Sensitivity of soot formation to strain rate in steady counterflow flames determines its response under unsteady conditions

Abstract

This work reports an original experimental investigation on the sooting characteristics of counterflow diffusion flames (CDFs) subject to sinusoidal oscillations of strain rates. The study was motivated by the necessity to consider transient responses of soot formation when using the laminar flamelet model to simulate soot formation in turbulent flames. Acoustic forcing was applied through loudspeakers installed at the inlet end of both the oxidizer and fuel nozzles of the counterflow burner to generate strain rate oscillations at the desired frequency and amplitudes. Particle image velocity and planar laser induced incandescence were used respectively to measure flow field and the soot volume fraction in a time-resolved manner. Counterflow flames of C2H4 and C3H8 fuels were tested with oxidizer streams of various oxygen mole fractions (XO). The experimental data revealed notable differences in unsteady soot responses between CDFs of C2H4 and C3H8 at the same XO. However, after matching the soot loadings of the two CDFs to have the same sensitivity to strain rate at steady states, their responses to unsteady strain oscillation became almost identical. We experimentally demonstrated that the unsteady response of soot formation to oscillating strain rates, as characterized by phase lag and amplitude attenuations, are strongly correlated with the sensitivity of the soot loading to strain rate under steady conditions. The results may have important implications for the development of transient flamelet models that incorporate unsteady effects on soot formation.