Revisit the relationship between LII signal and soot volume fraction - M2P2 Seminar

Abstract : The laser-induced incandescence (LII) technique has been increasingly used to measure soot volume fraction in both laminar and turbulent flames. The planar LII (PLII) method is a powerful tool to capture soot distribution in the laser sheet and can provide quantitative soot volume fraction with the help of a proper calibration based on the assumption that the LII signal intensity is proportional to soot volume fraction, i.e., SLII = C´fv. On the other hand, Melton showed in 1984 that SLII µ npdp3+154nm/ldet (np is the primary particle number density, dp is primary particle diameter, ldet is the LII signal detection wavelength) at the moment of the peak soot temperature in the high-fluence regime. This relationship has been interpreted as that the LII signal is approximately proportional to soot volume fraction when the detection wavelength is sufficiently long. In this presentation, the relationship between LII signal and soot volume fraction is revisited by conducting LII model calculations for polydisperse soot primary particles and different laser fluences. The results show that the linear relationship between LII signal and soot volume fraction is strictly valid only during the laser pulse or shortly after the laser pulse in the low-fluence regime where soot sublimation is negligible. At high-fluence regime, this linear relationship breaks down within the laser pulse. The fundamental reason for the departure of the linear relationship is the particle size dependence of the cooling processes through heat conduction or sublimation. This analysis of the relationship between LII signal and soot volume fraction has significant implications to LII measurement of soot volume fraction.