Some theoretical considerations in modeling laser-induced incandescence at low-pressures |
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Authors: | F Liu KJ Daun V Beyer GJ Smallwood DA Greenhalgh |
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Institution: | (1) Institute for Chemical Process and Environmental Technology, National Research Council Building M-9, 1200 Montreal Road, Ottawa, Ontario, Canada;(2) School of Engineering, Cranfield University, Cranfield, Bedford, MK43 0AL, UK |
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Abstract: | Laser-induced incandescence (LII) of nanoparticles at low pressures has received some attention in recent years as a particle
sizing technique or a technique for inferring the mean value of the absorption function of the particle material. In this
paper, we are concerned with some fundamental issues in the theory of LII with particular attention paid to those encountered
at very low pressures. The commonly adopted Rayleigh approximation for particle laser energy absorption and subsequent thermal
emission is critically evaluated against the Mie solution in the range of size parameter relevant to LII. The Rayleigh approximation
can cause significant error in particle laser energy absorption rate, especially when shorter wavelengths are used, and potentially
in the particle temperature inferred from the two-color LII. We also demonstrate that claims that low-pressure LII can be
used for particle sizing are flawed, due to the use of an incorrect expression for radiation heat loss rate from the particles
in this regime, and unjustified neglect of particle sublimation heat loss. Using the currently best available carbon sublimation
rate expression and physical parameters, the relative importance of heat conduction, thermal radiation, and sublimation heat
loss from an isolated carbon particle was investigated for different ambient pressures, particle temperatures and particle
diameters. To ensure particle radiation heat loss is dominant over conduction and sublimation the ambient pressure and the
particle temperature should be kept respectively lower than 10-4 atm and below about 2800 K. Under these conditions the effective temperature of a particle ensemble containing non-aggregated
polydisperse primary particles to the power of -4 is proportional to the mean value of the particle absorption function, provided
the particles are in the Rayleigh regime in the near infrared. The effect of aggregation on particle absorption and emission
is briefly discussed.
PACS 44.10.+i; 44.40.+a; 61.46Df |
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