Understanding the physical process of LII is central to practical implementation and accurate theoretical modelling of LII. The LII dependence upon laser fluence is shown to depend upon detection conditions thereby not providing direct information about the soot temperature or structural changes. Transmission electron microscopy, used to investigate the morphological changes induced in the soot at different laser fluences, shows increasing graphitization of the soot with increasing laser fluence. For laser fluences above 0.45-0.05 J/cm2 at 1064 nm, vaporization/fragmentation of soot primary particles and aggregates occurs. Optical measurements are performed using a second laser pulse to probe the effects of these changes upon the LII signal. With the exception of very low fluences, the structural changes induced in the soot lead to a decreased LII intensity produced by the second laser pulse. These two-pulse experiments also show that these changes do not alter the LII signal on timescales less than 1 7s for fluences below the vaporization threshold.
This paper reviews the optical techniques for in-cylinder combustion temperature measurement, particularly soot measurements in diesel engines. The review starts with the two-colour method for in-cylinder soot and combustion temperature measurement. The principle and implementation of the two-colour technique are described in detail. Both signal point and full-field temperature and soot measurements by the two-colour method are considered. In the second part, the soot diagnostics based on light scattering, especially the light extinction method for in-cylinder soot concentration measurements, are discussed. In the third part, optical techniques for spatially resolved two-dimensional measurements of soot particles in diesel engines are introduced. Since laser induced incandescence (LII) is a relatively new technique and is particularly suitable for the two-dimensional imaging of soot distribution, the operating principle and implementation of LII are discussed in detail. At the end of each part, examples are given to illustrate the understanding gained about diesel combustion as a result of the application of these optical techniques. This paper provides a comprehensive review for those who are interested in using optical diagnostics for in-cylinder soot and combustion temperature measurement in diesel engines. (C) 1998 Elsevier Science Ltd. All rights reserved.
Laser-induced incandescence from soot was analyzed with a time-dependent, numerical model of particle heating and cooling processes that includes spatial and temporal intensity profiles associated with laser sheet illumination. For volume fraction measurements, substantial errors result primarily from changes in gas temperature and primary soot particle size. The errors can be reduced with the proper choice of detection wavelength, prompt gating, and high laser intensities. Two techniques for primary particle size measurements, based on ratios of laser-induced incandescence signals from a single laser pulse, were also examined. Compared with the ratio of two integration times, the newly proposed ratio of two detection wavelengths is better suited for simultaneous volume fraction and size measurements, because it is less temperature sensitive and produces stronger signals with, however, a lower sensitivity to size changes.
CARS measurements of gas temperature profiles performed at low
pressure (about 1 Torr) in a PECVD RF reactor and in a CVD reactor reveal the
thermal accommodation phenomenon between the gas and the surfaces. A
one-dimensional thermal model has been developed to calculate the temperature
profiles in the PECVD and CVD reactors and the results are compared with the
experimental measurements. In addition to the thermal conduction and
accommodation, the model takes into account the radiative exchange between the
surfaces. The influence of the pressure on the temperature jump between the gas
and the surfaces was investigated in the CVD reactor. Thermal accommodation
probabilities for five gas/surface couples have been determined: 0.07â€“0.13 for
H2/stainless steel, 0.05+/-0.01 for H2/Si, 0.17+/-0.02 for H2/graphite, 0.38+/-0.03 for
N2/stainless steel and 0.26+/-0.02 for N2/graphite. In the PECVD reactor, the
influence of the electrical power deposited in the plasma on the temperature profile
between the electrodes was studied.