Ultra-lean hydrogen-enriched oscillating flames behind a heat conducting bluff-body: Anomalous and normal blow-off

This paper presents a numerical study of ultra-lean hydrogen-methane flames stabilized behind a rectangular, highly conducting metallic bluff body acting as a flame holder. Using high fidelity numerical simulations, we show that lean inverted steady flames exist below normal flammability limits. They have distinct stabilization mechanism from pure methane flames. These flames are blown-off for sufficiently small velocities, a phenomenon called anomalous blow-off. At even leaner conditions oscillating ultra-lean hydrogen-methane flames can be established. These oscillating flames exist within a rather small range of equivalence ratios and inflow velocities, and move to mean locations closer to the flame holder as the reactant flow is increased. We show that the oscillations are associated with the shedding of flame balls from the downstream end of a "residual flame" that remains attached. Unlike their steady counterparts, the oscillating flames exhibit blow-off at both low velocities (anomalous blow-off) and at sufficiently high inflow velocities (normal blow-off). We show that normal blow-off is linked to heat losses to the flame holder.