Laser-Induced Incandescence

Abstract Absolute calibration of laser-induced incandescence (LII) is demonstrated via comparison of LII signal intensities with gravimetrically determined soot volume fractions. This calibration technique does not rely upon calculated or measured optical characteristics of soot. The variation of the LII signal with grametrically measured soot volume fractions ranging from 0.078 to 1.1 ppm established the linearly of the calibration. With the high spatial and temporal resolution capabilities of laser-induced incandescence (LII), the spatial and temporal fluctuations of the soot field within a gravimetric chimney were characterized. Radial uniformity of the soot volume fraction, f υc , was demonstrated with sufficient averaging of the single laser-shot LII images of the soot field thus confirming the validity of the calibration method for imaging applications. As illustration, instantaneous soot volume fractions within a Re = 5000 ethylene/air diffusion flame measured via planar LII were established quantitatively with this calibration.

Increasingly, health researchers are discovering that soot itself is implicated directly in the numerous health effects attributed to particulate matter and poor air quality. Medical research over the past decade has revealed that microscopic soot particles are among the most harmful components of air pollution.
Environmental researchers have also identified black carbon as a key contributor to radiative forcing, which is important to climate change.
Due to its low reactivity and low volatility, soot can be reliably measured regardless of temperature and dilution conditions that affect many of the other constituents of particulate matter. .WHAT IS SOOT? Soot refers to the dry solid particles produced through the incomplete combustion of hydrocarbon fuels.
Other terminology used by specialists from a range of fields to describe similar or identical nanoparticles includes: Although the definitions for each may be specific to a field, LII is effective in measuring all of these.

APPLICATIONS
The LII 300 can be applied to measure soot in many applications, including: • On-road mobile emissions • Diesel engine exhaust • Gasoline engine exhaust • Diesel particulate filter performance • Advanced and alternative fuels, including biofuels • Gas turbine particulate emissions • Ambient air monitoring • Atmospheric black carbon levels • Urban air quality • Carbon black production For engine emissions, LII 300 may be reliably applied directly to raw exhaust or to dilute exhaust.

Laser Induced Incandescence Technology (LII)
involves measuring the thermal emission (incandescent light) emitted from particles heated by a pulsed laser to temperatures in the 2500 K to 4500 K range. LII is highly selective, responding only to the presence of black carbon, making it decidedly appropriate for measuring the nonvolatile particles produced as a combustion emission. This selectivity is due to the fact that the nonvolatile particles are primarily black carbon. Black carbon is the primary and most stable constituent of particulate matter emissions from combustion. BC absorbs laser radiation over a broad spectral range, and is refractory, so that the nanoparticles survive heating to the temperatures necessary for the incandescence to be detected. At these temperatures, all volatile components that may have been condensed on the BC particles will be promptly evaporated, and most other nonrefractory particles will have also evaporated or undergone sublimation. Due to this selectivity, LII does not measure the total particle mass, but only mass of nonvolatile particulate matter (nvPM.) A community of science has developed to advance the LII technique at a large number of organizations around the world, and a number of international workshops, to assess the improvements in the LII technique have been held since 2005 (www.liiscience.org).

Selected by SAE E31 committee as the standard for gas turbine nonvolatile particulate matter mass emissions measurements.
Turbine engine (helicopter) tests at Wright Patterson LII signal analysis results in the determination of the mass concentration, volume concentration, active surface area, and primary particle diameter of the particulates. In this context, primary particle diameter refers to the geometrical diameter of the spherical black carbon based particles formed during the combustion of hydrocarbon fuels; often these particles fuse together in the combustor to form a single nonspherical larger particle known as an aggregate. LII does not measure the size of the aggregates emitted from the combustor. The measurements made with LII are produced with each laser pulse at a 20-Hz rate, permitting online time-resolved data collection and reporting of results in real time.

Auto-compensating laser-induced incandescence (AC-LII) enhances the LII
technique by recording the temperatures of the irradiated particles during the measurement process. This technique automatically compensates for any changes in the experimental conditions, including fluctuations in local ambient temperature, variation in laser fluence, laser beam attenuation by the particulate matter, or desorption of condensed volatile material. The LII 300 instrument produced by Artium Technologies distinguishes itself from other LII instruments by being the only commercially available instrument offering AC-LII.
AC-LII is based on the use of a traceable calibration source to establish the spectral sensitivity of the instrument to incandescence. Measurement at two distinct spectral bands enables real-time two-color pyrometry to measure particle temperature. This automatically compensates for: • Fluctuations in local ambient temperature • Variation in laser fluence