Investigation of sooting in normal-gravity droplet combustion using light extinction and gravimetric techniques

Abstract

Normal-gravity soot volume fraction (fv) measurements performed for combustion of single toluene droplets under atmospheric and reduced pressures are presented. Because of the transient nature of the flame and the sooting region, a full-field light extinction/tomographic inversion technique was used. The reduction in sooting as a function of pressure was assessed by using the maximum soot volume fraction at various axial positions above the droplet. The maximum soot volume fraction was reduced by 70% when the pressure was reduced from 1 to 0.4 atm. The observed variation in sooting cannot be attributed solely to the changes in the adiabatic flame temperature (caused by reductions in pressure). The variations in residence time in the changing flow configurations caused by reductions in the system Grashof number also contribute to the reduction in sooting. The gravimetrically measured soot yield was also compared to the optical measurements of fv to determine the existence of a correlation between the two techniques. The total soot yield was proportional to the maximum fv and displayed the same linear variation when the pressure was reduced. Therefore, the nonintrusive light extinction measurements can be calibrated (with the gravimetric measurements) to not only provide fv within the flame (which is important for radiation characterization and soot/flame structure analysis) but to simultaneously measure the total particulate emission through the open-tipped flames (which is important in assessing the environmental impact of hazardous droplet combustion).