<p>Ultrafast
laser-based diagnostic techniques are powerful tools for the detailed
understanding of highly dynamic combustion chemistry and physics. The
ultrashort pulses provide unprecedented temporal resolution along with high
peak power for broad spectral range−ideal for nonlinear signal generation at
high repetition rate−with applications including next-generation combustors for
gas turbines, plasma-assisted combustion, hypersonic flows and rotating
detonation engines. The current work focuses on advancing (i) femtosecond (fs)
two-photon laser-induced fluorescence, and (ii) hybrid femtosecond/picosecond
vibrational and rotational coherent anti-Stokes Raman scattering (fs/ps RCARS
and VCARS) to higher pressures for the first time. </p><p>Quantitative single-laser-shot kHz-rate concentration
measurements of key atomic (O-atom) and molecular (CO) species is presented
using femtosecond two-photon laser-induced fluorescence (TP-LIF) for a range of
equivalence ratios and pressures in diffusion flames. A multitude of
signal-interfering sources and loss mechanisms−relevant to high-pressure fs
TP-LIF applications−are also quantified up to 20 atm to ensure high accuracy.
The pressure scaling of interferences take into account degradation, attenuation
and wave-front distortion of the excitation laser pulse; collisional quenching
and pressure dependent transition line-broadening and shifting; photolytic
interferences; multi-photon ionization; stimulated emission; and radiation
trapping. </p><p>Hybrid fs/ps VCARS of N<sub>2</sub> is reported for
interference-free temperature measurement at 1300-2300 K in high-pressure,
laminar diffusion flames up to 10 atm. A time asymmetric probe pulse allowed
for detection of spectrally resolved CARS signals at probe delays as early as
~200-300 fs while being independent of collisions for the full range of
pressures and temperatures. Limits of collisional independence, accuracy and
precision of the measurement is explored at various probe-pulse delays,
pressures and temperatures. </p><p>
</p><p>Additionally, a novel all diode-pumped Nd:YAG amplifier
design is presented for generation of time-synchronized ps-probe pulses for
hybrid fs/ps RCARS of N<sub>2</sub>. High-energy, nearly transform-limited,
single-mode, chirp-free ps probe-pulses are generated at variable pulsewidths.
The detailed architecture and characterization of the laser is presented. kHz-rate
RCARS thermometry is presented up to 2400 K. Excellent spatial, spectral, and
temporal beam quality allowed for fitting the theoretical spectra with a simple
Gaussian model for the probe pulse with temperature accuracies of 1-2%. </p>
<p><br></p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/11323040 |
| Date | 05 December 2019 |
| Creators | Kazi Arafat Rahman (8085560) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/NONLINEAR_ULTRAFAST-LASER_SPECTROSCOPY_OF_GAS-PHASE_SPECIES_AND_TEMPERATURE_IN_HIGH-PRESSURE_REACTING_FLOWS/11323040 |
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