Numerical Simulation on Effects of Laser Fluence on Temporal and Time Integrated LII-Process of Soot Particle

Abstract

The primary goal of this study is to analyze the mathematical model of Laser Induced Incandescence (LII) process applied to the measurement of diameter and temperature of a soot particle within flame. To simulate heating and cooling processes of a nanometer size particle, computer codes have been written to calculate the time dependent responses of the particle to laser pulse heating. The results include temperature, diameter, heat losses and LII-signal responses. At low laser fluence heating, the rate of heat losses from the particle is very low in comparison to the rate of heat gain by absorption and LII-signal being sensitive to the temperature of the particle. At high laser fluence heating, the particle sublimes and becomes smaller in diameter. The sublimation cooling is the most important form of heat loss for the particle and is at a comparable rate to the heat gain by absorption. The LII-signal is sensitive to both the temperature and diameter of the particle. Overall (or time integrated) heat transfer and LII-signal over the period of the laser pulse as a function of laser fluence has also been analyzed. The time integration of heat transfer modes over the duration of the laser pulse is represented in the form of an energy balance equation. It has been found that the absorption energy was, at the beginning, stored in the form of internal energy and later lost in the form of sublimation cooling as the laser energy fluence increased. At low laser fluence energy input, the magnitude of the time integrated LII-signal over the laser pulse duration increases with the laser fluence. At high laser fluence energy input, sublimation occurs and the diameter of the particle becomes smaller, undermining the LII-signal. These competing effects, between the internal stored energy gain against the sublimation energy lost, led to the threshold of the time integrated LII-signal at high laser fluence energy inputs. Once the threshold signal is reached, the overall LII-signal becomes less sensitive to the increasing of laser fluence.