  An unconfined strongly swirled flow is investigated to study the effect of
hydrogen addition on upstream flame propagation in a methane-air premixed flame
using Large Eddy Simulation (LES) with a Thickened Flame (TF) model. A
laboratory-scale swirled premixed combustor operated under atmospheric
conditions for which experimental data for validation is available has been
chosen for the numerical study. In the LES-TF approach, the flame front is
resolved on the computational grid through artificial thickening and the
individual species transport equations are directly solved with the reaction
rates specified using Arrhenius chemistry. Good agreement is found when
comparing predictions with the published experimental data including the
predicted RMS fluctuations. Also, the results show that the initiation of
upstream flame propagation is associated with balanced maintained between
hydrodynamics and reaction. This process is associated with the upstream
propagation of the center recirculation bubble, which pushes the flame front in
the upstream mixing tube. Once the upstream movement of the flame front is
initiated, the hydrogen-enriched mixture exhibits more unstable behavior; while
in contrast, the CH4 flame shows stable behavior.
