Experimental and kinetic studies of the effect of CO2 dilution on laminar premixed n-heptane/air flames

Abstract In the present study, experimental and kinetic studies of the effect of CO 2 dilution on laminar premixed n -heptane/air flames were performed via the outwardly propagating spherical flames and the simulation of one-dimensional planar flames. The laminar flame speeds and Markstein lengths of n -heptane/air/CO 2 mixtures at different CO 2 dilution rates were measured and the chemical effect of CO 2 was separated from its total effects by introducing a type of fictitious carbon dioxide (FCO 2 ) in the simulation. Results show that the laminar flame speeds of n -heptane/air/CO 2 mixtures nonlinearly decrease with the increase of CO 2 dilution rate. Moreover, as CO 2 dilution rate increases, the instability of laminar n -heptane/air/CO 2 flames decreases due to the decrease of the hydro-dynamic instability. The results of kinetic analysis show the increase of CO 2 concentration in the n -heptane/air flames decreases the peaks of mole fractions of the main radical species (OH, O, H, CH 3 and HO 2 ), net reaction rates of the main elementary reactions and flame temperature. Both the physical and chemical effects of CO 2 suppress the laminar flame speeds and flame temperatures and inhibit the productions of the main radical species and net reaction rates of the main elementary reactions, and the former dominates. The chemical effect of CO 2 remarkably promotes the production of CO via shifting the equilibrium of R24 CO + OH = CO 2  + H, and the third-body effect of CO 2 slightly increases the production of H 2 O shifting the equilibrium of R8 H 2 O + M = H + OH + M. The backward reactions of R24 and R8 enhanced by CO 2 addition compete with the important chain branching reaction R1 H + O 2  = O + OH for H atoms, which plays the chemical role in reducing the laminar flame speed and flame temperature.