Investigation of burning velocity of lean and rich premixed NH3/H2 turbulent flames using multi-scalar imaging

Journal article, 2024

Two groups of both lean and rich NH3/H2/O2/N2 turbulent premixed piloted jet flames with the same laminar flame speed are investigated using simultaneous multi-scalar laser diagnostics techniques including NH3/NH/OH planar laser induced fluorescence (PLIF) and Rayleigh scattering. One group uses air as the oxidizer and the other uses an adapted mixture of atomic N-to-H ratio of 1:3 associated with in-situ hydrogen production from ammonia cracking. Turbulent rms velocity uʹ is varied in a wide range by changing the bulk flow velocity. Global consumption speeds ST,G are evaluated by measuring the inlet bulk flow rates of reactants and areas of time-averaged flame fronts extracted using different flame marker contours, i.e., cNH3 = 0.3, cNH = 0.3 (inner contour), and cOH = 0.4, where c designates species number density normalized using its maximum value. The obtained results show the following trends to be emphasized. First, an increase in ST,G,NH3 by uʹ is more pronounced when compared to two other global consumption speeds (ST,G,NHi , where i indicates the inner contour, and ST,G,OH ) and is close to a linear dependence. The adapted mixture shows only a moderate influence on ST,G,NH3 but not on the other two. Second, ST,G,NH3 is weakly affected by variations in Lewis number Le. On the contrary, third, ST,G,NHi is significantly higher in lean mixtures characterized by Le < 1 when compared to rich mixtures characterized by Le > 1. Fourth, these variations in ST,G,NHi are reasonably well approximated introducing an empirical factor of Le−0.9 into fitting ST,G,NHi /SL onto uʹ/SL . The qualitative difference between sensitivities of ST,G,NH3 and ST,G,NHi to variations in Le is attributed to the fact that the NH3 and NH isosurfaces are localized to preheat and reaction zones, respectively, of the laminar flames. The reported experimental data imply different influence of differential diffusion on flame preheat and reaction zones, thus, calling for further research into the issue.