Radiation-driven flame weakening effects in sooting turbulent diffusion flames

Abstract

The objective of the present study is to use detailed numerical modeling to bring basic information on the structure of turbulent non-premixed flames under sooting and radiating conditions. The questions of flame weakening, flame extinction, and soot leakage are studied using direct numerical simulation. Simulations of ethylene–air combustion are performed in both a reference laminar counter-flow flame configuration and a momentum-driven turbulent wall-flame configuration. In the turbulent wall-flame configuration, the flame optical thickness is artificially increased in order to magnify the role played by luminous thermal radiation. The resulting optically thickened flames feature frequent and pronounced low-temperature, low-burning-intensity spots that may be interpreted as quasi-flame extinction events. The data analysis reveals that these flame weakening events are similar to radiation extinction phenomena previously observed in microgravity laminar flames, and are associated with low values of the fuel–air mixing rate and large values of the flame radiant fraction. These events also differ from previously made observations to the extent that they are associated with soot mass leakage across the flame and occur under a broader range of flame stretch conditions, thereby suggesting that turbulent flames are more susceptible to radiation extinction than their laminar counterparts.