Measurement and prediction of NOx emissions from unconfined propane flames from turbulent-jet, bluff-body, swirl and precessing jet burners

Abstract

The relationships among flame radiation, NOx emissions, and residence time are explored for unconfined propane turbulent diffusion flames with widely dissimilar mixing characteristics. Variations in mixing of a turbulent-jet were achieved at constant initial conditions using coaxial air through swirl and bluff-body burners and using a precessing jet burner to allow the separate effects of jet Froude number and flame temperature to be explored. A wide range of flame types were produced, from the strongly radiating and sooting flame of the precessing jet burner, which is buoyancy dominated, through recirculation-zonedominated bluff-body and swirl flames, for which partial quenching of the reactions occurs. It was found that the radiant fraction scales with residence time for all the flames, suggesting a relationship between Froude number and global flame temperature. The importance of flame temperature, and hence thermal NOx, in all flames is demonstrated. The NOx emission indices from these flames are well correlated with the residence time and non-adiabatic flame temperature, demonstrating that the role of flame radiation is significant. Departure from purely thermal NOx predictions for high-temperature recirculation-zone-dominated flames is attributed to the poor correlation of the reaction-zone volume with the flame volume of this class of flames. Departure for the most radiant, and hence lowest temperature, flames is attributed to increased relative significance of the prompt mechanism.