Effects of pressure on structure and extinction limits of counterflow nonpremixed water-laden methane/air flames

Structure and extinction limits of counterflow nonpremixed water (H₂O)-laden methane (CH₄)/air flames at various pressures are computationally investigated to better understand combustion processes of fuel having naturally high H₂O (vapor) content under elevated pressures. Using a detailed kinetic mechanism and a statistical narrow-band radiation model, the flame structure and extinction limits are predicted for elevated pressures and a wide range of flame strain rates and compared with those at atmospheric pressure. Results show that with increasing pressure the maximum flame temperature increases and the extinction limits are generally extended due to the reduced amount of dissociation and the enhanced radiation reabsorption of H₂O, indicating that flames can sustain more H₂O vapor at elevated pressure. The concentration of active radicals and the flame thickness decrease with increasing pressure. The observed flammable range of the H₂O to CH₄ molar ratio at elevated pressures is comparable to that found in self-sustained combustion of CH₄ hydrates at atmospheric pressure, and the chemical effects of H₂O addition on flame structure are insignificant. Elevated pressure enhances the formation of soot precursors such as acetylene (C₂H₂), implying an opposite tendency from the water addition effects. - Highlights: • Maximum flame temperature increases with increasing pressure. • Extinction limits are extended with elevated pressure. • Enhanced recombination and radiation reabsorption of H₂O cause these tendencies. • Elevated pressure enhances formation of soot precursor C₂H₂. • Thermal effect of H₂O addition on flame structure at elevated pressure is dominant.