Mixing and Reacting Flows
If we want to burn two gasses, we
have to mix them first. Therefore, mixing usually determines the
rate of combustion. Even if we premix the gases, such as in an
automobile engine, the reaction rate is still often limited by the
mixing of heat from the source of ignition.
So understanding mixing is a first step to understanding fuel
efficiency, pollution control, and other aspects of combustion.
The images below are from direct numerical simulations (DNSs) of
turbulent mixing and subsequent reaction. DNSs involve no models
and are often considered exact, but they are limited to simple
problems. The flow here is so simple that the reaction involves
only two gasses (red and blue) and gives off no heat. Yet it
us a lot about mixing.
tic Region Supercomputing
Turbulent mixing between read
and blue fluid.
Thin zone where reaction occurs.
The images below are slices from DNSs in
of fuel reacts with oxidant in a one-step, irreversible reaction. The
is incompressible, and the fluid properties are constant. The
show that the reaction zones are thin, even when the reaction is
and become thinner when the reaction rate depends on temperature.
Because the reaction zones are so thin, modeling techniques involving
which are common for non-reacting turbulence, cannot be used.
Return to Turbulence
The pink fuel and blue oxidant
mix. The reaction
will occur only where the two are mixed.
An isothermal reaction occurs
where the fluids have mixed. The
reaction zone is about as thick as the mixing zone.
If the reaction releases heat, the
temperature is highest where all
the fuel has been consumed.
Many species react faster at
higher temperature. The reaction
zones are very thin, since the reaction needs fuel, oxidizer, and high
temperature to occur.