Measurement of soot volume fraction by Cavity Ring-Down Extinction (CRDE) in elevated pressure premixed laminar ethylene/air flames

Abstract

The quantitative measurements of the soot volume fraction can deliver important validation data for detailed models of soot inception, growth, and oxidation in hydrocarbon-fueled flames – a process still not fully understood. Therefore, the sensitive and non-intrusive optical measurement of soot levels in fuel-rich steady flames is highly desirable. This work presents the first in situ Cavity Ring-Down Extinction (CRDE) setup for monitoring soot concentration levels in the low-ppb range in premixed ethylene/air stagnation flames stabilized on a flat flame burner at elevated pressure. The high-reflectivity CRD mirrors were mounted in nitrogen-flushed chambers attached to and separated from the flame area by small diameter apertures. CFD simulations supported the dimensioning of the purge flows and visualized possible perturbation of the cold- and hot-gas (from the flame) flows inside the burner housing. Within the uncertainties of the present experimental conditions and based on results from other labs for similar flame conditions, the pressure dependence of the evaluated soot volume fraction f_V was fitted to a power-law ansatz with an exponent n between 2.1 and 2.6 depending on the equivalence ratio. A global thermal rate constant for surface growth, \({k}_{\text{SG}}\approx 49\pm 20 {\text{s}}^{-{1}}\), was found for a measured flame temperature of 1750 ± 100 K in an atmospheric pressure flame, in gross agreement with results in the literature, which demonstrates the potential of the current setup for soot diagnostics in laminar premixed elevated pressure flames. A detection limit for f_V of above 40 ppt has been determined from a comparative measurement of CO₂ having a small absorption coefficient at the laser wavelength of 1064 nm.