An Exploratory Study of Soot Sample Integrity and Probe Perturbation in a Swirl-Stabilized Combustor

Abstract

In-flame measurements of soot particulate using conventional extractive and nonintrusive optical probes are compared for a swirl-stabilized combustor. Except for large (∼5μm) particulate present in the extracted samples, the soot particle size compares favorably with optically measured values, and the soot particle morphology reflects that formed in gas turbine combustors. Two, nonflame sources for the large particulate are suggested by the optical data: particles formed or elongated during transport subsequent to extraction, and particles attrited from upstream carbon deposits on a solid surface. The extractive probe produces a change in the local particle number density which varies from little change to a 70-fold suppression in reacting flow and a 200-fold increase in cold seeded flow depending on the location within the combustor of the optical sampling volume, the location of the extractive probe relative to the optical sampling volume, and the combustor operating conditions.

Optical Measurements of Soot Size and Number Density in a Spray-Atomized, Swirl-Stabilized Combustor

“…To validate the optical system, the optical probe was positioned at the entrance of an extractive probe used previously [5]. The extractive probe was located at an axial location (x/R = 5.0) and radial location (r/R = 0.83) well displaced from the centerline where flow perturbation was expected to be minimized.…”

“…Soot, a nonspherical scatterer with an index of refraction of some uncertainty, therefore requires special consideration. An evaluation of such effects, considered in an earlier study, concluded that the combined error was 20-30 percent with some broadening of the distribution [5].…”

“…The method adopted for the pointmeasurement of particle size and number density of soot particulate was scattering intensity ratioing. Figure 5 illustrates the optical configuration, a modification of that used previously [5]. This earlier work utilized collection optics at 10 and 5 deg off the optical axis.…”

“…Of particular interest are the mass emission, size distribution, and composition of the soot particulate. Extractive probe measurements have been used in the past to derive this set of information (e.g., [2, 3, and 4]) but, in complex flows where backmixing can exacerbate and widely distribute perturbations introduced by the presence of a physical probe, caution must be exercised in the use of such methods [5]. This may include limiting the measurements to the combustor exitplane, far downstream of the recirculation zone.…”

“…Such measurements must be nonintrusive and therefore rely on optical techniques. Although optical techniques have been successfully employed to measure soot in relatively simple flows (flat flames and diffusion flames), limited applications to complex flows have only recently been demonstrated [5,6]. In a recent study [6], we addressed the application of a nonintrusive, optical probe to a complex flow where the principal objective was to evaluate the performance of large angle intensity ratioing for optically measuring the local soot size and soot number density in the reaction zones of a complex flow.…”