Effects of CO 2 dilution on turbulent premixed flames at high pressure and high temperature

Abstract

Turbulent premixed flames of mixtures of CH 4 and air diluted with CO 2 at high pressure and high temperature were experimentally studied to clarify the effects of exhaust gas recirculation (EGR), especially the effects of CO 2 in EGR gases, on turbulent flame characteristics. The maximum CO 2 dilution ratio, defined as the ratio of the molar fraction of CO 2 to those of air and CO 2, 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, S T, normalized using laminar burning velocity, S L, became smaller when the mixture was diluted with CO 2. 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 CO 2 dilution. Moreover, the mean fuel consumption rate in the flame region decreased with increasing CO 2 dilution ratio. This effect was stronger than the decrease in mass fraction of fuel due to CO 2 dilution. The decrease in the smallest wrinkling scale of the flame front with increasing turbulence Reynolds number in the case of CO 2 dilution was more significant than that in the case of no CO 2 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.