Influence of initial pressure on flame propagation characteristics of natural gas-hydrogen-air mixture

Flame propagation of natural gas-hydrogen-air mixture under various initial pressures was studied in a constant volume combustion bomb using a high-speed schlieren photography system. Laminar burning velocities and mass burning fluxes were obtained at various initial pressures, hydrogen fractions and equivalence ratios. Flame stability and the influencing factors (Markstein length, density ratios and flame thickness) were also analyzed using the flame images at various hydrogen fractions, initial pressures and equivalence ratios. The results show that hydrogen fractions, initial pressures and equivalence ratios have combined influence on both un-stretched laminar burning velocity and flame instability. According to the flame propagation pictures taken by high-speed camera, flame stability decreases with the increase of initial pressure. For mixtures with a fixed equivalence ratio and hydrogen fraction, variation of initial pressure has less influence on density ratio than on flame thickness.