Effects of oxygen partial premixing on soot formation in ethylene counterflow flames with oscillating strain rates

The sooting characteristics of counterflow partially-premixed flames subject to monochromatic oscillation of strain rates were investigated. Strain rate oscillations with well controlled frequencies and amplitudes were imposed on a series of ethylene counterflow partially-premixed flames. Flow fields and soot volume fractions were measured non-intrusively in a spatially and temporally resolved manner. Numerical simulations were also performed to support the experimental observations. The results showed that oxygen partial premixing in the ethylene fuel stream notably increased soot production due to the enhanced temperature and oxidative pyrolysis that led to increased formation of benzene precursors. More importantly, despite these enhancing effects on soot loading, we demonstrated for the first time that partial premixing does not alter the sensitivity of soot formation to strain rate in steady counterflow flames. In addition, when subject to oscillating strain rates, the unsteady responses of partially-premixed flames were still seen to be independent of partial premixing. This indicates that our previous conclusion—the sensitivity of soot formation to strain rate in steady counterflow diffusion non-premixed flames determines its response under unsteady condition—still holds for partially-premixed flames. The results could be useful for the development of advanced flamelet model to quantitatively account for unsteady effects on soot formation in practical turbulent flames that frequently encounters partial premixing.