. 2.​4.​1, has shown, that the internal energy describes the energetic state of a fluid.
Achim Schmidt

### Chapter 6. Thermal Equation of State

Abstract
In this chapter, the correlation between the thermal state variables pressure p, specific volume v and temperature T is investigated. As already discussed in the previous chapter, thermal state values can easily be measured. Furthermore, according to Gibbs’ phase law for ideal gases two intensive state values, i.e. pressure and temperature, fix the state of a thermodynamic system unequivocally. Thus, a mathematical function to calculate the third thermal state value v must exist:
$$v=f(p, T)$$
Achim Schmidt

### Chapter 7. Changes of State

Abstract
So far thermodynamic systems have been discussed and state values to quantify their internal state have been categorised. In this chapter the focus will be on changes of state, i.e. bringing a system from an initial state to a new state.
Achim Schmidt

### Chapter 8. Thermodynamic Processes

Abstract
So far it has been clarified what a thermodynamic system is and how its state can be determined. Within conventional thermodynamics the principle of thermodynamic equilibrium forms the basis for thermodynamic calculations. However, due to external impacts a system can be shifted from one equilibrium state to another.
Achim Schmidt

### Chapter 9. Process Values HeatHeat and WorkWork

Abstract
Over the previous chapters the internal state of a system has been described. A thermodynamic system can change its state and several possible changes of state have been discussed. For the further understanding it is essential to be familiar with the terms reversibility/irreversibility as well as with the idea of a quasi-static change of state. These approaches have been clarified in the previous Chaps. 7 and 8: By external impacts a system’s thermodynamic equilibrium can be disturbed and the system moves into a new balanced state.
Achim Schmidt

### Chapter 10. State Value Versus Process Value

Abstract
The previous chapters have shown, that it is essential to distinguish between state and process values. State values specify the state of a thermodynamic system. Furthermore, they can be visualised in a pv-diagram for instance. The state of an ideal gas is fixed by two independent state values, so that each pv-pair in a pv-diagram defines the state of a system unambiguously.
Achim Schmidt

### Chapter 11. First Law of Thermodynamics

Abstract
In this chapter the causality between process variables and the change of state variables of a thermodynamic system is derived. The first section introduces the principle of equivalence between work and heat—an essential prerequisite in order to formulate the energy conservation principle known as first law of thermodynamics. Finally, the first law of thermodynamics is applied for closed respectively open systems as well as for thermodynamic cycles. Anyhow, thermodynamic cycles play an important role in the following chapters.
Achim Schmidt

### Chapter 12. Caloric Equations of State

Abstract
In the previous chapters the law of energy conservation has been thoroughly discussed. By this, it was possible to evaluate thermodynamic systems energetically. A distinction has been made between closed and open systems. However, next to thermal state values, such as pressure p or temperature T, that can be measured easily, a new category of state values has been introduced: These state values, i.e. specific internal energy u and specific enthalpy h for instance, can not be determined by a sensor and thus need to be calculated.
Achim Schmidt

### Chapter 13. Meaning and Handling of Entropy

Abstract
In the previous Chap. 12 the caloric equations of state have been introduced and derived thermodynamically. A new state value, the specific entropy s has been introduced. However, at that point it was not clear how to utilise entropy in order to evaluate thermodynamic systems. This chapter focuses on clarifying why entropy is beneficial. First, a comparison is made with the first law of thermodynamics, so that entropy balancing is comprehensible: In contrast to energy, entropy is not a conservation value, since entropy can be generated within a system. Nevertheless, entropy can be balanced. A distinction is made between closed/open systems and thermodynamic cycles. Furthermore, a new state diagram, i.e. the Ts-diagram, is derived. Such a state diagram visualises the process values specific heat q and specific dissipation $$\psi$$. Obviously, together with the pv-diagram, that illustrates the other process value, i.e. the specific work, it is an important diagram in thermodynamics. Finally, two new changes of state, namely the isentropic and polytropic change of state are treated in this chapter.
Achim Schmidt

### Chapter 14. Transient Processes

Abstract
Irreversibility plays an important role in evaluating thermodynamic processes. In case friction occurs, a system can not be operated reversibly. This has been treated in Chap. : A wire pendulum for instance does not reach its initial, starting position after a number of oscillations. The amplitude decreases by time until the pendulum finally stops in its rest position. This is due to dissipation, i.e. friction at the mounting point as well as due to interactions between environment and pendulum. Consequently, kinetic energy is dissipated and transferred to the environment from where it can not be gained back into the system. Consequently, the state value entropy in such a case rises due to dissipation. It has been shown, that the rate of generation of entropy is a quantitive measure for the degree of irreversibility.
Achim Schmidt

### Chapter 15. Second Law of Thermodynamics

Abstract
Though the balance of entropy has been denoted as Second Law of Thermodynamics in the previous chapters, its classical formulation comes along with thermodynamic cycles. Thus, the focus now is on these cycles. However, there are two different types of thermodynamic cycles: Clockwise cycles on the one hand convert heat into mechanical energy and are named thermal engines. Counterclockwise cycles on the other hand are fridges and heat pumps, that lift thermal energy from a lower to a higher temperature level. However, at that stage cycles are represented in a so-called black-box notation, i.e. the fluxes at its border are balanced without focussing on the detailed physical processes that run inside the machine.
Achim Schmidt

