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

Ammonia blends have great potential to be utilized in gas turbine combustors as carbon-free fuels. This paper investigates and compares the lean blow-off behavior of premixed bluff-body stabilized hydrocarbon flames and ammonia/hydrogen/nitrogen flames 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 which span a range of flame stability behaviour from far away from to near lean blow-off. The NH₃/H₂/N₂ flames blow off at leaner conditions as the hydrogen volume fractions increase. A NH₃/H₂/N₂ (70%/22.5%/7.5%) flame is slightly more resilient to lean blow-off compared with methane and propane flames at a velocity of 20 m/s despite having a significantly lower laminar flame speed. The flame structure for all fuel blends change from a ‘V-shape’ to ‘M-shape’ when approaching lean blow-off, as observed in the Abel deconvoluted OH* fields for methane and propane flames and $NH2*$ fields for ammonia fuel blends, which can result in incomplete reactions and finally trigger the lean blow-off. However, the strong OH* intensity in the shear layer near flame root for the NH₃/H₂/N₂ flames indicate that a robust reaction is maintained in this region for this fuel blend, increasing flame stability. The flame and recirculation zone lengths both decrease with equivalence ratio. Widely-distributed positive curvature along the flame surface of the ammonia fuel blend flames which have a Lewis number less than unity, may also enhance combustion for these flames. As blow-off is approached hydro-carbon flame fronts undergo increasing strain rates along their flame fronts, which may be due to their migration towards the inner shear layer region of the flow. In comparison the strain rates along NH₃/H₂/N₂ flames fronts do not change significantly as blow-off is approached due to less dramatic changes to the flame shape. The faster consumption rates of hydrogen than ammonia near the flame root for the ammonia blend flames, and the lower temperature loss compared with the adiabatic temperature may also contribute to the stabilization of ammonia blends near lean blow-off.