Tech Briefs

Using Laser-Induced Incandescence To Measure Soot in Exhaust

This system incorporates several improvements over prior LII soot-measuring systems.

An instrumentation system exploits laser-induced incandescence (LII) to measure the concentration of soot particles in an exhaust stream from an engine, furnace, or industrial process that burns hydrocarbon fuel. In comparison with LII soot-concentration-measuring systems that have been described in prior NASA Tech Briefs articles, this system is more complex and more capable.

Like the other systems, this system includes a pulsed laser and associated optics that shape and aim a laser beam through an exhaust stream. The laser beam heats entrained soot particles to incandescence. Light from the glowing soot particles is collected by two bandpass- filter-and-photodetector assemblies for measurement of the intensity of the incandescence as a function of time in two wavelength bands. On the basis of the established principle of twocolor pyrometry, the instantaneous temperature of the glowing soot particles is determined from the ratio between the instantaneous intensities in the two wavelength bands.

The heating of the soot particles by absorption of laser light and the subsequent cooling of the particles through incandescence (and, when applicable, through evaporation of volatile materials from their surfaces) are complex nanoscale processes that can be represented by a computational model in which, during the decay of incandescence following the laser pulse, the time-dependent absolute intensities and the time-dependent temperature depend, further, on the volume concentration and surface area of the soot particles. In this system, the model is inverted to obtain the number density and size of the primary soot particles. The mass density of soot averaged over the probe volume can then be calculated from the volume concentration.

Calibration of the photodetectors and the optical components that precede them is necessary for determining absolute intensities. In this system, calibration is performed by use of a strip-filament lamp or other extended light source that has a known radiance traceable to that of a standard source maintained by the National Institute of Standards and Technology.

Uniform heating of all soot particles in the probe volume and in a sheath volume surrounding the probe volume is necessary to ensure accuracy. To satisfy this requirement, (1) the laser beam is expanded into a sheet of finite thickness that is perpendicular to the viewing axis of the detecting optics, and (2) the detecting optics include an iris that defines the probe volume as a cylindrical central, mid-thickness region within the beam.

In prior LII systems, the laser fluence is so great that soot particles are heated to temperatures above the sublimation temperature of carbon (about 4,000 K). This was done to produce an LII signal that was somewhat independent of laser fluence, making it unnecessary to measure the temperatures of soot particles. Unfortunately, the loss of mass through sublimation alters the very quantity (mass density of soot) that one seeks to measure. Also for prior LII systems, the laser fluence required to reach sublimation temperatures is dependent upon the initial particle temperature, and is affected by condensed species such as volatile organic compounds and water. In this self-calibrating system, the intensity measurements are used to adjust the laser fluence to keep the laser-heated soot particles below the sublimation temperature.

This work was done by William D. Bachalo and Subramanian V. Sankar of Artium Technologies, Inc. for Glenn Research Center.

Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Commercial Technology Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-17479-1.