  In the current study, the influence of turbulent mixing and local reaction
rates on deflagration to detonation transition (DDT) was investigated using a
state-of-the-art large eddy simulation (LES) strategy. Specifically, detonation
attenuation by a porous medium, and the subsequent re-initiation for
methane-oxygen, a moderately unstable mixture, was considered. The purpose of
the investigation was to validate the numerical strategy with previous
experimental observations, and to determine what specific roles turbulent
mixing and shock compression have on flame acceleration during the final stages
of DDT. The modelling procedure adopted was a grid-within-a-grid approach: The
compressible linear eddy model for large eddy simulation (CLEM-LES). It was
found that average turbulent fluctuations greater than the laminar flame speed
were required in order to maintain wave velocities above the Chapman-Jouguet
(CJ)-deflagration velocity threshold, a precursor requirement for DDT to occur.
It was also found that sufficient turbulent burning on the flame surface was
required in order to drive pressure waves to sufficiently strengthen the
leading shock wave, locally, in order to trigger auto-ignition hot spots in the
wave front. These local explosion events, which were found to burn out through
turbulent surface reactions, drive transverse pressure waves outward. Upon
subsequent shock reflections or interactions of the transverse waves, new local
explosion events occurred, which further strengthened the adjacent leading
shock wave above the CJ-detonation speed. Eventually, through this process, the
wave sustained the CJ-detonation speed, on average, through the cyclic
mechanism of local explosion events followed by turbulent surface reactions.
Finally, combustion of the flame acceleration process was found to lie
predominantly within the thin-reaction zones regime.
