SOOT VOLUME FRACTIONS IN THE OVERFIRE REGION OF TURBULENT-DIFFUSION FLAMES

Overfire soot volume fractions and mixture fractions, flame heights, and characteristic flame residence times, were measured for turbulent acetylene, propylene, ethylene and propane diffusion flames burning in still air. Test conditions ranged from highly buoyant pool-like flames to buoyant jet flames, using three burners (with exit diameters of 5, 50, and 234 mm) and a wide range of fuel flow rates. Soot generation efficiencies (the percentage of fuel carbon converted to soot and emitted from the flame) were uniform throughout the overfire region for a given flame condition. Soot generation efficiencies increased with increasing flame residence times but tended to approach asymptotic values for residence times roughly ten times longer than residence times at the normal smoke point. Within the asymptotic region, soot volume fractions are directly related to mixing levels, analogous to the laminar flamelet concept for nonpremixed flames, which offers substantial simplifications for analysis of the continuum radiation properties of the overfire region. © 1990.