Effects of Stretch and Preferential Diffusion in Laminar Syngas Premixed Flames

Syngas is considered a promising renewable fuel because of its superior emission characteristics and high flexibility with respect to its production and utilization. However, its composition can vary widely, which poses challenges with its use in practical systems. In this regard, fundamental studies dealing with its combustion and emission behavior can provide valuable input for designing such systems. This paper reports a computational study on the structure, thermal-diffusive instability, and extinction of syngas premixed counterflow flames. One-dimensional (1D) and two-dimensional (2D) simulations are performed using validated computational fluid dynamics (CFD)–based codes and a chemistry model with 52 species and 544 reactions. Results focus on the effects of stretch, preferential diffusion, and radiation on flame characteristics. The effect of preferential diffusion is to increase and decrease the peak heat release rate with strain rate for lean and rich syngas flames, respectively. Similarly, the flame temperature is higher and lower than the adiabatic flame temperature for lean and rich flames, respectively. In addition, for lean flames, the flame temperature exhibits nonmonotonic variation, first increasing and then decreasing with strain rate as the flame approaches extinction. At low strain rates ($k<25{\mathrm{s}}^{-1}$), the effect of radiation is to reduce flame temperature and burning rate and to cause extinction at very low strain rates. Thus, flame radiation leads to two extinction limits in terms of strain rates. Temporal evolution of the extinction process at high strain rates indicates that extinction starts locally near the centerline, and this region subsequently spreads radially until the flame extinguishes globally. Simulations are also used to examine correlations between stretch and burning rate. Lean flames are found to be diffusively unstable ($Ma<0$), and their propensity toward instability increases as the mixture becomes leaner or $H_2$ content in syngas increases. Conversely, rich syngas flames are diffusively stable ($Ma>0$), and their propensity toward instability decreases as the mixture becomes richer or $H_2$ content increases. These results are consistent with previous studies dealing with spherical flames burning $H_2$ and hydrocarbon fuels.