Unstretched unburned flame speed and burned gas Markstein length of diluted hydrogen/air mixtures

Fundamental combustion characteristics of H₂/air flames with the addition of actual H₂/air combustion residuals (a mixture of 65% N₂ + 35% H₂O by mole) are examined experimentally and numerically at 1–2 bar, 373–473 K, equivalence ratio of 0.7, and dilution ratios of 0–40%. Spherically expanding flame measurements at constant pressure show that flame speed and adiabatic flame temperature drop almost linearly with increasing diluent level. Detailed numerical simulations and analyses of sensitivity coefficients reveal that this is because of the low chemical reactivity of the dilution mixture. On the other hand, the change in burned gas Markstein length with the dilution mixture addition is found more complex and cannot be represented with a linear trend. Experimental flame speed data are compared with results of chemical kinetic analyses obtained by several chemical mechanisms in order to assess the accuracy of these models.