Assessment of soot volume fractions from laser-induced incancescence by comparison with extinction measurements in laminar, premixed, flat flames

Abstract

For the validation of detailed chemical models for soot formation, and for their application to turbulent flames, 2-D measuring techniques are necessary to derive locally resolved soot volume fractions, particle sizes, and number densities. In one-dimensional laminar flames, these particle properties are derived from Rayleigh-scattering/extinction techniques. The extinction technique, as a line-of-sight method, is not appropriate for investigation of three-dimensional, nonstationary, turbulent systems. Therefore, for turbulent flames, other measuring techniques have to be developed that can be employed in combination with, for example, Rayleigh scattering. One technique that has been applied to obtain soot volume fractions in laminar flames is Laser-Induced Incandescence (LII) [1]. The major task in applying this technique is to clarify in which way the relative LII signals should be calibrated to yield absolute soot volume fractions. In this work, the LII technique is investigated systematically in laminar, premixed, ethyne/argon/oxygen flames using a pulsed, high-power Nd:YAG laser. Therefore, the LII signals are compared with soot volume fractions obtained using extinction. The systematic investigation of the LII technique under different detection conditions (camera gate width and gate delay times) shows that there are inconsistencies between calibrated LII signals and soot volume fractions from extinction. The aim of this work is to explain these inconsistencies by a numerical simulation of the LII signal, based on mass and energy balance equations of the heated soot particles for different flame conditions. Additionally, a sensitivity analysis with respect to different parameters of the balance equation is performed. The results indicate that geometric irregularities of the soot particles probably have to be taken into account for correct prediction of soot volume fractions using LII.