Two-Angle Ratio Scattering (STAR) Method for Real-Time Measurement of Agglomerate Soot Concentration and Size: Theory

Abstract

The Scattering by Two-Angle Ratio (STAR) light scattering method described in this article is based on a new analysis of a well-established theory in the literature that describes black carbon soot as fractal agglomerates. Previous analyses have shown that the measured scattering signals at two appropriate angles can be used to determine the mean agglomerate particle size and mass concentration. The current work shows that a dimensionless invariant function of the two-angle scattering ratio can be defined (C m #), for computation of the mass concentration and mean agglomerate size. In addition, the three soot optical properties can be combined into one overall soot property constant, S p , which, based on literature measurements of each component property, is also near invariant for a variety of fuel and combustor conditions. Research literature on optical parameters has been used (without resorting to arbitrary calibrations) to compute values of the gas phase soot mass concentration. In a companion article (Holve et al. 2011), measurements of diesel and gas turbine engines show that the model for interpreting scattering measurements from STAR is in reasonable agreement with gravimetric measurements.

Two-Angle Ratio Scattering (STAR) Method for Real-Time Measurement of Agglomerate Soot Concentration and Size: Experimental Measurements

“…The primary objective of this work has been to develop a fast response and robust optical instrument based on fundamental RDG and PFA theories that can quantitatively measure gas phase engine exhaust soot concentrations and agglomerate particle size. The measurements of mass concentration, agglomerate size, and agglomerate and primary particle number presented here are based on the analysis model developed by (Holve 2011), combined with the assumption that the soot primary particle parameter, S p , is near invariant. Soot properties are based on recent measurements of the refractive index, which give consistent values for both scattering and absorption.…”

“…The application of STAR for the measurement of exhaust emissions is subject to stringent requirements based on EPA 40CFR Part 1065 (henceforth "1065"), ISO16183, and others. For these conditions, we used the Siris geometry shown in Figure 1 with a large angle detector of 120 degrees that uses a different value of C m # as described by Holve (2011). The Siris instrument was integrated with the BG-3 partial flow sampling system (PFSS, patent no.…”

“…In a companion article (Holve 2011), a rationale and theory for using Scattering by Two-Angle Ratio (STAR) have been developed to more accurately determine the size and mass concentration of agglomerate soots based on light scattering (patent no. 7782459).…”

“…Equations (1) and(2) allow explicit estimation of light scattering measurement uncertainties due to uncertainties in the knowledge of the instrument and soot properties (Holve 2011). The resulting equations are generally applicable to soot from both diesel engines and gas turbines (GTs).…”