Laser-Induced Incandescence for Soot Diagnostics: Theoretical Investigation and Experimental Development

Abstract: Laser-induced incandescence, LII, is a laser-diagnostic technique that can be used to measure the volume fraction and the sizes of soot particles suspended in a gas, such as within a combustion process or in its exhausts. The technique is based on the facts that the time-decay of the radiation from laser-heated soot particles is directly related to the particle size in the probed volume, and that the time-integrated radiation is related to the soot volume fraction. In the present work, the properties of LII were investigated, such as its dependence on different parameters, how it can be used to calculate soot properties and how it relates to and can be combined with other techniques. Both theoretical and experimental studies were carried out. An LII model was used for the theoretical work and its predictions under various conditions were investigated. The studies addressed different questions with regard to the sensitivity of LII to various parameters. Simulations were made to determine the best choices for the laser system settings when measuring the soot volume fraction under different conditions. This resulted in general suggestions for what practices are best under various conditions. It was also found that if the two modes of a bimodal particle size distribution are of about the same magnitude the smaller mode has no significant effect on the LII particle size evaluated. Studies of soot aggregates consisting of ramified clusters of spherical primary particles were carried out using numerically constructed aggregates, together with a Monte Carlo algorithm for calculating heat conduction. The effect of different degrees of bridging between the primary particles on primary particle size as evaluated with use of LII was examined, and found to be noticeable but relatively small. A laser system was designed for use in the experimental studies and evaluation routines that use our LII model were designed and implemented, to be able to evaluate particle sizes from experimental LII signals. The experimental setup was designed so as to possess properties suitable for obtaining well-controlled LII particle size measurements. That setup was employed, with slight variations, for investigating the properties of LII. It was also applied in different laboratory soot sources for measurement of particle sizes and of soot volume fraction. The same setup was used for studying the effects of the beam profile, which was found to have only a marginal effect on the evaluated size, other uncertain parameters involved in the LII technique being of greater importance in this respect. Two collaborative projects were also carried out for investigating how LII could be used for determining particle sizes both of cold soot aggregates obtained from a flame-based soot generator and of industrial carbon black samples. With use of the soot generator it was investigated, when the primary particle distributions were similar, if the differing decay rates of the LII signals could be explained by the differing degrees of aggregation. This could be an effect of the increase in shielding between the primary particles with an increase in aggregate size, as predicted by theory. The LII sizes as determined for the carbon black samples were closely correlated with the equivalent particle sizes as determined by industrial surface-area analysis of the particles. The properties of a flat ethylene/air flame on a McKenna burner were also investigated, as a function both of height and of radial position. The absorption function for soot was found to increase with an increase in height from 6 to 14 mm above the burner, due probably to the changes that occurred in soot composition from nascent soot to more mature soot. In the central part of the flame, at a height of 10 mm above the burner, the temperatures and the soot particle sizes were found to be nearly constant.