Page 334 - Advanced thermodynamics for engineers

CHAPTER
15
COMBUSTION AND FLAMES

15.1 INTRODUCTION
Combustion is the mechanism by which the chemical (bond) energy (see Chapter 11) in a ‘fuel’ can be
converted into thermal energy, and possibly mechanical power. Most combustion processes require at
least two components in the reactants – usually a fuel and an oxidant. The chemical bonds of these
reactants are rearranged to produce other compounds referred to as products. The reaction takes place
in a flame. There are three parameters which have a strong influence on combustion: temperature,
turbulence and time. In designing combustion systems attention must be paid to optimising these
parameters to ensure that the desired results are achieved. In reciprocating engines the time available
for combustion is limited by the operating cycle of the engine, and it is often necessary to increase the
turbulence to counterbalance this effect. In furnaces the time available for combustion can be increased
by lengthening the path taken by the burning gases as they traverse the chamber.
There are two basically different types of flame: premixed and diffusion. An example of premixed
flames occurs in conventional spark-ignition (petrol, natural gas, hydrogen) engines: see Chapter 16.
Ignition is initiated by means of a spark, which ignites a small volume of the charge in the vicinity of
the spark plug; this burning region then spreads through the remaining charge as a flame front. This
type of combustion mechanism can be termed flame traverses charge, and once combustion has
commenced it is very difficult to influence its progress. Diffusion flames occur in situations of het-
erogeneous mixing of the fuel and air, when fuel-rich and fuel-lean regions of mixture exist at various
places in the combustion chamber. In this case the progress of combustion is controlled by the ability
of the fuel and air to mix to form a combustible mixture – it is controlled by the diffusion of the fuel and
air. An example of this type of combustion is met in the diesel engine, where the fuel is injected into
the combustion chamber late in the compression stroke: see Chapter 16. The momentum of the fuel jet
entrains air into itself, and at a suitable temperature and pressure part of the mixture spontaneously
ignites. A number of ignition sites may exist in this type of engine, and the fuel–air mixture then burns
as the local mixture strength approaches a stoichiometric value. This type of combustion is controlled
by the mixing (or diffusion) processes of the fuel and air. Other examples of diffusion combustion are
gas turbine combustion chambers, and boilers and furnaces.
These different combustion mechanisms have an effect on how the energy output of the combustion
process can be controlled. In the homogeneous, premixed, combustion process the range of air–fuel
ratios over which combustion will occur is limited by the flammability of the mixture. In early petrol
engines the mixture strength varied little, and was close to stoichiometric at all operating conditions

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