Premixed Flames

Abstract

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.