Premixed flames refer to the combustion mode that takes place when a fuel and oxidizer have been mixed prior to their combustion. Premixed flames are present in many practical combustion devices. Two such applications are a home heating furnace and a spark ignited internal combustion engine. In premixed flame combustors, the fuel and oxidizer are mixed thoroughly before being introduced into the combustor. Combustion is initiated either by ignition from a spark or by a pilot flame, creating a ‘flame’ that propagates into the unburned mixture. It is important to understand the characteristics of such a propagating flame in order to design a proper combustor. Some relevant engineering questions arise, such as: How fast will the flame consume the unburned mixture? How will flame propagation change with operating conditions such as equivalence ratio, temperature, and pressure? From a fire protection viewpoint, how can flame propagation be stopped? Topics covered in this chapter include: (1) the physical processes in a premixed flames, (2) flame speed and flame thickness, (3) flammability limits, (4) flame quenching, (5) minimum energy for sustained ignition and subsequent flame propagation, and (6) turbulent premixed flames.
KeywordsEquivalence Ratio Flame Propagation Flame Temperature Premix Flame Flame Speed
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