Laser-induced incandescence for soot particle size and volume fraction measurements using on-line extinction calibration

A novel technique for two-dimensional measurements
of soot volume fraction and particle size has been developed.
It is based on a combined measurement of extinction
and laser-induced incandescence using Nd:YAG laser wavelengths
of 532 nm and 1064 nm. A low-energy laser pulse at
532 nm was used for extinction measurements and was followed
by a more intense pulse at 1064 nm, delayed by 15 ns,
for LII measurements. The 532-nm beam was split into a signal
beam passing the flame and a reference beam, both of
which were directed to a dye cell. The resulting fluorescence
signals, from which the extinctionwas deduced, together with
the LII signal, were registered on a single CCD detector.
Thus the two-dimensional LII image could be converted to
a soot volume fraction map through a calibration procedure
during the same laser shot. The soot particle sizes were evaluated
from the ratio of the temporal LII signals at two gate
time positions. The uncertainty in the particle sizing arose
mainly from the low signal for small particles at long gate
times and the uncertainty in the flame temperature. The technique
was applied to a well-characterized premixed flat flame,
the soot properties of which had been previously thoroughly
investigated.