Differential diffusion effects in a turbulent hydrogen oxygen, H2-O2,
nonpremixed flame are investigated using DNS.
A generalized Burke-Schumann formulation
(Sanchez, Linan, and Williams, 1997) that allows for
differing mass and thermal diffusivities as well as finite-rate chemistry
is used. The formulation is based on a three-step reduced mechanism
which assumes partial equilibrium of the two-body chain-carrying reactions
that results in a chemical mechanism with H as the only intermediate
species. The flow field is incompressible decaying isotropic turbulence.
Preferential diffusion of H2 and the intermediate species
H are considered. The objectives are to study
the effect of differential diffusion of both fuel and intermediate species, and
the effects of reaction rate, dilution and turbulence on differential
diffusion.
The effects of differential diffusion are characterized
by the difference in element mixture fractions, Delta=ZH - ZO.
Comparisons are made with the limiting cases of no reaction and a
single-step, infinite rate reaction.
An evolution equation for Delta is derived for each of the three cases.
With nonunity LeH2 and unity LeH, the peak conditional average
temperature increases over the unity Lewis number case. But with
nonunity LeH2 and LeH,
the peak conditional temperature decreases below
the unity Lewis number case. The effects of H2 and H differentially
diffusing are found to be counteracting.
For the nonreacting case, conditional Delta has a reverse "S" shape that is
characteristic of differential diffusion. With reaction, the flame modifies
the shape so
there is a peak at the stoichiometric position. Varying the reaction rate is
found to have little effect on Delta though reacting scalars that are used
in the calculation of Delta are affected.
With increasing dilution, the peak of the conditional average of Delta also
increases.
The shape of the conditional average of Delta for the lowest dilution was
found to approach the
reverse "S" shape of the nonreacting case. With more dilution, Delta
evidenced more of a peak at the stoichiometric position.
Increasing turbulence was found to reduce the conditional average of Delta
though the fluctuations of Delta increased.