Effect of gravity and pressure on soot formation in laminar inverse diffusion flames at elevated pressures

Inverse diffusion flame (IDF) configuration, where the oxidizer is surrounded by fuel, is commonly used in reforming of hydrocarbon fuels for hydrogen production through the autothermal reforming and partial oxidation processes. Understanding the mechanism of soot formation in IDF is crucial for achieving efficient and environmentally friendly hydrogen production. In this study, high-fidelity numerical simulations were conducted to investigate the effects of pressure and gravity on the soot formation in a laminar IDF configuration at pressures up to 20 bar. The chemical kinetic models with detailed polycyclic aromatic hydrocarbons (PAH) pathways and an empirical reactive soot inception model are employed. The simulation results agreed well with experimental measurements, showing consistency in flame height, PAH concentration, and soot volume fraction. The simulations accurately reproduced the spatial distributions of PAHs and soot in the IDF, and quantitatively captured the linear increase in peak soot volume fraction with pressure. The linear relationship is mainly attributed to the linear increase in the PAH concentration, driven by changes in density due to pressure increase. Moreover, compared to zero gravity condition, higher flame temperature and radical concentrations were observed in normal gravity, leading to higher soot formation rates. However, buoyancy accelerates fluid movement, reducing residence time and ultimately suppressing soot formation in normal gravity conditions.