Effects of CO2 dilution on turbulent premixed flames at high pressure and high temperature

Abstract Turbulent premixed flames of mixtures of CH4 and air diluted with CO2 at high pressure and high temperature were experimentally studied to clarify the effects of exhaust gas recirculation (EGR), especially the effects of CO2 in EGR gases, on turbulent flame characteristics. The maximum CO2 dilution ratio, defined as the ratio of the molar fraction of CO2 to those of air and CO2, was 0.1. The mixture was preheated up to 573 K and the maximum pressure was 1.0 MPa. Bunsen-type turbulent premixed flames of the mixtures were stabilized in a high-pressure chamber. OH-PLIF visualizations of the flames were performed. By analyzing the OH-PLIF images, turbulent burning velocity, mean volume of turbulent flame region, and mean fuel consumption rate were calculated. Results showed that the turbulent burning velocity, ST, normalized using laminar burning velocity, SL, became smaller when the mixture was diluted with CO2. When the turbulent flame region was defined as the region between 〈c〉 = 0.1 and 〈c〉 = 0.9, the mean volume of the flame region increased in the case of CO2 dilution. Moreover, the mean fuel consumption rate in the flame region decreased with increasing CO2 dilution ratio. This effect was stronger than the decrease in mass fraction of fuel due to CO2 dilution. The decrease in the smallest wrinkling scale of the flame front with increasing turbulence Reynolds number in the case of CO2 dilution was more significant than that in the case of no CO2 dilution, corresponding well to the scale relation due to turbulence and intrinsic flame instability proposed previously. These results, as well as the previously reported effects of the profiles of the heat-release region on combustion oscillation, imply that exhaust gas recirculation for high-pressure, high-temperature turbulent premixed flames is effective for restraining combustion oscillation of premixed-type gas-turbine combustors.