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 CO2 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/CO2 mixtures at different CO2 dilution rates were measured and the chemical effect of CO2 was separated from its total effects by introducing a type of fictitious carbon dioxide (FCO2) in the simulation. Results show that the laminar flame speeds of n-heptane/air/CO2 mixtures nonlinearly decrease with the increase of CO2 dilution rate. Moreover, as CO2 dilution rate increases, the instability of laminar n-heptane/air/CO2 flames decreases due to the decrease of the hydro-dynamic instability. The results of kinetic analysis show the increase of CO2 concentration in the n-heptane/air flames decreases the peaks of mole fractions of the main radical species (OH, O, H, CH3 and HO2), net reaction rates of the main elementary reactions and flame temperature. Both the physical and chemical effects of CO2 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 CO2 remarkably promotes the production of CO via shifting the equilibrium of R24 CO + OH = CO2 + H, and the third-body effect of CO2 slightly increases the production of H2O shifting the equilibrium of R8 H2O + M = H + OH + M. The backward reactions of R24 and R8 enhanced by CO2 addition compete with the important chain branching reaction R1 H + O2 = O + OH for H atoms, which plays the chemical role in reducing the laminar flame speed and flame temperature.