Combustion

Frontmatter

1. Introduction, Fundamental Definitions and Phenomena

Abstract

Combustion is the oldest technology of mankind; it has been used for more than one million years. At present, about 90% of our worldwide energy support (e. g., in traffic, electrical power generation, heating) is provided by combustion; therefore it is really worthwhile studying this process.

Jürgen Warnatz, Ulrich Maas, Robert W. Dibble

2. Experimental Investigation of Flames

Abstract

Computer simulations (which are treated in detail in some of the following chapters) are increasingly part of the discovery and design process. One can expect this growing trend to continue. Because of (not in spite of) the increasing level of sophistication of the numerical simulations, guidance from increasingly sophisticated experiment is a necessity. Such guidance is needed for several reasons:

• A first reason for comparison with experiment is that the discovery of previously unknown chemical reactions or physics may emerge. It is through this iterative comparison between simulations and experiment that progress is made (investigation).
• Secondly, in the interest of obtaining approximate solutions in an acceptable time, the simulations must be done on modelled equation sets where one has knowingly left out or simplified terms in the equations. With experience, one learns what terms may safely be neglected on the basis that they contribute little to the features of interest. This experience is obtained through comparison of numerical predictions with experiment (validation).

In general, the experiments supply measurements that critically test an aspect of what the simulations predict, namely the velocity, temperature, and concentrations of species. In the past, progress was made by inferences made from intrusive probe measurements. As the models are steadily improving, the details demanded from the simulations steadily increase. Accordingly, increasingly detailed experiments are needed for the understanding of the fundamental physics and chemistry that is then embodied into the model.

Jürgen Warnatz, Ulrich Maas, Robert W. Dibble

3. Mathematical Description of Premixed Laminar Flat Flames

Abstract

If a chemically reacting flow is considered, the system at each point in space and time is completely described by specification of pressure, density, temperature, velocity of the flow, and concentration of each species. These properties can be changing in time and space. These changes are the result of fluid flow (called convection), chemical reaction, molecular transport (e. g., heat conduction, diffusion, and viscosity), and radiation. A mathematical description of flames therefore has to account for each of these processes (Hirschfelder et al. 1964).

Jürgen Warnatz, Ulrich Maas, Robert W. Dibble

4. Thermodynamics of Combustion Processes

Abstract

In Chapter 3, the example of a flat one-dimensional flame was used to show that several ingredients are needed for the solution of the conservation equations. One of these ingredients is the thermodynamic properties of each species, i. e., enthalpy H, entropy S, and heat capacities c _p of each species as a function of temperature and pressure. In this chapter it will be shown how H and S are generated and used. For example, one can predict the final temperature of a flame and the species composition at this final temperature using thermodynamics.

Jürgen Warnatz, Ulrich Maas, Robert W. Dibble

5. Transport Phenomena

Abstract

Molecular transport processes, i. e., diffusion, heat conduction, and viscosity, have in common that the corresponding physical properties are transported by the movement of the molecules in the gas. Diffusion is the mass transport caused by concentration gradients, viscosity is the momentum transport caused by velocity gradients, and heat conduction is the energy transport caused by temperature gradients. Additionally, there are other phenomena such as mass transport caused by temperature gradients (thermal diffusion or Soret effect) or energy transport caused by concentration gradients (Dufour effect). The influence of the latter is usually very small and is often neglected in the simulation of combustion processes. A detailed discussion of the transport processes can be found in the books of Hirschfelder et al. 1964 or of Bird et al. 1960.

Jürgen Warnatz, Ulrich Maas, Robert W. Dibble

6. Chemical Kinetics

Abstract

The thermodynamic laws discussed in Chapter 4 allow the determination of the equilibrium state of a chemical reaction system. If one assumes that the chemical reactions are fast compared to the other processes like diffusion, heat conduction, and flow, thermodynamics alone allow the description of the system locally (see, e. g., Section 13.1). In most cases, however, chemical reactions occur on time scales comparable with that of the flow and of the molecular transport processes. Therefore, information is needed about the rate of chemical reactions, i. e., the chemical kinetics. Thus, the basic laws of chemical kinetics shall be discussed in the following, which are based on macroscopic observation. The chapter will show that these macroscopic rate laws are a consequence of the underlying microscopic phenomena of collisions between molecules.

