The diffusion is the rate-controlling process in turbulent diffusion flames. The characteristic time scales
for convection and diffusion are normally much larger than the characteristic time scale of the combustion
processes. The assumption of infinitely fast chemistry, or local chemical equilibrium, is therefore
justified in some cases. This means that the modelling can be significantly simplified.
However, there are situations where the assumption of fast chemistry does not hold, i.e. reactions
involving CO and NOx, where the finite rate chemistry must somehow be represented. Local quenching, lift-off and blow-off can occur when the local diffusion time scale is of the same order of magnitude as that of the chemical timescale of the reactions.
The assumption of local chemical equilibrium makes it possible to predict the flame length of a jet flame. Central to this is the introduction of mixture fraction, a conserved scalar, which is independent of chemistry. All scalars, such as density, temperature, species concentrations, are uniquely related to the mixture fraction.
Combustion regimes
There are four regimes in non-premixed turbulent combustion
- Flame extinction
- Separated flamelets
- Connected flame zones
- Connected reaction zones
For large mixture fraction fluctuations, Z'_{st} > (\Delta Z)_f , a regime of separated flamelets exists.
The reaction is connected when Z'_{st} < (\Delta Z)_f , i.e. when the mixture fraction fluctuation is smaller than or equal to the diffusion thickness in mixture fraction space.
Content: Gas fire
