=Paper=
{{Paper
|id=None
|storemode=property
|title=Effect of laser pulse duration on laser-induced incandescence soot
|pdfUrl=https://ceur-ws.org/Vol-865/LiuPulse.pdf
|volume=Vol-865
}}
==Effect of laser pulse duration on laser-induced incandescence soot==
<pdf width="1500px">https://ceur-ws.org/Vol-865/LiuPulse.pdf</pdf>
<pre>
          Effect of Laser Pulse Duration on Laser-Induced
                       Incandescence of Soot


                      Fengshan Liu, Gregory J. Smallwood
National Research Council, Building M-9, 1200 Montreal Road, Ottawa, ON., Canada, e-mail:
             Fengshan.Liu@nrc-cnrc.gc.ca; Greg.Smallwood@nrc-cnrc.gc.ca


        The phenomenon of laser-induced incandescence (LII) has been utilized to
develop versatile diagnostic techniques for measurements of combustion-generated
soot concentration and primary particle size in many combustion applications. Many
aspects in the implementation of LII have been investigated, such as the excitation
laser wavelength and spatial profile, signal detection timing (prompt or delayed) and
temporal width, and signal detection wavelengths. One potentially important
parameter in LII practice that has received little attention in the LII community is the
laser pulse duration. This is perhaps due to the fact that almost all the LII
experiments conducted in the last two decades employed Q-switched, flashlamp
pumped Nd:YAG lasers as the light source, which all provide a similar pulse duration
in a narrow range between 5 and 10 ns FWHM. Few LII experiments were conducted
using a much long laser pulse (microsecond) or much a shorter one (picoseconds).
Picosecond laser pulses were shown theoretically to provide advantages over
nanosecond ones in the determination of primary particle size distribution using low-
fluence LII in flames at elevated pressures.
        In this study the effect of laser pulse duration on the temperature histories of
primary soot particle of different sizes was numerically investigated in both low- and
high-fluence regimes under conditions of a typical atmospheric pressure laminar
diffusion flame. The laser pulse durations considered in this study vary from 100 ps
to 6 ns FWHM. Such laser pulse durations correspond to those of a typical
Ti:Sapphire pulsed laser operated at 780 nm. Such laser is capable of generating
laser pulses with duration from picoseconds to nanoseconds, depending on if the
regenerative amplifier is seeded by the femtosecond oscillator.
        Numerical results show that at a fixed laser fluence in the low-fluence regime
with decreasing the laser pulse duration the effect of heat conduction on the peak
soot temperature is suppressed. Under the conditions considered here heat
conduction lowers the peak soot temperature of a 30 nm soot particle by about 70 K
when the laser pulse is 6 ns FWHM. As the laser pulse duration decreases, the
differences in the peak temperatures reached by particles of different sizes become
smaller, due to the effective separation of the volumetric heating process and the
surface dependent heat conduction cooling. In high-fluence regime, the peak soot
temperature increases more significantly with decreasing the laser pulse duration.
There seem no advantages using a shorter laser pulse than the commonly employed
6 ns FWHM one in the high-fluence regime.
        In summary, a shorter laser pulse offers advantages over the typical 5 to 10
ns duration one in some low-fluence LII applications where it is desirable to suppress
the effect of heat conduction, such as particle size determination in high pressures
and evaluation of E(m). However, it does not seem to offer advantages in high-
fluence regime.




             5th international workshop on Laser-Induced Incandescence
              May 9-11, 2012, Palais des Congrès, Le Touquet, France

</pre>