The behaviour of NH[formula omitted]/H[formula omitted]/N[formula omitted], CH[formula omitted] and C[formula omitted]H[formula omitted] turbulent premixed bluff-body stabilized flames near lean blow-off

Abstract

The lean blow-off (LBO) behaviour of turbulent premixed bluff-body stabilized flames was investigated. Fuels with a range of Lewis numbers were used to examine differential diffusion characteristics, including NH/H/N (70%/22.5%/7.5% by volume), CH and CH. Simultaneous OH-PLIF and PIV were employed to study flame structure changes as the flames approach LBO, and quantify curvature and hydrodynamic strain rates along the flame surface. Large Eddy Simulation (LES) was also conducted to quantify the consumption rates of each fuel at some time before and during the blow-off transient. At moderate inlet velocities the ammonia blended flames are more resilient to LBO in comparison with the methane and propane flames. The flame structure of the ammonia blended flames is much more fragmented than the hydrocarbon flames, even relatively far from LBO, with a higher intensity heat release rate distribution close to the flame base. Higher positive flame curvatures were observed in the ammonia blended flames, which is likely to contribute to the strong anchoring of these flames even in regions of high strain close to the flame base. The consumption rate for these flames is shown to increase locally near the flame base when approaching LBO. Furthermore, the lower strain rates experienced on the surface of NH/H/N flame compared with the other two flames may also delay blow-off. The CH-air flames feature more spatial regions along the shear layers with predominantly negative curvature, which can also enhance the reaction due to the Lewis number of these flames exceeding unity. However in these flames the consumption rate decreases locally in the highly strained regions when ramping to LBO, potentially contributing to their lower relative LBO resilience. For CH-air flames, the curvature, hydrodynamic strain rates and consumption rates do not change much when ramping to LBO, which offer no input to modify the LBO.