  A high-pressure hydrogen micromix combustor has been investigated using
direct numerical simulation with detailed chemistry to examine the flame
structure and stabilisation mechanism. The configuration of the combustor was
based on the design by Schefer [1], using numerical periodicity to mimic a
large square array. A precursor simulation of an opposed jet-in-crossflow was
first conducted to generate appropriate partially-premixed inflow boundary
conditions for the subsequent reacting simulation. The resulting flame can be
described as a predominantly-lean inhomogeneously-premixed lifted jet flame.
Five main zones were identified: a jet mixing region, a core flame, a
peripheral flame, a recirculation zone, and combustion products. The core
flame, situated over the jet mixing region, was found to burn as a thin
reaction front, responsible for over 85% of the total fuel consumption. The
peripheral flame shrouded the core flame, had low mean flow with high
turbulence, and burned at very lean conditions (in the distributed burning
regime). It was shown that turbulent premixed flame propagation was an
order-of-magnitude too slow to stabilise the flame at these conditions.
Stabilisation was identified to be due to ignition events resulting from
turbulent mixing of fuel from the jet into mean recirculation of very lean hot
products. Ignition events were found to correlate with shear-driven
Kelvin-Helmholtz vortices, and increased in likelihood with streamwise
distance. At the flame base, isolated events were observed, which developed
into rapidly burning flame kernels that were blown downstream. Further
downstream, near-simultaneous spatially-distributed ignition events were
observed, which appeared more like ignition sheets. The paper concludes with a
broader discussion that considers generalising from the conditions considered
here.
