Introduction

Premixed combustion requires that fuel and oxidizer be completely mixed before combustion is allowed to take place. Examples of practical applications are spark-ignition engines, lean-burn gas turbines, and household burners. In all three cases fuel and air are mixed before they enter into the combustion chamber. Such a premixing is only possible at sufficiently low temperatures where the chain-branching mechanism that drives the reaction chain in hydrogen and hydrocarbon oxidation is unable to compete with the effect of three-body chain-breaking reactions. Under such low temperature conditions combustion reactions are said to be “frozen.” At ambient pressures the crossover from chain-branching to chain-breaking happens when the temperature decreases to values lower than approximately 1,000K for hydrogen flames or lower than approximately 1,300K for hydrocarbon flames (cf. Peters, 1997). The frozen state is metastable, because a sufficiently strong heat source, a spark for example, can raise the temperature beyond the crossover temperature and initiate combustion.

Once fuel and oxidizer have homogeneously been mixed and a heat source is supplied it becomes possible for a flame front to propagate through the mixture. This will happen if the fuel-to-air ratio lies between the flammability limits: Flammable mixtures range typically from approximately ϕ = 0.5 to ϕ = 1.5, where ϕ is the fuel-air-equivalence ratio defined by (3.12) in Chapter 3. Owing to the temperature sensitivity of the reaction rates the gas behind the flame front rapidly approaches the burnt gas state close to chemical equilibrium, while the mixture in front of the flame typically remains in the unburnt state. Therefore, the combustion system on the whole contains two stable states, the unburnt (index u) and the burnt gas state (index b).