Premixed flame
Since the fuel and oxidiser—the key chemical reactants of combustion—are available throughout a homogeneous stoichiometric premixed charge, the combustion process once initiated sustains itself by way of its own heat release.
The majority of the chemical transformation in such a combustion process occurs primarily in a thin interfacial region which separates the unburned and the burned gases.
The premixed flame interface propagates through the mixture until the entire charge is depleted.
The inner structure of a laminar premixed flame is composed of layers over which the decomposition, reaction and complete oxidation of fuel occurs.
In the presence of volumetric heat transfer and/or aerodynamic stretch, or under the development intrinsic flame instabilities, the extent of reaction and, hence, the temperature attained across the flame may be different from the AFT.
, the planar, adiabatic flame has explicit expression for the burning velocity derived from activation energy asymptotics when the Zel'dovich number
Let the unburnt conditions far ahead of the flame be denoted with subscript
[4] Second order correction to this formula with more complicated transport properties were derived by Forman A. Williams and co-workers in the 80s.
Flame stretch can happen due to the straining by outer flow velocity field or the curvature of flame; the difference in the propagation speed from the corresponding laminar speed is a function of these effects and may be written as: [8][9] where
is the unit normal on the flame surface pointing towards the unburnt gas side,
In practical scenarios, turbulence is inevitable and, under moderate conditions, turbulence aids the premixed burning process as it enhances the mixing process of fuel and oxidiser.
If the premixed charge of gases is not homogeneously mixed, the variations on equivalence ratio may affect the propagation speed of the flame.
In some cases, this is desirable as in stratified combustion of blended fuels.
The wrinkling process increases the burning velocity of the turbulent premixed flame in comparison to its laminar counterpart.
as: which is defined such that the level-sets of G represent the various interfaces within the premixed flame propagating with a local velocity
This, however, is typically not the case as the propagation speed of the interface (with resect to unburned mixture) varies from point to point due to the aerodynamic stretch induced due to gradients in the velocity field.
Under contrasting conditions, however, the inner structure of the premixed flame may be entirely disrupted causing the flame to extinguish either locally (known as local extinction) or globally (known as global extinction or blow-off).
Such opposing cases govern the operation of practical combustion devices such as SI engines as well as aero-engine afterburners.
The prediction of the extent to which the inner structure of flame is affected in turbulent flow is a topic of extensive research.
Here, the pre-mixed gases flow in such a way so as to form a region of stagnation (zero velocity) where the flame may be stabilized.
In this configuration, the flame is typically initiated by way of a spark within a homogeneous pre-mixture.
The subsequent propagation of the developed premixed flame occurs as a spherical front until the mixture is transformed entirely or the walls of the combustion vessel are reached.
Since the equivalence ratio of the premixed gases may be controlled, premixed combustion offers a means to attain low temperatures and, thereby, reduce NOx emissions.
Due to improved mixing in comparison with diffusion flames, soot formation is mitigated as well.
Premixed combustion has therefore gained significance in recent times.
The uses involve lean-premixed-prevaporized (LPP) gas turbines and SI engines.