### Chapter 16. ExergyExergy

Abstract
So far the law of energy conservation, i.e. the first law of thermodynamics, has been discussed, that quantified the changeability of energy. However, this changeability is limited, so that thermal energy can not be converted completely into mechanical energy in a steady state process. The second law of thermodynamics can be applied to evaluate the constraints of energy conversion. In Chap. 15 a clockwise Carnot machine was introduced: A machine that operates between two thermal reservoirs in order to convert thermal energy into maximum mechanical work. Its efficiency in best case is given by the minimum and maximum temperature the machine is working in-between. This principle is essential to understand the thermodynamic idea of exergy as maximum working capability of any form of energy. The significance of the exergy is presented in this chapter.
Achim Schmidt

### Chapter 17. Components and Thermodynamic Cycles

Abstract
Although many technical components, that are required to run energy conversion processes, have already been discussed previously, the focus in this chapter is on thermal turbo-machines as well as on heat exchangers. However, in this chapter the technical components are treated in steady state operation. In order to quantify the efficiency of turbine and compressor the so-called isentropic efficiency is defined. In addition, relevant thermodynamic cycles are introduced and discussed. Cyclic processes have been discussed in the previous chapters as well, but mostly in a black-box notation. A distinction has been made between clockwise cycles, i.e. thermal engines, and counterclockwise cycles, i.e. cooling machines respectively heat pumps. This chapter concludes the first part of this book.
Achim Schmidt

### Chapter 18. Single-Component Fluids

Abstract
This chapter deals with single-component fluids, i.e. pure fluids not being mixed with other fluids. In Part I idealised single-component fluids have already been introduced: Ideal gases, that follow the well-known thermal and caloric equations of state, and incompressible liquids, whose specific volume remains constant. For incompressible liquids the caloric equations of state can be adapted easily. The behaviour of real fluids, however, deviates from the idealised models. In particular, real fluids can change their state of aggregation.
Achim Schmidt

### Chapter 19. Mixture of Gases

Abstract
So far single component fluids, i.e. fluids of one chemical composition, have been treated. Part I has focused on ideal gases and incompressible liquids. In Chap. 18 fluids underlying a phase change have been investigated, i.e. a fluid can occur in solid, liquid and gaseous state—however, its chemical composition remains constant even when a phase change takes place.
Achim Schmidt

### Chapter 20. Humid Air

Abstract
In the previous chapter it has been clarified how to handle mixtures of ideal gases. Definitions for characterising the mixture’s composition have been introduced as well as the irreversibility of the mixing of different ideal gases. However, in this chapter the focus is on a technical relevant mixture of fluids: humid air.
Achim Schmidt

### Chapter 21. Steady State Flow Processes

Abstract
Steady state flow processes have already been discussed when introducing the first law of thermodynamics for open systems, see Sect. 11.​3. Figure 21.1 shows an example for a simple open system with a single inlet and a single outlet. Characteristic for open systems in steady state is, that the mass inside the system remains constant with respect to time, so that the mass flux into the system needs to be balanced by the mass flux out of the system, i.e.
\begin{aligned} \boxed {\dot{m}_{1}=\dot{m}_{2}}. \end{aligned}
Achim Schmidt

### Chapter 22. Thermodynamic Cycles with Phase Change

Abstract
Though thermodynamic cycles have already been introduced in Sect.  17.​2, this chapter covers cycles, in which the working fluid is subject to phase changes. If a system after several changes of state finally reaches the initial state, it is called a thermodynamic cycle. Thus, all state values reach their initial value, i.e.
Achim Schmidt

### Chapter 23. Combustion Processes

Abstract
In Part I ideal gases and incompressible liquids have been introduced and the basic thermodynamic correlations have been derived. Part II has shown, that real fluids can be subject to a change of aggregate state and that many thermodynamic cycles are based on phase change, e.g. a steam power plant. Furthermore, Part II covered mixture of fluids, e.g. humid air or mixtures of ideal gases. However, within these mixture each component was stable and not part of a chemical reaction, i.e. decomposition of the present atoms and molecules. In Part III the focus now is on chemical reacting systems: First, the stoichiometry of a chemical reaction is investigated, i.e. the principle of mass conservation is applied to reactants and products of a chemical reaction. In doing so it is possible to predict the composition of the products. This is important for instance, when the composition of an exhaust gas of a combustion process needs to fulfil technical thresholds. Second, a chemical reacting system is investigated in terms of an energy balance. In this chapter the heating-value approach is followed with focus on technical combustions, i.e. combustions based on fossil fuels. In Chap. 24 another energetic approach based on absolute enthalpy respectively entropy is introduced. Major advantage of this method is, that the irreversibility of chemical reactions can be quantified.
Achim Schmidt

### Chapter 24. Chemical Reactions

Abstract
This chapter examines chemical reactions more generally than the previous chapters. In Chap. 23 the focus has been on the combustion of fossil fuels and the lower/upper heating value has been introduced to handle a chemical decomposition energetically. However, this method used to be rather impractical, since the oxidisation has to be split into several parts and a distinction has been required whether condensation occurs. Now, a more straightforward method is preferred by introducing the so-called absolute specific enthalpy/entropy. In doing so, the specific enthalpy does not only imply a caloric effect, as it has been done in part I and II, but it includes the specific chemical bonded energy as well.
Achim Schmidt