The main alternative methodology to the one described in the previous section is known as the
laminar flamelet approach. In the correct combustion regimes, the chemical timescale for the
combustion reactions is much shorter than any flow timescale, i.e. the high Damköhler number
regime. In other words the thickness of the flame (lF , l) is much smaller than any
characteristic flow length, and in particular, much smaller than the Kolmogorov lengthscale
which is the shortest turbulent length scale in the flow. This implies that the
flame can be considered as a 2-d sheet separating regions of burnt and unburnt gas
(for premixed combustion) or regions of fuel and air (non-premixed combustion).
This flame front propagates through the flow the laminar flame speed sL, which
is relatively easy to specify (from experiment or modelling), and it also interacts
passively with the flow, being transported by the mean flow and wrinkled by the
turbulence. This motion can be linked to the flow characteristics as computed from, say, a
standard k -
model, thus providing a complete combusting flow simulation. The
detailed modelling will vary depending on case, but the framework works very well for
premixed, non-premixed and partially premixed combustion. Figure 2 shows this
diagramatically.
In detail, the location of the flame sheet is specified by some form of indicator function. For instance in premixed combustion, a progress variable c can be defined as a normalised temperature or normalised mass fraction
One final point concerns the modelling of the correlation term . Correlation
terms of this form crop up frequently in turbulent combustion modelling, and are
frequently modelled using the gradient transport assumption. This is the assumption that
this sort of term is essentially a diffusive effect and can be modelled in this way, as
![]() | (VII.9) |