Emissions in short-gated ns/ps/fs-LIBS for fuel-to-air ratio measurements in methane-air flames

Journal Article · Wed May 12 00:00:00 EDT 2021 · Applied Optics

DOI:https://doi.org/10.1364/AO.418453· OSTI ID:1842240

Gragston, Mark; Hsu, Paul; Jiang, Naibo; Roy, Sukesh; Zhang, Zhili

A study of short-gated 10 nanosecond (ns), 100 picosecond (ps), and 100 femtosecond (fs) laser induced breakdown spectroscopy (LIBS) was conducted for fuel-to-air ratio (FAR) measurements in an atmospheric Hencken flame. The intent of the work is to understand which emission lines are available near the optical range in each pulse width regime and which emission ratios may be favorable for generating equivalence ratio calibration curves. The emission spectra in the range of 550–800 nm for ns-LIBS and ps-LIBS are mostly similar with slightly elevated atomic oxygen lines by ps-LIBS. Spectra from fs-LIBS show the lowest continuum background and prominent individual atomic lines, though have significantly weaker ionic emission from nitrogen. A qualitative explanation based on assumed local thermodynamic equilibrium and electron temperatures calculated by the ${\mathrm{N}}_{\mathrm{I}\mathrm{I}}\left(565\mathrm{n}\mathrm{m}\right)$ and ${\mathrm{N}}_{\mathrm{I}\mathrm{I}}\left(594\mathrm{n}\mathrm{m}\right)$ emissions is presented. In studying line emission ratios for FAR calculation, it is found that ${\mathrm{H}}_{\mathrm{\alpha \; <\#comment/>}}\left(656\mathrm{n}\mathrm{m}\right)/{\mathrm{N}}_{\mathrm{I}\mathrm{I}}\left(568\mathrm{n}\mathrm{m}\right)$ is best for FAR measurements with ns-LIBS and remains viable for ps-LIBS, while ${\mathrm{H}}_{\mathrm{\alpha \; <\#comment/>}}\left(656\mathrm{n}\mathrm{m}\right)/{\mathrm{O}}_{\mathrm{I}}\left(777\mathrm{n}\mathrm{m}\right)$ is optimal for the ps-LIBS and fs-LIBS cases. Due to low continuum background and short time delay for spectra collection, fs-LIBS is very promising for high-speed FAR measurements using short-gated LIBS.

• Sponsoring Organization: USDOE
• Grant/Contract Number: SC0021382
• OSTI ID: 1842240
• Alternate ID(s): OSTI ID: 1853781
• Journal Information: Applied Optics, Journal Name: Applied Optics Journal Issue: 15 Vol. 60; ISSN APOPAI; ISSN 1559-128X
• Publisher: Optical Society of America
• Country of Publication: United States
• Language: English