Experimental evaluation of dimensionless extinction coefficient of soot in co-flow laminar diffusion flames at elevated pressures

Dimensionless extinction coefficient (K_e) is one of the most important optical parameters of soot since it affects the accuracy of laser extinction measurement of soot volume fraction. In literature, K_e values have been measured for soot from laminar and turbulent flames with different fuels at atmospheric pressure. However, relevant studies about the measurement of K_e values in flames at elevated pressures are still lacking. In this study, a simultaneous gravity sampling and laser extinction system (GSLE) capable of operating in a combustion chamber was proposed to evaluate the value of K_e in laminar diffusion flames at elevated pressures. Methane and two straight-chain alkanes (n-heptane and n-dodecane) were selected as fuels for comparative studies at 0.1–0.8 MPa and 0.1–0.5 MPa, respectively. The morphology properties of soot samples were analyzed quantitatively by transmission electron microscopy (TEM). The results show the K_e values of in-flame soot for methane, n-heptane, and n-dodecane range from 6.1 to 9.7, 6.5 to 10.7, and 7.4 to 10.9 at the corresponding experimental pressures, respectively. The K_e values of in-flame soot for all three fuels increase with pressure. The above results conclude that using a constant K_e value measured at atmospheric pressure may result in larger errors in the measurement of soot volume fraction in flames at elevated pressure. The quantitative analysis of TEM images shows that the significant variation of soot morphology parameters with pressure is one of the main factors for the variation of K_e value with pressure.