The burning velocity model (BVM), also known as turbulent flame closure (TFC), is used to close the combustion source term for reaction progress.
 | (7–47) |
 | (7–48) |
Where 
is the density of the
unburnt mixture. Note that the molecular diffusion term for the reaction
progress is removed from the transport equation. The diffusive exchange
of species and energy, which makes the flame proceed in space, is
already accounted for by the source term 
. However, turbulent
transport is a convective process and is modeled using the Eddy Diffusivity
approximation.
The model is completed with a closure for the turbulent burning
velocity 
. Accordingly, this
type of model is called Turbulent Burning Velocity Model (BVM). The
concept has two significant advantages compared to models based on
molecular reaction rates:
In a given configuration,
typically
varies by only 1 order of magnitude. In contrast, molecular reaction
rates occurring in combustion of hydro-carbonates typically vary in
the computational domain by several orders of magnitude.
can be measured directly
in experiments, that is, data is available for the quantity that is modeled.
Further, the burning velocity directly determines target quantities of a simulation, such as flame position. Thus, it is easier to derive and fine-tune accurate models for the burning velocity model than to do so for approaches based on molecular reaction rates.