Optics: measuring and testing – By particle light scattering
Reexamination Certificate
2004-01-13
2004-10-26
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By particle light scattering
C356S338000, C356S336000, C356S315000
Reexamination Certificate
active
06809820
ABSTRACT:
FIELD OF INVENTION
The present invention relates to a method and apparatus for analysis of submicron-sized particles, such as soot, over a wide range of particle concentrations with high temporal and spatial resolution in particular, it relates to improvements in the Laser-Induced Incandescence technique (LII for short) for improved measurement accuracy by the use of a laser beam of low fluence and/or a good laser energy profile.
BACKGROUND OF INVENTION
The presence of particulate matter, such as soot particles, in the environment has brought about an increased interest in the development of methods and devices for the determination of particulate concentration and its average sizes. Soot in particular has been the subject of study for measurement. However, all small particles pose an important area of interest and concern, particularly for environmental and health reasons. The emission of soot from engines, power generation facilities, incinerators, or furnaces, for example, represents a loss of useful energy and further is a serious environmental pollutant and a health risk. However, the presence of soot in flames can also have positive effects. For example, the energy transfer from a combustion process is largely facilitated by the radiative heat transfer from soot. Thus, to understand soot formation and develop control strategies for soot emission or formation, measurements of soot concentrations are necessary. Other applications include characterization of metal nanoparticles and ceramic nanoparticles. The characterization can be used for monitoring, regulatory compliance, process control production of value-added nanoparticles, and many other applications. LII is a good diagnostic tool for measurements of particulate as the LII signal is proportional to particle volume fraction and is also related to particle sizes.
Current techniques for measuring diesel particulate concentration include the Bosch Smoke Number and direct mass sampling. In the Bosch Smoke Number method particles are collected on filter paper from a portion of the exhaust stream and the light reflection from the collected sample is measured. This is compared against a calibration chart to determine the mass flow. Since sufficient sample material must be collected over time, this method requires a long period for sample collection and has a poor time and spatial resolution. Thus this method cannot provide diagnostic information about the formation of particles in the combustion cycle. The direct mass sampling method is the official regulatory method of the EPA and measures the mass of soot from a difference of the mass of the soot on a filter and the mass of the filter alone. This method, however, has a limited accuracy, particularly for low emission vehicles. Both methods suffer a loss in accuracy when the source produces lower emissions, and require significantly longer testing for low emission combustors.
The measurement of soot particle concentrations has been greatly improved by the development of LII, which can provide concentration information with high temporal and spatial resolution. Previous techniques could not detect small concentrations and could not provide accurate time resolved information regarding soot formation.
LII exposes a volume of gas containing refractory particles, which are particles capable of absorbing laser light energy with an evaporation temperature sufficiently high to produce measurable incandescence, to a pulsed, focused, high-intensity laser light. The particles absorb laser energy, heating to temperatures far above the surrounding gas. At these elevated temperatures (in a range of 4000-4500 K in the case of soot) the particles incandesce strongly throughout the visible and near infrared region of the spectrum. In the past, the regime in which evaporation was the predominant heat loss mechanism limited the maximum particle temperature. For example, any further increase in laser light energy resulted in an increase in the evaporation rate rather than an increase in particle temperature. In accordance with Planck's radiation law, any material gives off energy in the form of radiation having a spectrum and magnitude influenced by its temperature. The higher the temperature is, the greater the intensity is and the shorter the peak wavelength is. Thus the radiative emission at these elevated temperatures increases in intensity and shifts to blue (shorter) wavelengths, compared with that of the surrounding medium. Thus the LII signal is readily isolated from any natural flame emission. Because of the rapid time scale and good spatial resolution, as well as its large dynamic range, LII is well suited as an optical diagnostic to measure soot volume fraction and the particle sizes in turbulent and time varying combustion devices. What was not appreciated heretofore was that optimum results could be achieved by controlling the maximum temperature to be less than a temperature such that evaporation never becomes the predominant heat loss mechanism for a majority of particles within a sample. Therefore, in accordance with this invention, it has been found that optimum results can be obtained by ensuring that no more than 5% of the total solid volume of the particles to be analyzed should be evaporated. Stated differently, preferably 95% of the total solid volume v; of the particles should not evaporate. In a most preferred embodiment less than 2% and preferably 1% or less of the total solid volume of the particles will be evaporated.
Hence, it is an object of this invention to provide a system wherein at least a majority of particles in a sample are heated such that they incandesce and do not significantly evaporate losing a substantial quantity of their solid volume, thereby cooling by way of conduction to a surrounding gas, or medium, rather than through significant evaporation as occurred in the past.
There is an important distinction that is made between “the invention” and the prior art described heretofore. By way of example, the prior art system above, heated a majority of soot particles in a sample, to elevated temperatures between 4000-4500K. At these elevated temperatures two cooling mechanisms were at play; evaporation, and conduction. It was believed at the time, that an advantage of heating particles to these high temperatures was that they incandesced strongly; another advantage was that the LII signal generated was relatively independent of laser fluence, although for unknown reasons; and it was believed that this was an optimum condition.
In patent application WO 97/30335 in the names of Alfred Leipertz et al., published Aug. 21, 1997, a laser-induced incandescence technique is described for determining a primary particle size. The technique taught by Leipertz includes the measurement of the incandescence at two discrete points in time after the laser light pulse, from which a ratio is generated to calculate the particle size according to a mathematical model. However, this technique has been shown to be prone to inaccuracies. Leipertz et al. sample the two measurements at a point of decay where they assume a linear change. This, however, is unlikely to happen until significant cooling has occurred and most of the signal has passed. Thus the signals measured by Leipertz et al are very weak and are highly influenced by noise. Laser fluence (spatial energy density) over the volume measured is also critical to the subsequent temperature decay. It is critical for accuracy to know the energy density profile over the volume. This factor is assumed without verification by the technique of Leipertz et al. Further error is introduced by the detection method, which uses spectrally broadband detectors to measure the signal. The Leipertz et al technique, as a result of these introduced errors, does not provide a good measurement of particle size.
Attempts to characterize particle size are also disclosed in a paper “Soot diagnostics using laser-induced incandescence in flames and exhaust flows” by R. T. Wainner and J. M. Seitzman, published in 1999, by the American Institut
Gulder Omer L.
Liu Fengshan
Smallwood Gregory J.
Snelling David R.
Font Frank G.
MacLean Doug
National Research Council of Canada
Nguyen Sang H.
Teitelbaum Neil
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