Figure 1. A second Raman/Rayleigh line-imaging detection unit allows researcher to simultaneously measure polarized and depolarized components of Raman scattering signals, helping to overcome fluorescence interference from soot precursors in hydrocarbon flames.
Allowing single-shot measurements of all major species in nonsooting flames of simple fuels, such as H2 and CH4, spontaneous Raman scattering has been a key Combustion Research Facility (CRF) laser diagnostic technique for fundamental turbulent-combustion studies. However, laser-induced fluorescence from soot precursors tends to interfere significantly with Raman scattering signals—limiting this method’s utility. Such fluorescence is unpolarized, while the Raman-scattered light mainly retains the laser beam’s polarization. Therefore, simultaneously measuring, with good precision, the signal’s orthogonal polarization components can greatly suppress this soot-precursor interference, or eliminate it, by subtraction.
Figure 2. Average Raman spectra from the peak-interference region of a steady laminar flame illustrate the ability to greatly suppress unpolarized fluorescence interference by subtracting the depolarized spectrum from the polarized spectrum.
Working with Rob Barlow and Bob Harmon (both in Sandia’s Reacting Flow Research Dept.) in the Turbulent Combustion Laboratory, Gaetano Magnotti (Reacting Flow Research Dept.) implemented an approach to simultaneously measure the two polarization components. This approach entails adding a second Raman/Rayleigh line-imaging detection system, as shown Figure 1, that is a mirror image of the original, precisely aligned to match magnification. The two Raman detectors have orthogonally oriented linear polarizers, and each system uses a high-speed (21,000 rpm) rotating shutter developed at Sandia by Duane Sunnarborg (retired) and Chris Carlen of the Engine Combustion Department. These shutters provide 3.9 µs (FWHM) gating, and they are locked in frequency and phase with less than 70 ns rms phase error, allowing for good precision in the simultaneous signal measurement.
This new polarized-depolarized detection capability significantly extends the range of flames that can be measured accurately using Raman/Rayleigh methods, particularly for flames of methane or other light hydrocarbons that produce high levels of soot-precursor fluorescence interference. This polarization separation and subtraction approach also eliminates interference from chemiluminescence, which can be an issue for measurements in intense, high-Reynolds-number flames.
Read the full article in CRF News.