Probe effects in soot sampling from a burner-stabilized stagnation flame

Abstract

Probe sampling of soot particles in laminar premixed flames is a common method for characterizing nascent soot formation. Probe intrusiveness into the flame can introduce significant uncertainty in interpretation of experimental data and comparison with numerical results. The aim of the present work is to study the probe-induced effects on soot sampling in a burner-stabilized stagnation (BSS) flame by numerical simulations. The thermophoretic effect is investigated first under non-reactive conditions. The relevant model formulation was tested against experimental data from the literature. Soot size distributions and global properties in the burner stabilized-stagnation flame configuration are studied using both a one-dimensional stagnation flow model and two-dimensional axisymmetric simulations using detailed kinetics and transport. A benchmark burner-stabilized stagnation flame fed with ethylene (Camacho et al., 2015 ) was employed as the target for detailed investigation, focusing on the quantification of the orifice flow effect on the soot size distribution. The results show that the orifice flow can introduce a notable impact on the local flow field, temperature, and particle residence time. Soot measurements have to be shifted some millimeters upstream from the stagnation surface because of the impact of the orifice on the local flow and temperature field. The extent of the spatial shift was quantified by comparing one-dimensional stagnation flow and two-dimensional axisymmetric simulations. The results showed that the spatial shift is weakly dependent on fuel chemistry, but it exhibits stronger dependencies on burner to stagnation separation, pressure drop across the orifice, unburned gas velocity, and the orifice diameter. The extent of spatial shift is parameterized with respect to these experimental parameters.