Flame and Combustion

J.F. Griffiths, J.A. Barnard

• Publisher: Routledge
• Year: 2019
• Print ISBN: 9781138434561
• eBook ISBN: 9781351448420
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

An introduction for postgraduate and undergraduate students to the chemical and physical principles of flame and combustion phenomena. This book should be of interest to undergraduate/postgraduate chemists; chemical engineers; undergraduate/postgraduate mechanical engineers and environmental scientists; and industrial combustion technologists.

1 Introduction

1.1 Introduction and background

Combustion provided early man with his first practical source of energy; it gave him warmth and light, it extended the range of foodstuffs which he could digest, and it enabled him to ‘work’ metals. Throughout the world today, combustion still provides more than 95% of the energy consumed and, despite the continuing search for alternative energy sources, there is little doubt that combustion will remain important for a very considerable time to come. The use of combustion fuels remains especially attractive where convenient ‘energy storage’ is required, as in transport applications, for example.

It is becoming increasingly important to ensure that combustion processes are utilised in the most efficient manner. Foremost are the need to minimise waste of energy, to avoid unnecessary emissions of carbon dioxide as a contributor to the ‘greenhouse effect’, and to minimise adverse effects on the environment, which may arise through pollutant emissions. Reserves of fossil fuels are also becoming depleted to varying extents, so that fuels themselves are becoming less accessible, and more expensive to recover. Thus, the study of combustion processes is an important area of scientific endeavour, and it is destined to remain so having regard to the requirements of society and to the demands of national and international legislation which control the environmental impact.

Combustion phenomena arise from the interaction of chemical and physical processes. The heat release originates in chemical reactions, but its exploitation in combustion involves heat transport processes and fluid motion. Thus, the theoretical interpretation draws heavily on physics, fluid mechanics and applied mathematics. Numerical analysis often forms a vital bridgehead between experiment and theory.

The aim of this book is to introduce many of the different aspects of combustion and its environmental impact, and to address the interdisciplinary nature of combustion at a sufficiently elementary level that an understanding may be gained by readers from a range of academic and technical backgrounds. In order to concentrate the information in a manageable way, much detailed material has been omitted and experimental evidence in support of scientific arguments has been excluded. Generalisations sometimes have to be made to maintain clarity. The reader is asked to bear these points in mind, in the knowledge that more specialised texts are available which address each topic in considerably greater depth. There is an extremely rich ‘combustion literature’ available which comprises learned journals and conference proceedings.

The most comprehensive coverage from conferences is to be found in the proceedings of the biennial ‘International Combustion Symposia of The Combustion Institute’. However, more selective material can be drawn from the proceedings of international conferences organised regularly by such bodies as The Institution of Mechanical Engineers (IMechE), The Society of Automotive Engineers (SAE) and the Society for Industrial Applied Mathematics (SIAM). The combustion orientated journals which cover the subject widely are led by Combustion and Flame, Combustion Science and Technology, and Combustion, Explosion and Shock Waves. Combustion topics are reviewed in Progress in Energy and Combustion Science. More specialised areas are addressed in other journals, such as Fuel, Fire Safety Engineering, Fire and Materials or the Journal of Loss Prevention. There is a wealth of information about combustion to be drawn from many chemistry, physics, engineering or mathematics series. The intention of the references and suggestions for further reading given here is to provide a route into the literature related to particular topics.

1.2 Survey of combustion phenomena

Combustion begins in chemistry with a self-supported, exothermic reaction. The physical processes involved are principally those which pertain to transport of material and energy. The conduction of heat, the diffusion of chemical species and the bulk flow of gas may all develop from the release of chemical energy in an exothermic reaction as a consequence of the thermal and concentration gradients that are set up in the vicinity of the reaction zone. It is the interaction of these various processes that leads to the phenomenon that is observed. Other effects, such as light emission, depend on specific chemical processes which may have only a negligible bearing on the main phenomenon.

The chemical reaction usually involves two components, one of which is termed the fuel and the other the oxidant (normally air), because of the part each plays in the reaction. The simplest circumstances for combustion to take place are when the two gaseous, premixed components, are introduced to a container maintained at a uniformly controlled temperature. If the vessel is hot enough, measurable exothermic oxidation of the fuel will occur. If the heat produced by the reaction is transported sufficiently rapidly to the container walls by conduction and convection, a steady (or stationary state) reaction is maintained. This balance of the heat release and loss rates, such that the reaction proceeds smoothly to completion, is usually referred to as slow reaction or slow oxidation, although there is no absolute criterion of reaction rate alone from which we could answer the question: ‘How slow is slow?’.

