Experimental and numerical studies on burning velocities and Markstein numbers of lean laminar H₂/CH₄/air flames

Experimental and numerical studies on the effects of hydrogen concentration on the unstretched laminar burning velocities and the Markstein numbers of premixed H₂/CH₄/air flames were conducted at equivalence ratio of 0.8. The experiments were conducted in a constant volume bomb at mixture temperature of 350 K and various initial mixture pressures up to 0.50 MPa. The mole fraction of hydrogen in the binary fuel was varied from 0 to 1.0. The unstretched laminar burning velocity increased non-linearly with increase in fraction of H₂. It decreased with increase in initial mixture pressure. For each mixture pressure, the sum of concentrations of H and OH radicals in the numerically simulated flame were found to correlate strongly to the unstretched laminar burning velocity. The Markstein number obtained from the experiments varied non-monotonically with increasing hydrogen fraction at initial mixture pressure of 0.10 MPa. It decreased with increase in initial mixture pressure. Analytical evaluation of the Markstein number suggested that the Markstein number may vary non-monotonically with hydrogen fraction due to non-monotonic variation of the effective Lewis number and may decrease with increase in mixture pressure due to increase in Zeldovich number. The propensity of flame instability varied non-monotonically at 0.10 MPa.