A Mie scattering investigation of the effect of strain rate on soot formation in precessing jet flames

Abstract

This is an experimental study of soot formation in precessing jet flames. The Mie diagnostic technique was implemented to provide qualitative visualisation of the zones of soot formation. A range of conditionally sampled experiments was carried out. The characteristic Reynolds number based on the nozzle diameter, was varied from 4329 to 11223 and the Strouhal number based on the nozzle diameter, was varied from 0.0042 to 0.0245. The nozzle diameter was fixed at 5 mm and the jet exit angle at 45 deg. Experimental data were collected and used to show the tendencies in the formation of soot at different experimental conditions. It was found that the relative soot intensity increases with increase in both Re and St numbers. The instantaneous images reveal that soot is predominantly formed in sheets of varying thickness. Very little soot is observed in the near nozzle region, which is consistent with the idea that the formation of soot in appreciable quantities is kinetically limited. Readily observable are very broad regions of low signal spanning much of the flame. These broad regions are more prevalent in the high St number flames where strain rates are lower and residence times are longer. The experimental results support the hypothesis that low strain in a diffusion flame promotes soot formation and high emissivity (i.e., soot formation correlates inversely with flame strain).