Above a certain temperature of the container, which depends on the physical properties of the reactants and the size and shape of the container, the rate of energy release from the chemical reaction may exceed the rate at which it can be transported to the vessel walls by the various heat transfer processes. The temperature within the system then increases, and the rate of reaction (and therefore the rate of heat release) also increases. This acceleration of reaction rate leads to a further increase in temperature, and the combined effects culminate in an explosion. The term ‘explosion’ refers to the violent increase in pressure which must accompany the rapid self-acceleration of reaction, usually manifest physically by its damaging consequences.

This type of explosion is driven solely by the rate of energy release through thermal feedback. The state of self-acceleration is termed ignition and the phenomenon described here is, therefore, called a thermal ignition or thermal explosion. Thermal ignition will be discussed in more quantitative detail in Chapter 8. The present description illustrates, perhaps in the simplest way of all, how an interaction between the ‘physics’ (i.e. the heat transport processes) and the ‘chemistry’ (determined here solely by the rate of heat of release and how it is affected by the temperature) governs whether or not explosion will occur. The underlying properties that make such an event possible are that reaction rates normally increase exponentially with temperature, whereas the heat transfer often depends almost linearly on temperature.

In a propagating combustion wave, called a deflagration or flame, reaction is initiated by a spark or other energy stimulus. Reaction is then induced in the layer of reactant mixture ahead of the flame front by two possible mechanisms, that is by heat conduction or by diffusion of reactive species from the hot burned gas or reaction zone behind the flame front. Thus, the thermal and chemical reaction properties of the combustion system may still drive the reaction but now there is a spatial structure, and both the heat and mass transport processes have to be described within that framework.

If the premixed reactants are forced to flow towards the flame front, and their velocity is equal to the rate at which the flame would propagate into stagnant gas, i.e. the burning velocity, the flame itself would come to a standstill. This is put into practice in combustion applications involving burners, the design of the appliance being aimed at holding the flame in one position and render it stable towards small disturbances.

An alternative to the premixed flame is the diffusion flame, in which the separate streams of fuel and oxidant are brought together and reaction takes place at their interface. The candle flame must surely be the commonplace example, although the description of it is complicated by the role of heat from the flame acting in a supplementary context of causing melting and evaporation of the wax which then burns in the gas-phase. The supply of air to the reaction zone is sustained by the convection currents set up by the flame itself. This flow also provides cooling to the sides of the cup of the melted fuel [1]. Flame processes are introduced in Chapter 3 and combustion in turbulent flames is discussed in Chapter 4.

The velocities of premixed flames are limited by transport processes, for example, heat conduction and species diffusion. The velocities cannot exceed the speed of sound in the reactant gas. However, it is often found that a propagating combustion wave undergoes a transition to a quite different type of wave, a detonation wave, which travels at a velocity much higher than the speed of sound. In this type of wave the chemical reaction is initiated by a supersonic compression, or shock wave, travelling through the reactants. The chemical energy that is released in the hot, compressed gases behind the shock front provides the driving force for the shock wave. It is necessary to consider the chemistry of the system only to the extent that it provides a source of energy at a rate which is governed by the prevailing temperature, pressure and reactant concentrations. These phenomena are discussed in more detail in Chapter 5.

1.3 Summary of principles and overview of applications

The phenomena described so far are not necessarily restricted to gaseous media. Most of them occur also in liquids and solids, and in dispersions of one phase within another (e.g. in droplet mists or dust clouds). Combustion can also occur at the interface between bulk phases (heterogeneous combustion), as discussed in later chapters.

Illustrated in this introductory summary are three broad subject areas within combustion, namely (i) chemical kinetics and spontaneous processes in an essentially homogeneous reactant mixture, (ii) flame propagation, and (iii) detonation and shock. There are many ramifications which may either subdivide or extend the scope of each, or which may be relevant to more than one area. Discussion of the applications may make these divisions still more diffuse. Nevertheless, from these various examples the self-sustaining characteristics inherent to combustion systems (a feature that sets the discipline of combustion apart from most other aspects of chemistry) should be emerging more clearly. Heat and free radicals originate in the chemistry itself, whereas the physical processes, heat and mass transport in particular, enable these sources to be used to promote (i.e. sustain) the reaction without any othe...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Glossary
• 1 Introduction
• 2 Physicochemical principles
• 3 Flames
• 4 Flame theory and turbulent combustion
• 5 Detonations
• 6 High temperature and flame chemistry (T > 1000 K)
• 7 Low temperature chemistry (T 1000 K)
• 8 Thermal ignition
• 9 Isothermal chain branching and chain-thermal interactions
• 10 Ignition, extinction and oscillatory phenomena
• 11 Aspects of mixed and condensed phase combustion
• 12 Combustion hazards
• 13 Internal combustion engines and fuels
• 14 Combustion and the environment
• Answers to problems with numerical solutions
• References
• Index