Experimental investigation of internal structures of NH3/H2/O2/N2 premixed jet flames using multi-scalar imaging

Journal article, 2024

Ammonia-hydrogen (NH3-H2) blends stand as a promising carbon-neutral fuel alternative. This study delves into the variation in the internal structure of NH3-H2 flames with varying Lewis numbers (Le) and turbulence intensities. Simultaneous multi-scalar imaging of NH3/NH/OH was performed to characterize the structures of turbulent jet flames with the same unstretched laminar flame speed (30.8 cm/s) but different H2 contents. Internal flame structures were characterized by isocontours of NH3, NH (both inner and outer edges, NHi and NHo), and OH to delimit the reaction zone. The parallelism of the selected isocontours (i.e., NH3, NHi, and NHo) with the OH baseline was quantified by calculating the probabilities, P(δp), of the selected isocontours overlapping with the OH baseline shifted by a distance of δp along its normal direction. On the one hand, results have revealed that increasing turbulence disrupts the parallelism of the selected isocontours with the OH baseline, leading to a larger and FWHM, as well as a reduced skewness of the P(δp) profiles. On the other hand, flames with higher H2 content and lower Le exhibit great resistance to turbulence but experience enhanced wrinkling due to the differential diffusion effect, which can also cause deviations in scalar parallelism at low turbulence intensity. For flames with reduced Le numbers, the NH3 isocontours are observed to be located even downstream of the NHi isocontours, which belong to the reaction zone. The extent of parallelism for the selected isocontours within the reaction zone is found to respond to turbulence and Le differently, deviating from an idealized picture of the flamelet manifold.