Raman scattering measurements in flames using a tunable KrF excimer laser

Area of Science:

• Combustion diagnostics
• Laser spectroscopy
• Physical chemistry

Background:

• Accurate measurements of species concentration and temperature are crucial for understanding combustion processes.
• Traditional methods often face limitations in temporal and spatial resolution or are affected by fluorescence interference.

Purpose of the Study:

• To demonstrate single-pulse, spatially resolved concentration and temperature measurements in H(2)-air flames using spontaneous vibrational Raman scattering.
• To minimize fluorescence interference from major species (O(2), N(2), H(2)O, H(2)) across all flame stoichiometries.

Main Methods:

• Utilized a narrow-band tunable KrF excimer laser (248.623 nm) as the Raman scattering source.
• Determined optimal laser tuning from fluorescence excitation spectra to minimize OH and O(2) interference.
• Employed single-pulse N(2) Stokes/anti-Stokes ratio for temperature measurements and a time-averaged technique matching theoretical spectra.

Main Results:

• Achieved photon-statistics-limited precisions of typically 5% for instantaneous measurements.
• Demonstrated successful measurements for O(2), N(2), H(2)O, and H(2) across fuel-lean to fuel-rich conditions.
• Obtained promising Raman flame spectra in CH(4)-air flames with good signal-to-noise ratios.

Conclusions:

• Single-pulse Raman scattering with a tunable KrF laser enables accurate, instantaneous measurements in H(2)-air flames with minimal fluorescence interference.
• The developed techniques show significant promise for UV Raman measurements in hydrocarbon flames.