A Study of Flame Dynamics and Structure in Premixed Turbulent Planar NH3/H2/Air Flames

Ammonia NH3 has received considerable attention as a near future carbon-free synthetic fuel due to its economic storage/transportation/distribution, and its potential to be thermally decomposed to hydrogen H-2. Since the boiling temperature and condensation pressure of NH3 is comparable to propane C3H8, it could be employed in marine engines running on C3H8 making the combustion processes carbon neutral. To promote the low burning velocity and heat of combustion of ammonia, it is required to enrich the pure ammonia with hydrogen. In this study, two quasi direct numerical simulation (quasi-DNS) cases with detailed chemistry (31 species and 203 reactions) and the mixture-averaged transport model are examined to study the planar ammonia/hydrogen/air flames under decaying turbulence. The reactants temperature and pressure are set to 298 K and 1 atm, respectively. The initial turbulent Karlovitz number is changed from 4.3 to 16.9 implying that all the test conditions are within the thin reaction zones combustion regime. Results indicate that the density-weighted flame displacement speed S-d*, on average, is higher than the unstrained premixed laminar burning velocity S-L(0) value for both test cases. This suggests that the flame elements propagate faster than its laminar flame counterpart. Furthermore, the flame stretch factor defined as the ratio of the turbulent to the laminar burning velocity divided by the ratio of the wrinkled to the unwrinkled flame surface area is higher than unity, i.e., the Damkohler's first hypothesis is not valid for these flame conditions. This indicates that the local flamelet velocity value, on average, is higher than the unstrained premixed laminar burning velocity. In addition, results show that the mean value of the local equivalence ratio for the turbulent conditions is higher than its laminar counterpart due to the preferential diffusion of hydrogen and the turbulent mixing. Furthermore, the net production rate of hydrogen is negatively correlated with the flame front curvature suggesting that the local burning rate is intensified under positively curved regions.