Introduction To Combustion

Warren C. Strahle, William A. Sirignano, William A. Sirignano

• Publisher: Routledge
• Year: 2020
• ISBN:
  9781000141184
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Engineering General
• Index: Technology & Engineering

This book presents basic information about combustion, mostly in the form of examples. It is a textbook for a one-semester or one-quarter course for juniors or seniors in mechanical, aerospace, chemical, or civil engineering.

1


INTRODUCTION AND REVIEW

1.1. GENERAL INTRODUCTION

The generation of heat on earth has come largely from a process called combustion, and this will continue well into the next century. The process involves a chemical transformation between a substance or substances called fuels and other chemicals called oxidizers. No alteration of the nuclei of the entering substances is involved, but bonds involving the electrons of the molecules and atoms take place. This bonding causes heat to be liberated, or, in some cases, heat is required to form the bonds. The case of heat liberation is the most interesting, since it is this heat energy that can be usefully exploited. In addition to fuel and oxidizer, often an ignition source is needed. For example, one of the earliest forms of combustion, the forest fire, uses wood as the fuel, the air as the oxidizer and the ignition source can be a spark (lightning).¹

Combustion has a wide variety of uses. It is used for power generation or creation of thrust in engines of all types. Combustion is used as a heat source for chemical processing, general heating and drying operations. It is useful in waste incineration, melting operations and welding. Combustion is involved in explosions for both useful and hostile purposes. The use of combustion has harmful effects, however. Pollutants and greenhouse gases are produced as well as waste heat and unwanted explosion and fires.

The science and engineering of combustion is the subject of this book. Combustion is a complex subject to master, requiring knowledge of many sub-disclipines of physics, chemistry and physical chemistry. Examples are fluid mechanics, quantum mechanics, statistical mechanics, kinetic theory, chemical kinetics, electromagnetic radiation, and heat and mass transfer. Fortunately, a working engineering knowledge can be attained without going too far into the details of these disciplines, and such an approach will be followed in this book.

Combustion can involve all phases of matter—solid, liquid and gas. For example, both solids and liquids are used in rocket engines, and gases are used in an oxy-acetylene welding torch. Although most of the material covered in this book will deal with the gas phase, the solid phase is covered, for example, in connection with solid rocket propellants and the liquid phase is dealt with when droplet burning is considered.

It is presumed, insofar as this book is concerned, that the student is at least a late junior in engineering or physics. Assumed is that first year physics, chemistry and calculus are past history and that first courses in thermodynamics and compressible fluid flow have been taken. However, with regard to compressible fluid flow, the necessary concepts are developed in this book, so that lack of knowledge here is perhaps not too crucial. The chemistry needed will be developed as necessary, but it is presumed that the student is familiar with the notions of atoms and molecules. Welcome to the study of combustion; it is a fascinating field.

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¹For an interesting history of man’s use of combustion see F. J. Weinberg, Fifteenth Symposium (International) on Combustion. Pittsburgh: The Combustion Institute, 1974, pp. 1-20.

1.2. CHEMISTRY AND PHYSICS REVIEW

We shall often be concerned about the atomic weight and molecular weight of atoms and molecules. These weights are quoted relative to the carbon 12 atom. The number 12 is the number of particles in the nucleus, consisting, in this case, of six positively charged protons and six neutral neutrons. Carbon 12, being electrically neutral, has six negatively charged electrons, giving it the atomic number 6. The atomic weight or molecular weight of any other atom or molecule is twelve times the ratio of its actual weight to the weight of the carbon 12 atom. It should be clear that we are really considering masses, not weights. However, the literature is full of the term “weight,” so by convention we adopt it here.

The proton and neutron have almost, but not exactly, the same mass and the electron has a nearly negligible mass compared to the proton. The consequence is that almost any atom or molecule will have its atomic or molecular weight very nearly an integer. As an example, the oxygen 16 atom has an atomic weight of 15.995, which is 16 for practical purposes.

There are also naturally occurring isotopes of the elements which have differing nucleus makeup from the most commonly occurring ...