Flame thermometry using laser-induced-grating spectroscopy of nitric oxide

Area of Science:

• * Combustion diagnostics
• * Laser spectroscopy
• * Thermometry

Background:

• * Accurate temperature measurements are crucial for understanding and controlling combustion processes.
• * Nitric oxide (NO) is a key species in combustion, influencing both emissions and flame dynamics.
• * Laser-induced thermal-grating scattering (LITGS) offers a non-intrusive method for probing combustion environments.

Purpose of the Study:

• * To systematically investigate laser-induced thermal-grating scattering (LITGS) for thermometry in air-fed combustion.
• * To analyze the contributions of LITGS and degenerate four-wave mixing (DFWM) to the scattered signal.
• * To derive temperature values in a CH4/O2/N2 flame using LITGS signals.

Main Methods:

• * Utilized a pulsed laser system to excite the (0,0) γ-bands of NO at 226.21 nm.
• * Studied the time-domain contributions of LITGS and DFWM for NO in N2 buffer gas up to 4 bar.
• * Applied LITGS to measure temperature in a laminar, pre-mixed CH4/O2/N2 flame at 1 and 2.5 bar.

Main Results:

• * Derived temperature values in the range of 1600–2400 K, consistent with adiabatic flame temperatures.
• * Achieved a pressure-dependent uncertainty of ±1.8% at 1 bar to ±1.4% at 2.5 bar.
• * Estimated a minimum detectable NO concentration of 80 ppm for a 5 s measurement at 10 Hz.

Conclusions:

• * LITGS is a viable technique for accurate, non-intrusive thermometry in combustion environments.
• * The method demonstrates good performance across a range of pressures and temperatures.
• * LITGS shows potential for sensitive NO concentration measurements in flames.