Experimental and numerical investigation of the synergistic effect on soot formation in ethanol/n-heptane laminar diffusion flames at elevated pressures

Abstract

As a carbon-neutral fuel, ethanol is commonly used as a partial replacement for gasoline, but its impact mechanism on the soot formation of gasoline remains unclear, especially under elevated-pressure conditions. In this work, the soot formation of ethanol/n-heptane was experimentally and numerically investigated in laminar diffusion flames in the pressure range of 1.2–5 bar. For all experimental conditions, the total carbon flow rate in the fuel was maintained at 0.353 mg/s with 0%, 10%, 20%, and 30% ethanol contributions. The soot volume fraction and flame temperature were obtained by laser-induced incandescence coupled with light extinction and color-ratio soot pyrometry technique. In the considered pressure range, the synergistic effect on the soot formation was observed with 10% ethanol addition and became more pronounced at higher pressures. The soot formation was suppressed when the amount of ethanol was increased to 20% or 30%. Numerical simulations are performed using a reduced n-heptane/ethanol chemical mechanism and a soot model based on the bivariate Hybrid Method of Moments. The synergy effect was successfully predicted by the model. The simulation results indicate that the reason for the synergistic effect is the increase in acetylene and methyl caused by ethanol pyrolysis, which increases the mass fraction of soot precursor species and consequently promotes PAH condensation.