Flame/turbulence interaction in ammonia/air premixed flames at high karlovitz numbers.

Turbulent premixed flames subjected to extreme levels of turbulence exhibit reaction zone broadening and thinning. Instantaneous flame structures visualized using advanced laser diagnostic methods have shown significantly different behaviours of flame/turbulence interaction in jet flames stabilized on burners of different sizes. This paper aims to reconcile the controversy about flame broadening on burners of different sizes. Premixed ammonia/air flames are investigated owing to the importance of ammonia in the future carbon-free energy system. Due to the low laminar flame speed of ammonia/air mixture, the intensity of turbulence (u ′ / S L) can be 5 times higher than that of methane/air flames at the same jet velocities. Planar laser-induced fluorescence (PLIF) imaging and large eddy simulation based on detailed chemical kinetics are carried out to systematically study the structures of ammonia/air premixed jet flames under extreme levels of turbulence conditions, with u ′ / S L up to 240 and Karlovitz number up to 3019. Consistent with the observation in methane/air jet flames reported in the literature, the reaction zones of premixed ammonia/air jet flames show significantly different behaviour on burners of different sizes under similar Karlovitz number conditions. It is found that reaction zone broadening in jet flames is a spatially evolving process, not only depending on the local eddy/flame interaction but also on the upstream history. The onset of distributed reaction zones is found to take place at positions located several jet orifice diameters above the burner and thereafter eddies are self-produced in the reaction layer due to the continuous vortex stretch interacting in the layer and broadening the reaction zone. The flames on a large burner have turbulence eddies of larger integral length scales that do not fit directly inside the reaction zones, and as such the eddies could not broaden the reaction zones. [ABSTRACT FROM AUTHOR]