10 - 10 Thermodynamics of Combustion Combustion is an...

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10 Thermodynamics of Combustion Combustion is an oxidation process and is usually exothermic (i.e. releases the chemical (or bond) energy contained in a fuel as thermal energy). The most common combustion processes encountered in engineering are those which convert a hydrocarbon fuel (which might range from pure hydrogen to almost pure carbon, e.g. coal) into carbon dioxide and water. This combustion is usually performed using air because it is freely available, although other oxidants can be used in special circumstances, e.g. rocket motors. The theory that will be developed here will be applicable to any mixture of fuel and oxidant and any ratio of components in the products; however, it will be described in terms of commonly available hydrocarbon fuels of the type used in combustion engines or boilers. The simplest description of combustion is of a process that converts the reactants available at the beginning of combustion into producrs at the end of the process. This model presupposes that combustion is a process that can take place in only one direction and it ignores the true statistical nature of chemical change. Combustion is the combination of various atoms and molecules, and takes place when they are close enough to interact, but there is also the possibility of atoms which have previously joined together to make a product molecule separating to form reactants again. The whole mixture is really taking part in a molecular ‘barn dance’ and the tempo of the dance is controlled by the temperature of the mixture. The process of molecular breakdown is referred to as dissociation; this will be introduced in Chapter 12. In reality a true combustion process is even more complex than this because the actual rate at which the reactions can occur is finite (even if extremely fast). This rate is the basic cause of some of the pollutants produced by engines, particularly NO,. In fact, in most combustion processes the situation is even more complex because there is an additional factor affecting combustion, which is related to the rate at which the fuel and air can mix. These ideas will be introduced in Chapter 15. Hence, the approach to combustion in this chapter is a simplified one but, in reality, it gives a reasonable assessment of what would be expected under good combustion conditions. It cannot really be used to assess emissions levels but it can be extended to this simply by the introduction of additional equations: the basic approach is still valid. The manner in which combustion takes place is governed by the detailed design of the combustion system. The various different types of combustion process are listed in Table 10.1, and some examples are given of where the processes might be found. There is an interdependence between thermodynamics and fluid mechanics in combustion, and this interaction is the subject of current research. This book will concentrate on the thermo-
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Thermodynamics of combustion 183 dynamics of combustion, both in equilibrium and non-equilibrium states.
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10 - 10 Thermodynamics of Combustion Combustion is an...

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