Effects of hydrogen concentration on stoichiometric H₂/CH ₄/air premixed turbulent flames

Outwardly propagating stoichiometric flames of H₂/CH ₄/air were studied in a constant volume fan-stirred combustion chamber in order to investigate the effects of hydrogen concentration on the turbulent burning velocities. The experiments were conducted at mixture temperature of 350 K and mixture pressure of 0.10 MPa. The mole fraction of hydrogen in the binary fuel was varied from 0 to 1.0 for turbulence intensities equal to 1.23, 1.64 and 2.46 m/s. Laminar flames of the mixtures were first investigated to obtain the unstretched laminar burning velocities and the associated Markstein numbers. The unstretched laminar burning velocity increased non-linearly with increase in hydrogen fraction. The Markstein number and the effective Lewis number of the mixtures varied non-monotonically with hydrogen mole fraction. The Markstein number was used to investigate the influence of thermo-diffusive effects on the turbulent burning velocity. The turbulent burning velocity increased with increase in hydrogen mole fraction at constant turbulence intensity. This was primarily because of the increase in the unstretched laminar burning velocity with increase in hydrogen mole fraction. However, due to decrease in the relative effect of turbulence on the flame, the ratio of turbulent burning velocity to the unstretched laminar burning velocity decreased with increase in hydrogen mole fraction for fixed turbulence intensities.