Cars investigations in flames

Abstract

Coherent anti-Stokes Raman spectroscopy (CARS) investigations in a variety of flames are described. These studies were directed toward finding solutions to previously identified or newly encountered shortcomings of CARS and to provide a foundation on which to base future development. CARS is generated by mixing a 10 pps, frequency-doubled neodymium “pump” laser with a spectrally broadband, laser-pumped, “Stokes” dye laser. This approach obviates the requirement to frequency scan the dye laser and generates the entire CARS spectrum with each pulse permitting “instantaneous” measurements of medium properties. A crossed-beam, phase-matching technique, termed BOXCARS, is used which leads to greatly enhanced and unambiguous spatial resolution in contrast to the conventionally employed collinear phase-matching approaches. Using this technique, moderate resolution (∼ 1.25 cm−1) CARS spectra from hot N2, obtained by scanning the spectrum in premixed laminar flames, show excellent agreement with computer generated model predictions. Lower resolution (2.7 cm−1) collinear phase-matched CARS spectra of flame N2 have been obtained in a single 10 nanosecond pulse using an optical multichannel analyzer. These single pulse spectra also display good agreement with predicted spectra and demonstrate the feasibility of single pulse thermometry. Measurements in a highly sooting, laminar propane diffusion flame revealed the existence of a coherent spectral interference arising from electronic resonance CARS generation from C2, produced by the laser vaporization of the soot. Reduction of the Stokes laser bandwidth and use of polarization filters permitted low distortion N2 CARS spectra to be obtained. These spectra, when computer fitted, allowed determination of the temperature marking the first measurement in highly sooting flames by a remote, spatially precise diagnostic technique. CARS species sensitivity was examined in a study of flame CO detectability levels. Very good agreement between CARS CO spectra and the computer model was obtained at the 4 percent CO level. With the fluctuations in the experimental apparatus it was difficult to detect CO below the 1–2 percent level. The computer calculations indicate that CO would be barely detectable to about 0.5 percent using conventional CARS approaches.