Quantitative measurements of thermo-chemical states in turbulent lean and rich premixed NH3/H2/N2-air jet flames

Premixed piloted jet flames are an ideal generic configuration to examine the impact of turbulence on thermochemical states for staged-combustion systems, like rich-quench-lean technologies, which have been proposed for ammonia combustion to minimize emissions. The current study aims to gain fundamental insights on the internal scalar structure of such premixed and rich-lean stratified ammonia-hydrogen flames. Turbulent premixed NH3/H-2/N-2-air jet flames, stabilized by a large, lean pilot flame (phi= 0.57), were investigated over a range of lean to rich global equivalence ratios (phi(global) = 0.8, 1.2, and 1.6), employing simultaneous 1D Raman/Rayleigh spectroscopy with a novel calibration approach for NH3. The quantitative scalar data of instantaneous flame structures and thermo-chemical states are analyzed with emphasis on the NH3-H-2 interaction and its effects on differential diffusion. In the transition from lean to rich jet flames, the spatial flame structures reveal the presence of residual H-2 in the products, while a significant minimization of the NH3 slip is observed. The remaining H-2 undergoes turbulent mixing with the hot exhaust gas causing additional heat release and elevated temperatures compared to 1D adiabatic flame simulations. The local oxygen concentration is found to be a determining factor in the interaction between thermal cracking and oxidation of NH3. Due to the formation of H-2 as a result of NH3 cracking on the one hand and the oxidation reactions and diffusion of H-2 on the other hand, a relatively high H-2 concentration is still observed at relatively high temperatures despite the presence of O-2. This interplay between in situ cracking, diffusion, turbulent mixing, and oxidation reactions leads to a zone of stratified combustion, so that overall a two-stage combustion characteristic is observed, showing premixed combustion primarily within the jet flow and stratified combustion in the mixing zone with the pilot exhaust gas.