Abstract
KEYWORDS:
Laser-Induced Incandescence, soot, volume fraction, particle diameter, Diesel engine.
ABSTRACT:
The Laser-Induced Incandescence is a technique allowing to study the soot formation in flames as well as inside the combustion chambers of the Diesel engines. The technique is based on the blackbody radiation of the soot when irradiated with a laser beam or a laser sheet of sufficient high energy, the soot temperature being brought up to the particles vaporization temperature. The emitted signal is proportional to the soot volume fraction within the measurement volume while its temporal decay is the image of the soot particle cooling and it gives an information regarding the soot mean diameter in the measurement volume if one fits an appropriate modelled curve on the experimental one.
An analysis of soot formation in an isooctane/air diffusion flame was performed, the purpose being to determine the soot volume fractions and the particles mean diameter. Pairs of temporal LII signals detected at the two wavelengths of interest have been recorded for several locations of the measurement volume in the flame. Assuming that the particles are heated at the same temperature by the laser sheet, one can estimate their surface temperature from the ratio between the two signals recorded for each location of the measurement volume in the flame. The laser heated particles temperature together with the LII signals expressed in absolute units allow us to estimate the soot volume fraction value. A calibration of the receiving optics with a lamp at steady known temperature is needed, this calibration helping us to express the LII signal in absolute radiance units. In order to determine the particles mean diameter, the experimental signal decay curves should be compared to the modelled ones.
Images of the incandescent soot in the combustion chamber of a Direct Injection Diesel Engine has been acquired in order to show the spatial distribution of the soot as well as the evolution of the soot concentration in a cycle.