Effects of CO₂ on the laminar burning velocities of toluene reference fuel (TRF) with increasing initial temperatures and pressures

Spherically expanding flames of TRF/air mixtures diluted with different levels of CO₂ content at elevated temperatures and pressures were experimentally studied in this work to clarify the effects of exhaust gas recirculation (EGR), especially the effects of CO₂ in EGR gases on the laminar flame characteristics of gasoline fuel. The experiments have been performed using a constant volume combustion bomb with schlieren method. The laminar burning velocities (LBV) and markstein lengths of the TRF flame with different dilution level of CO₂ (0–20%) at the elevated temperature of 373 K, 443 K and pressures of 1 bar, 3 bar and 5 bar have been studied. Numerical decoupling simulations were then applied to investigate the individual dilution, thermal and chemical effects of CO₂, respectively. The CO₂ addition has a suppressing effect on the LBV of the TRF fuel at all the tested conditions. The LBV of the TRF mixture diluted with 20% CO₂ reduced to about 60% of the non-diluted flame at 373 K and atmospheric pressure. The LBV were promoted by the elevation of initial temperature, while inhibited by the promotion of initial pressure. The increasing of initial temperature extended both the lean and rich limits, while the elevation of initial pressure would facilitate the combustion on the leaner flame. The Markstein length had a slight increase, indicating a small increase of flame instability, with the increase of the of CO₂ dilution ratio for the rich flames and decreased for the lean flames. The crossover equivalence ratio, φ*, was witnessed to shift slightly towards the leaner side, while the critical equivalence ratio, φ_c, moved to the richer side. Among the three factors, the dilution effect played the dominant role and the chemical effect came up the next. The thermal effect took up the least portion, yet non-negligible. In addition, the chemical effect showed a more sensitive respond to the temperature variation.