Jürgen Warnatz, Ulrich Maas, Robert W. Dibble

7. Reaction Mechanisms

Abstract

In Chapter 6 it has been shown that the combustion of a simple fuel like hydrogen (global reaction 2 H₂ + O₂→ 2 H₂O) requires nearly 40 elementary reactions for a satisfactory chemical mechanism. For combustion of hydrocarbon fuels, as simple as methane CH₄, the number of elementary reactions in the chemical mechanism is much larger. In some cases several thousands of elementary reactions (e. g., in the case of autoignition of the Diesel fuel with the typical component cetane C₁₆H₃₄; see Chapter 16) influence the overall process.

Jürgen Warnatz, Ulrich Maas, Robert W. Dibble

8. Laminar Premixed Flames

Abstract

Measurements of laminar flame velocities and the experimental determination of concentration- and temperature profiles in laminar flame fronts were introduced in Chapter 2. A challenge to the combustion scientist is to construct a model that will match the observed concentration- and temperature profiles and allows prediction of events for which there are no measurements. Chapter 3 introduced a simple model, consisting of a system of partial differential equations derived from the conservation of mass, enthalpy, and species mass. In these equations, the terms dealing with thermodynamics were the subject of Chapter 4. The terms dealing with transport properties were the subject of Chapter 5. In the equation system, the source or sink of species and energy are the chemical reactions discussed in Chapters 6 and 7. The model now is complete; one only needs to solve the system of partial differential equations with appropriate boundary conditions.

Jürgen Warnatz, Ulrich Maas, Robert W. Dibble

9. Laminar Nonpremixed Flames

Abstract

In the previous chapter, premixed flames were discussed. In these flames, the fuel and oxidizer are mixed first with combustion occurring well after mixing. Nonpremixed flames were introduced as a basic flame type in Chapter 1. In nonpremixed flames, fuel and oxidizer react as they mix; examples of nonpremixed flames are given in Table 1.2. In this chapter, the standard model of laminar nonpremixed flames is developed. The extension of this model to a quantitative description of turbulent nonpremixed flames is the subject of Chapter 14.

Jürgen Warnatz, Ulrich Maas, Robert W. Dibble

10. Ignition Processes

Abstract

The discussion of premixed flames (Chapter 8) and nonpremixed flames (Chapter 9) assumed that the flames were at a steady state. The solutions are time-independent. The time-dependent process of starting with reactants and evolving in time towards a steadily burning flame is called ignition. Ignition processes are always time-dependent. Examples of ignition processes include induced ignition (such as occurs in gasoline engines induced by a spark), autoignition (such as occurs in Diesel engines), and photoignition caused by photolytic generation of radicals.

Jürgen Warnatz, Ulrich Maas, Robert W. Dibble

11. The Navier-Stokes-Equations for Three-Dimensional Reacting Flows

Abstract

In the previous chapters, the conservation equations for one-dimensional flames were developed, solution methods discussed, and results presented. In this chapter, general three-dimensional conservation equations are derived for mass, energy, and momentum; these are the Navier-Stokes equations for reactive flow.

Jürgen Warnatz, Ulrich Maas, Robert W. Dibble

12. Turbulent Reacting Flows

Abstract

In previous chapters, premixed and nonpremixed reacting flows have been studied on the assumption that the underlying fluid flow is laminar. In most combustion equipment, e. g., engines, boilers, and furnaces, the fluid flow is usually turbulent. In turbulent flows, mixing is greatly enhanced. As a consequence, the combustion chamber is, for example, much smaller than possible with laminar flows. In spite of the widespread use of turbulent combustion, many questions are still open here.

