Ultrafast laser-absorption spectroscopy for single-shot, mid-infrared measurements of temperature, CO, and CH₄ in flames

Abstract

This Letter describes the development of an ultrafast (i.e., femtosecond), mid-infrared (mid-IR), laser-absorption diagnostic and its initial application to measuring temperature, CO, and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:msub> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">C</mml:mi> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mn>4</mml:mn> </mml:msub> </mml:mrow> </mml:math> in flames. The diagnostic employs a Ti:sapphire oscillator emitting 55 fs pulses near 800 nm that were amplified and converted into the mid-IR though optical parametric amplification at a repetition rate of 5 kHz. The pulses were directed through the test gas and into a high-speed mid-IR spectrograph to image spectra across a <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo>≈</mml:mo> </mml:mrow> <mml:mn>30</mml:mn> <mml:mspace width="thinmathspace"/> <mml:mspace width="thinmathspace"/> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">n</mml:mi> <mml:mi mathvariant="normal">m</mml:mi> </mml:mrow> </mml:math> bandwidth with a spectral resolution of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo>≈</mml:mo> </mml:mrow> <mml:mn>0.3</mml:mn> <mml:mspace width="thinmathspace"/> <mml:mspace width="thinmathspace"/> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">n</mml:mi> <mml:mi mathvariant="normal">m</mml:mi> </mml:mrow> </mml:math> . Gas properties were determined by least-squares fitting simulated absorbance spectra to measured single-shot absorbance spectra. The diagnostic was validated with measurements of temperature, CO, and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:msub> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">C</mml:mi> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mn>4</mml:mn> </mml:msub> </mml:mrow> </mml:math> 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 column density were acquired near 4.9 µm in a laser-ignited HMX (i.e., 1,3,5,7-tetranitro-1,3,5,7-tetrazoctane) flame and exhibited <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mn>1</mml:mn> <mml:mo>−</mml:mo> <mml:mi>σ</mml:mi> </mml:math> precisions of 0.4% and 2.3%, respectively, at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo>≈</mml:mo> </mml:mrow> <mml:mn>2700</mml:mn> <mml:mspace width="thinmathspace"/> <mml:mspace width="thinmathspace"/> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">K</mml:mi> </mml:mrow> </mml:math> . Further, temperature and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:msub> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">C</mml:mi> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mn>4</mml:mn> </mml:msub> </mml:mrow> </mml:math> column density measurements were acquired near 3.3 µm in a partially premixed <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:msub> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">C</mml:mi> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mn>4</mml:mn> </mml:msub> </mml:mrow> </mml:math> -air flame produced by a Hencken burner and exhibited <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mn>1</mml:mn> <mml:mo>−</mml:mo> <mml:mi>σ</mml:mi> </mml:math> precisions of 0.3% and 1% respectively, at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo>≈</mml:mo> </mml:mrow> <mml:mn>1000</mml:mn> <mml:mspace width="thinmathspace"/> <mml:mspace width="thinmathspace"/> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">K</mml:mi> </mml:mrow> </mml:math> .

Topics & Concepts