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

Abstract

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 $\left(u^{\prime }\right)$ and Karlovitz number $\left(Ka\right)$ are varied in a wide range by changing the inlet bulk flow velocity. The Lewis number $\left(Le\right)$ is varied by changing the hydrogen volume fraction in the fuel blends. Laminar flame speed $\left(S_L\right)$, flame thickness $\left({\delta }_L\right)$, and Zel'dovich number $\left(Ze\right)$ are varied by enriching air with oxygen. Turbulent burning velocities $\left(U_T\right)$ 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 $U_{T,\text{NH}3}$, which is associated with an isosurface within flame preheat zones, is weakly affected by variations in $Le$ or ${\delta }_L$, the three other burning velocities, which are associated with isosurfaces within flame reaction zones, are significantly higher in flames characterized by smaller $Le$ or ${\delta }_L$. Third, for NH and OH marked flame fronts, the increase in $U_T$ 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 $u^{\prime }/S_L=234$ and $Ka=1670$ do not show any sign of levelling-off of $U_T\left(u^{\prime }\right)$-curves but imply significant influence of differential diffusion on $U_T$ (for NH and OH isosurfaces). Fifth, the experimental data on $U_T/S_L$ can be well approximated using power-law fits with respect to $u^{\prime }/S_L$ and $L/{\delta }_L$ and substituting major characteristics of unperturbed laminar flames, i.e., $S_L$ and ${\delta }_L$, with the counterpart characteristics of highly strained laminar flames. This finding supports leading point concept of premixed turbulent combustion.

Novelty and Significance Statement

Three different species (NH₃, NH, and OH) are simultaneously measured in lean NH₃/H₂/O₂/N₂ turbulent jet flames adapting two aligned laser diagnostics systems. Turbulent burning velocities $U_T$ are quantified using areas of mean isoscalar contours associated with these species and are compared with one another. Whie $U_{T,\text{NH}3}$ is insensitive to variations in laminar flame thickness or Lewis number, the other burning velocities are significantly increased with decreasing the thickness or $Le$. These measured results imply, for the first time to the authors’ knowledge, qualitatively different behaviors of turbulent burning velocities associated with flame preheat (NH₃) and reaction (NH and OH) zones. Moreover, the experimental data indicate a significant influence of differential diffusion on $U_T$ at Karlovitz numbers as high as 1670 and are well approximated within the framework of leading point concept.