Thermodynamics

Principles and Applications

İsmail Tosun

• Editore: WSPC
• Anno: 2015
• ISBN:
  9789814696951
• Argomento: Tecnologia e ingegneria
• Categoria: Ingegneria meccanica

This eminently readable introductory text provides a sound foundation to understand the abstract concepts used to express the laws of thermodynamics. The emphasis is on the fundamentals rather than spoon-feeding the subject matter. The concepts are explained with utmost clarity in simple and elegant language. It provides the background material needed for students to solve practical problems related to thermodynamics. Answers to all problems are provided.

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Contents:

• Introduction
• Calculation of Work in Reversible and Irreversible Processes
• Pressure-Volume-Temperature ( PVT ) Relations for Pure Substances
• The First Law of Thermodynamics
• The Second Law of Thermodynamics
• Power and Refrigeration Cycles

Readership: Researchers, academics, professionals, undergraduate and graduate students in chemical engineering, mechanical engineering and energy studies.

Chapter 1

Introduction

1.1Basic Concepts

Problems encountered in science and engineering can be solved by the application of the following basic concepts:

• Conservation of mass,

• Conservation of momentum,

• First law of thermodynamics (or conservation of energy),

• Second law of thermodynamics.

The first three basic concepts indicate that mass, momentum, and energy are all conserved quantities. A conserved quantity is one that can be transformed from one form to another. However, transformation does not alter the total amount of that quantity.

Note that half of the basic concepts come from the subject of thermodynamics. The word thermodynamics comes from the combination of the Greek words “therme”, meaning heat, and “dunamis”, meaning power. It is the study of energy and its transformations. Energy can be transformed into heat and work. Therefore, the subject of thermodynamics deals with how to convert energy efficiently into work.

The first law of thermodynamics is a statement of the conservation of energy, i.e., although energy can be transferred from one system to another in many forms, it can neither be created nor destroyed. Therefore, the total amount of energy available in the universe is constant. The second law of thermodynamics, on the other hand, tells us that as the energy is converted from one form to another its ability to produce useful work decreases. In other words, energy conversion leads to degradation of energy¹ as shown in Fig. 1.1. The efficiency of a conversion process,

, defined by

Fig. 1.1 Degradation of energy during energy conversion.

(1.1-1)

is always less than 1. If a series of energy conversions is used, then the overall efficiency is the product of the efficiencies of individual processes, i.e.,

(1.1-2)

Both the first and second laws of thermodynamics are in agreement with all human experience and, as a result, are also called laws of nature. Any feasible process in mother nature should satisfy both the first and second laws of thermodynamics.

1.2Definitions

1.2.1System

Any region that occupies a volume and has a boundary is called a system. The volume outside the boundary is called the surroundings of the system. The sum of the system and its surroundings is called the universe. Thermodynamics considers systems only at the macroscopic level. It is convenient to distinguish between three general types of systems:

• Isolated system: These are the set of systems that exchange neither mass nor energy with the surroundings. For example, the universe is an isolated system.

• Closed system: These are the set of systems that exchange energy (in the form of heat and work) but not mass with the surroundings.

• Open system: These are the set of systems that exchange both mass and energy with the surroundings.

The equations available to analyze closed and open systems are different from each other. Therefore, one should properly define the system before solving the problem.

1.2.2Property, state, and process

To describe and analyze a system, some of the quantities that are characteristic of it must be known. These quantities are called properties and comprise volume, mass, temperature, pressure, etc. Thermodynamic properties are considered to be either extensive or intensive. When the property is proportional to the mass of the system, the property is extensive, i.e., volume, kinetic energy, potential energy. On the other hand, when the property is independent of the mass of the system, the property is intensive, i.e., viscosity, refractive index, density, temperature, pressure, mole fraction. An easy way to determine whether a property is intensive or extensive is to hypothetically divide the system into two equal parts with a partition. Each part will have the same value of intensive property, i.e., temperature, pressure, and density as the original system, but half the value of the extensive property, i.e., number of moles and volume. In other words, while the extensive properties are additive, intensive properties are not.

Specific (or molar) properties are extensive properties divided by the total mass (or total moles) of the system, i.e.,

(1.2-1)

If φ represents any extensive property, then Eq. (1.2-1) is expressed as

(1.2-2)

where m and n are the total mass and moles, respectively. Note that all specific (or molar) properties are intensive.

A complete list of the properties of a system describes its state. Consider a function

w = f (x,y)

(1.2-3)

in which there are three variables: w is dependent; x and y are independent. Obviously, once the values of x and y are specified, the value of w is automatically fixed. In thermodynamics we would say that “the state of the system, w, is fixed when the ther...