Acetylene measurement in flames by chirp-based quantum cascade laser spectrometry

Area of Science:

• Spectroscopy
• Laser Technology
• Combustion Chemistry

Background:

• Mid-infrared (mid-IR) spectroscopy is crucial for chemical analysis.
• Quantum cascade lasers (QCLs) offer tunable mid-IR light sources.
• Real-time monitoring of combustion byproducts like acetylene is important for safety and efficiency.

Purpose of the Study:

• To design and characterize a mid-IR spectrometer based on a pulsed distributed-feedback quantum cascade laser.
• To evaluate the spectrometer's performance for real-time absorption measurements of acetylene.
• To demonstrate the system's application in analyzing acetylene concentrations within a flame.

Main Methods:

• A pulsed distributed-feedback quantum cascade laser was employed for frequency down-chirp scanning over a 6.5 cm(-1) spectral region.
• The system was calibrated and tested by measuring acetylene spectra in a 16 cm absorption cell at atmospheric pressure.
• Detection limits were determined using acetylene at low concentrations.
• The spectrometer was then applied to measure acetylene in an ethylene-air opposed flow flame.

Main Results:

• The spectrometer's chirp behavior was extensively characterized.
• The system achieved a minimum detectable absorbance peak of approximately 1.5 x 10(-4) absorbance units.
• A minimum detectable concentration-length product of 2.4 parts per million meter was achieved for acetylene.
• Acetylene concentrations were successfully measured as a function of height in an opposed flow flame and correlated with fuel pre-injection temperature.

Conclusions:

• The developed mid-IR spectrometer is highly sensitive and accurate for real-time acetylene detection.
• The system demonstrates significant potential for in-situ combustion diagnostics.
• This technology can provide valuable insights into fuel breakdown processes and flame chemistry.