Experimental investigation of NH3-H2 jet flames adopting multi-scalar imaging: Comparison of turbulent burning velocities obtained using different flame-front markers

Journal article, 2025

A series of lean NH3/H2/O2/N2 pilot jet flames is investigated using simultaneous planar laser-induced fluorescence (PLIF) imaging of NH3/NH/OH species. Turbulent rms velocity (uʹ) and Karlovitz number (Ka) are varied in a wide range by changing the inlet bulk flow velocity. The Lewis number (Le) is varied by changing the hydrogen volume fraction in the fuel blends. Laminar flame speed (SL), flame thickness (δL), and Zel’dovich number (Ze) are varied by enriching air with oxygen. Turbulent burning velocities (UT) are evaluated by measuring the inlet mass flow rates and mean flame front areas associated with different flame markers, i.e., NH3, NH, and OH. The obtained results show the following trends. First, there are significant quantitative differences between burning velocities measured using different flame front markers. Second, while UT,NH3, which is associated with an isosurface within flame preheat zones, is weakly affected by variations in Le or δL, the three other burning velocities, which are associated with isosurfaces within flame reaction zones, are significantly higher in flames characterized by smaller Le or δL. Third, for NH and OH marked flame fronts, the increase in UT with decreasing Le is attributed to differential diffusion effects, which are more pronounced for NH3/H2/air mixtures characterized by a higher Ze when compared to the counterpart mixtures enriched with oxygen. Fourth, the data measured at the highest uʹ/SL = 234 and Ka = 1670 do not show any sign of levelling-off of UT(uʹ)-curves but imply significant influence of differential diffusion on UT (for NH and OH isosurfaces). Fifth, the experimental data on UT/SL can be well approximated using power-law fits with respect to uʹ/SL and L/δL and substituting major characteristics of unperturbed laminar flames, i.e., SL and δL, with the counterpart characteristics of highly strained laminar flames. This finding supports leading point concept of premixed turbulent
combustion.