Investigation of the near-field structure and stability of non-premixed NH[formula omitted]/H[formula omitted]/N[formula omitted] jet flames at various pressure and co-flow conditions

Abstract

Inspired by the attractiveness of ammonia as an energy carrier coupled with the relative lack of experimental data at industrially-relevant conditions for ammonia/ammonia-blended flames, the current work was undertaken to help fill gaps in our understanding. This study reports the structure and stability of non-premixed, simulated cracked ammonia jet flames at three different cracking ratios (CR) (40%, 50%, and 60%) and pressures (1, 2, and 3 bar), and two different co-flow temperatures (295 and 573 K) using a newly commissioned high-pressure and temperature combustion duct. Measurements of qualitative OH and NO via PLIF were conducted to study the near-field flame structure. The results revealed that the detachment velocity increases monotonically with CR, however for a given CR, the detachment velocity decreases with pressure. For CR 55%, the jet flame exhibits a stable lifted flame behavior whereas blowout occurs directly after detachment for CR 55%. The visible flame length/size tends to slightly increase with pressure for the unheated co-flow cases, while the opposite trend was observed for the heated co-flow cases. As CR increases, the flame length/size slightly decreases regardless the pressure and co-flow temperature. It was observed that increasing CR increases the intensity of OH radicals while the NO intensity is reduced. It is suggested that the mismatch between peak NO and OH locations, which increases with CR, promotes the thermal-NO pathway rather than the fuel-NO pathway. Overall, increasing the pressure and co-flow temperature leads to a significant decrease in the peak NO and OH mismatch. Furthermore, as the pressure increases, both OH and NO intensities decrease when the co-flow is at room temperature. Nevertheless, increasing the pressure in the heated co-flow flames results in decreasing OH intensity while increasing that of the NO. This discrepancy between co-flow temperatures with pressure indicates the need for further investigation.