Soot Size Determination In A Swirl Stratified Premixed Ethylene-Air Flame By A Sampling Probe And Elastic Light Scattering

Abstract

An experimental study was performed to measure the size and number density distributions of soot particles produced in turbulent, stratified (high spatial equivalence ratio gradient), swirled (rotating flow) premixed ethylene-air flames at atmospheric pressure. Soot particle size and number density measurements are initially performed using a Scanning Mobility Particle Sizer (SMPS). For this purpose, two-stage sampling and dilution device designed for hot gas analysis was used. The dilution rate of the sampling apparatus was evaluated to ensure a most representative probing. In a second step, the ex-situ measurements were replaced by single-shot measurements recorded with a high cadency planar multi-angle light scattering (2D-MALS) laser diagnostic technique, based on the relationship between aggregate size and light scattering angle. In a third step, results obtained by the SMPS as well as by the optical diagnostic were compared. Because a direct confrontation of the obtained results obtained is not feasible due to difference in nature of both techniques, a two-step post-processing methodology as developed. For this purpose, the median electric mobility diameters measured by SMPS were converted into diameters of gyration by means of a semi-empirical conversion tool. Moreover, a time average data processing was applied to the soot size distributions recorded by 2D-MALS, considering that SMPS measurements performed over a relatively large acquisition time converge to a single value. Results obtained in stratified swirled premixed flames by SMPS exhibit an overall good agreement with the optical measurements. This good similarity between the results provides a remarkably high degree of confidence in the quality of the measurements for both measurement systems investigated. Furthermore, the results suggest that a comparison of soot aggregate size and number density measurements between a sampling probe and a light scattering technique is still possible in turbulent flames at atmospheric pressure. While the current work has focused on the performances of SMPS and the 2D-MALS approach on a one-to-one basis, future work will also address the application of these diagnostics to high-pressure two-phase flames for which measurements are made in confined environments.