Investigation of nitrogen dilution effects on the laminar burning velocity and flame stability of syngas fuel at atmospheric condition

Abstract

The objective of this investigation was to study the effect of dilution with nitrogen on the laminar burning velocity and flame stability of syngas fuel (50% H 2–50% CO by volume)–air (21% O 2–79% N 2 by volume) mixtures. The syngas fuel composition considered in this work comprised x% N 2 by volume and ( 100 − x )% an equimolar mixture of CO and H 2. The proportion x (i.e., %N 2) was varied from 0 to 60% while the H 2/CO ratio was always kept as unity. Spherically expanding flames were generated by centrally igniting homogeneous fuel–air gas mixtures in a 40-L cylindrical combustion chamber fitted with optical windows. Shadowgraphy technique with a high-speed imaging camera was used to record the propagating spherical flames. Unstretched burning velocity was calculated following the Karlovitz theory for weakly stretched flames. Also, Markstein length was calculated to investigate the flame stability conditions for the fuel–air mixtures under consideration. Experiments were conducted for syngas fuel with different nitrogen proportions (0–60%) at 0.1 MPa (absolute), 302 ± 3 K , and equivalence ratios ranging from 0.6 to 3.5. All the measurements were compared with the numerical predictions obtained using RUN-1DL and PREMIX with a contemporary chemical kinetic scheme. Dilution with nitrogen in different proportions in syngas resulted in (a) decrease in laminar burning velocity due to reduction in heat release and increase in heat capacity of unburned gas mixture and hence the flame temperature, (b) shift in occurrence of peak laminar burning velocity from ϕ = 2.0 for 0% N 2 dilution to ϕ = 1.4 for 60% N 2 dilution, (c) augmentation of the coupled effect of flame stretch and preferential diffusion on laminar burning velocity, and d) shift in the equivalence ratio for transition from stable to unstable flames from ϕ = 0.6 for 0% N 2 dilution to ϕ = 1.0 for 60% N 2 dilution. The present work also indicated that if the fuel mole fraction in the wide range of fuel–air mixtures investigated is less than 22%, then those fuel mixtures are in the unstable regime with regard to preferential diffusion.