Simultaneous temperature and sensitive two-species concentration measurements by single-shot CARS

Area of Science:

• Physical Chemistry
• Spectroscopy
• Laser Diagnostics

Background:

• Accurate in-situ measurements of temperature and species concentrations are crucial for understanding combustion and plasma processes.
• Coherent anti-Stokes Raman scattering (CARS) is a powerful laser-based technique for non-intrusive thermometry and species sensing.

Purpose of the Study:

• To perform simultaneous, spatially and temporally resolved measurements of nitrogen (N2) and oxygen (O2) mole fractions and temperature using CARS.
• To evaluate the capabilities and limitations of CARS for precise mole fraction measurements in turbulent environments and flames.

Main Methods:

• Utilized a crossed-beam CARS (BOXCARS) setup with broadband and narrowband Stokes lasers and a frequency-doubled Nd:YAG laser.
• Obtained single-shot N2 spectra for temperature and N2 mole fraction using broadband CARS.
• Deduced O2 mole fractions from a specific rovibrational Q-line using narrowband CARS.

Main Results:

• Achieved a single-shot detectivity limit better than 0.4% for O2 at 2200 K and atmospheric pressure.
• Demonstrated the capability for 2-D probability density function measurements in an isothermal jet and a laminar flame.
• Identified significant challenges including signal generation away from the focal point, beam disruption by turbulence, and Stark effect, leading to mole fraction errors.

Conclusions:

• While CARS is effective for thermometry and detecting species, precise mole fraction measurements face considerable challenges.
• Turbulence and the Stark effect are identified as major sources of error in mole fraction determination.
• Referencing the mole fraction of nitrogen (N2) can mitigate errors caused by turbulence in premixed flames.