Lean Blow-Off Behaviour of Premixed Bluff-Body Stabilized Hydrocarbon-Air Flames and Ammonia/Hydrogen/Nitrogen-Air Flames

The lean blow-off (LBO) behavior of turbulent premixed bluff-body stabilized hydrocarbon flames and ammonia/hydrogen/nitrogen flame is investigated and compared both experimentally and numerically. Simultaneous high-speed PIV and OH-PLIF are employed to resolve temporal flame and flow field information, allowing the curvature and hydrodynamic strain rates along the flame surfaces to be calculated. OH* and NH2* chemiluminescence images are also used to examine flame structures at the same bulk flow velocity but at four equivalence ratios from far away from to near LBO. A NH3/H2/N2 (70%/22.5%/7.5%) flame is slightly more resilient to LBO compared with methane and propane flames at 20 m/s. The hydrocarbon flame structures change from 'V-shape' to 'M-shape' when approaching lean blow-off, resulting in incomplete reactions and finally trigger the LBO. However, the strong OH* intensity in the shear layer near flame root for the ammonia blend flames indicate a robust reaction which can increase flame stability. Widely-distributed positive curvature along the flame surface of the NH3/H2/N2 flames (Le<1) may also enhance combustion. The less strain rates change along NH3/H2/N2 flames fronts due to less dramatic changes to the flame shape and position can extend the stability limits. Furthermore, the faster consumption rates of hydrogen near the flame root for the ammonia blend flames, and the lower temperature loss compared with the adiabatic temperature also contribute to the stabilization of ammonia blends near lean blow-off.