  An a priori analysis of a DNS database of turbulent lean premixed methane
flames is presented considering the relative effects of turbulence and LES
filtering, along with a potential modelling approach for LES with finite-rate
chemistry. The leading-order effect was found to be due to the filter
operation; flame response to turbulence was a secondary effect, and manifested
primarily as an increase in standard deviations about conditional means. It was
found that the radicals O, H and OH were less impacted by the filter than other
high-temperature radicals, which were significantly reduced in magnitude by the
filter. By considering reaction path diagrams, key reactions have been
identified that are responsible for disparities between the desired filtered
reaction rates and the reaction rates evaluated using quantities available in
LES calculations (i.e. the filtered species and temperature). More
specifically, the hydrogen abstraction reactions that take CH4 to CH3 (by O, H
and OH) were found to have particularly enhanced reaction rates, and dominate
the reaction path. Under the conditions presented, reaction paths were found to
be independent of turbulence intensity. In general, the filtered reaction rates
from the DNS were found to align more closely with the filtered laminar profile
than the reaction rate of filtered species and temperature, (but disparities
were found to decrease with increasing Karlovitz number). A simple model for
scaling reaction rates is considered based on filtered laminar flame profiles,
and the resulting reaction paths demonstrate proof-of-concept of a simple
approach for formulating a reaction rate model for LES with finite-rate
chemistry.