Jürgen Warnatz, Ulrich Maas, Robert W. Dibble

13. Turbulent Nonpremixed Flames

Abstract

Turbulent nonpremixed flames are of interest in practical applications. They appear in jet engines, Diesel engines, steam boilers, furnaces, and hydrogen-oxygen rocket motors. Except for the turbulent premixed combustion in many spark-ignited engines (Otto cycles), most combustion is turbulent nonpremixed.

Jürgen Warnatz, Ulrich Maas, Robert W. Dibble

14. Turbulent Premixed Flames

Abstract

This chapter will discuss turbulent premixed flames. The distinction between premixed flames and nonpremixed flames is made clear by reviewing the ideal case of each. The ideal nonpremixed flame has fast (equilibrium) chemistry that rapidly adjusts to the local mixture fraction; the mixture fraction is constantly changing. The unburnt gas in an ideal premixed flame is completely mixed before chemistry begins. Then the ideal premixed flame has a delta function PDF for mixture fraction with chemistry that suddenly evolves from unburnt to burnt at the interface between reactants and products; the interface propagates with a speed υ_L.

Jürgen Warnatz, Ulrich Maas, Robert W. Dibble

15. Combustion of Liquid and Solid Fuels

Abstract

Prior to this chapter, this book has focused on combustion between fuel and oxidizer in the gas phase. However, in many practical combustion processes the fuel starts as a liquid, or as a solid, which is then burnt by a gaseous oxidizer. Examples of the combustion of liquids include that in jet aircraft engines, Diesel engines, and oil fired furnaces. Examples of combustion of solids include that of coal, wood (in forest and building fires), plastics, and trash.

Jürgen Warnatz, Ulrich Maas, Robert W. Dibble

16. Low Temperature Oxidation, Engine Knock

Abstract

A detailed knowledge of the combustion processes in internal combustion engines is required if one wishes to further improve on the remarkable development of engine technology and related fuel technology. Such improvements aim at the efficient use of fuels with a minimum amount of pollutant emissions. In the specific case of the spark ignited engine, i. e., the Otto engine, a thermodynamic analysis of the engine cycle shows that overall efficiency η will increase with increasing compression ratio ε(η ≈ 1 − 1/ε ^κ−1; κ = C _p /C _V ). Furthermore, the absolute power output increases when more mass is inducted on each intake stroke. Unfortunately, as the compression ratio is increased, the onset of a phenomenon called engine knock occurs, being ruinous to the engine. Understanding the chemical basis of knock is the subject of this chapter.

Jürgen Warnatz, Ulrich Maas, Robert W. Dibble

17. Formation of Nitric Oxides

Abstract

With the steady increase in combustion of hydrocarbon fuels, the products of combustion are distinctly identified as a severe source of environmental damage. The major combustion products are carbon dioxide and water. These products were, until recently, considered harmless. Now, even the carbon dioxide is becoming a significant source in the atmospheric balance, and concerns of a global greenhouse effect are being raised.

Jürgen Warnatz, Ulrich Maas, Robert W. Dibble

18. Formation of Hydrocarbons and Soot

Abstract

Hydrocarbons and soot predate NO _x (Chapter 17) as pollutants from combustion. In earlier times, smoke from the factory smokestack was a sign of prosperity. In time it became a nuisance and finally a health concern. The remedy to the appearance of soot and smoke are the three “t’s” of combustion: time, temperature, and turbulence (Babcock and Wilcox, 1972). By allowing for more time at high temperatures with good mixing, one is usually assured of oxidizing soot and other hydrocarbons. However, these conditions also lead to a larger production of NO.

Jürgen Warnatz, Ulrich Maas, Robert W. Dibble

19. References

Jürgen Warnatz, Ulrich Maas, Robert W. Dibble

Backmatter