Ultrafast-laser-absorption spectroscopy for single-shot, mid-infrared measurements of temperature, CO, and CH$_4$ in flames

TL;DR

This paper introduces a femtosecond mid-infrared laser-absorption diagnostic capable of single-shot measurements of temperature, CO, and CH$_4$ in flames with high precision and validation against known standards.

Contribution

The development of a high-speed, femtosecond mid-IR laser-absorption system for accurate, single-shot flame diagnostics at 5 kHz is a novel advancement.

Findings

Validated with static-gas cell measurements showing 0.7%-1.8% accuracy.

Achieved 0.4% precision in temperature and CO measurements near 2700 K.

Achieved 0.3% precision in CH$_4$ and temperature measurements near 1000 K.

Abstract

This manuscript describes the development of an ultrafast (i.e.,femtosecond), mid-infrared, laser-absorption diagnostic and its initial application to measuring temperature, CO, and CH$_4$ in flames. The diagnostic employs a Ti:Sapphire oscillator emitting 55-fs pulses near 800 nm which were amplified and converted into the mid-infrared (mid-IR) though optical parametric amplification (OPA) at a repetition rate of 5 kHz. The pulses were directed through the test gas and into a high-speed mid-infrared spectrograph to image spectra across a $\approx$30 nm bandwidth with a spectral resolution of $\approx$0.3 nm. Gas properties were determined by least-squares fitting a spectroscopic model to measured single-shot absorbance spectra. The diagnostic was validated with measurements of temperature, CO, and CH$_4$ in a static-gas cell with an accuracy of 0.7% to 1.8% of known values. Single-shot, 5 kHz measurements of temperature and CO were acquired near 4.9 $\mu$m in a laser-ignited HMX (i.e., 1,3,5,7-tetranitro-1,3,5,7-tetrazoctane) flame and exhibited a 1-$\sigma$ precision of 0.4% at $\approx$2700 K. Further, CH$_4$ and temperature measurements were acquired near 3.3 $\mu$m in a partially premixed CH$_4$-air flame produced by a Hencken burner and exhibited a precision of 0.3% at $\approx$1000 K.