Temperature and Thermal Diffusivity Diagnostics in Laminar Methane Flames Using Infrared Four-Wave Mixing Techniques.

Area of Science:

• Combustion diagnostics
• Laser spectroscopy
• Thermometry

Background:

• Four-wave mixing techniques like CARS, LIGS, and DFWM are valuable for combustion diagnostics due to high S/N, coherent signals, and spatial resolution.
• Accurate measurement of flame temperature and thermodynamic properties is crucial for combustion research and simulation.

Purpose of the Study:

• To demonstrate combined LIGS and DFWM measurements for precise thermometry in premixed laminar CH4/O2/N2 flames.
• To achieve accurate temperature measurements across a wide range of equivalence ratios (0.6 to 1.5).
• To extract other thermodynamic parameters like speed of sound and thermal diffusivity.

Main Methods:

• Utilized nano-second pulsed lasers to generate mid-infrared pump beams (near 3 µm) for exciting water rovibrational transitions.
• Employed LIGS for fuel-lean flames and DFWM for fuel-rich flames, leveraging LIGS for DFWM calibration.
• Applied a theoretical model to LIGS signal time waveforms for extracting speed of sound and thermal diffusivity.

Main Results:

• LIGS achieved temperature precision better than 16 K (0.8%) in fuel-lean flames.
• DFWM achieved temperature precision better than 90 K (4.5%) in fuel-rich flames, compensating for gas composition uncertainties.
• Extracted local speed of sound and thermal diffusivity with precisions better than 0.5% and 1.3%, respectively.

Conclusions:

• The combined LIGS and DFWM approach provides accurate thermometry across a broad range of flame conditions.
• This dual-technique method offers potential for precise diagnostics of thermodynamic parameters in flames.
• The high-precision measurements contribute valuable data for flame research and computational fluid dynamics simulations.