  Turbulent mixing is a physical process of fundamental importance in
high-speed premixed flames. This mixing results in enhanced transport of
temperature and chemical scalars, leading to potentially large changes in flame
structure and dynamics. To understand turbulent mixing in non-reacting flows, a
number of classical theories have been proposed to describe the scaling and
statistics of dispersing fluid particle pairs, including predictions of the
effective, or turbulent, eddy diffusivity. Here we examine the validity of
these classical theories through the study of fluid particle pair dispersion
and eddy diffusivity in highly turbulent premixed methane-air flames at a
Karlovitz number of approximately 140. Using data from a direct numerical
simulation and a higher-order Lagrangian tracking algorithm, particle pair
centroids are seeded at different initial temperatures and separations, and
then integrated forward in time. We show that scaling relations and results
developed for pair dispersion in non-reacting flows remain relevant in this
high-intensity premixed flame, and we identify the impacts of heat release on
dispersion and eddy diffusivity.
