15 - 15 Combustion and flames 15.1 Introduction Combustion...

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15 Combustion and flames 15.1 Introduction Combustion is the mechanism by which the chemical (bond) energy 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 flume. 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. In these engines the fuel and air are mixed (often homogeneously) during the admission process to the engine, either by a carburettor or low pressure inlet manifold fuel injection system. 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 flume traverses charge (FTC), and once combustion has commenced it is very difficult to influence its progress. The diffusion flame occurs in situations of heterogeneous 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. 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 bums 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.
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292 Combustion andflames 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
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This note was uploaded on 03/09/2010 for the course MECHANICAL ME9802701 taught by Professor Prof.william during the Spring '10 term at Institut Teknologi Bandung.

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15 - 15 Combustion and flames 15.1 Introduction Combustion...

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