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

Abstract

Two groups of both lean and rich NH/H/O/N turbulent premixed piloted jet flames with the same laminar flame speed are investigated using simultaneous multi-scalar laser diagnostics techniques including NH/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 is varied in a wide range by changing the bulk flow velocity. Global consumption speeds 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., , (inner contour), and , where designates species number density normalized using its maximum value. The obtained results show the following trends to be emphasized. First, an increase in by is more pronounced when compared to two other global consumption speeds (, where i indicates the inner contour, and ) and is close to a linear dependence. The adapted mixture shows only a moderate influence on but not on the other two. Second, is weakly affected by variations in Lewis number . On the contrary, third, is significantly higher in lean mixtures characterized by when compared to rich mixtures characterized by . Fourth, these variations in are reasonably well approximated introducing an empirical factor of into fitting onto . The qualitative difference between sensitivities of and to variations in is attributed to the fact that the NH 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.