| OCR Text |
Show COHERENT ANTI-STOKES RAMAN SCATTERING MEASUREMENTS OF TEMPERATURE AND SPECIES CONCENTRATIONS IN COAL-PARTICLE-LADEN COMBUSTION FLOWS* ROBERT P. LUCHT, Combustion Research Facility, Sandia National Laboratories, Livermore, California 94550, United States ABSTRACT Coherent anti-Stokes Raman scattering (CARS) spectra have been obtained for oxygen and nitrogen in a coal-seeded laminar flow reactor. Gas phase temperatures were determined from nitrogen CARS soectra. Significant spectral interferences were observed in the oxygen spectra due to laser-induced coal particle breakdown. KEYWORDS Laser diagnostics; CARS; combustion; temoerature; oxygen. INTRODUCTION Coherent anti-Stokes Raman scattering (CARS) is a laser diagnostic which has been applied in a variety of hostile combustion environments for measurements of gas phase temperature and species concentrations. In a CARS measurement, two laser beams, the pump and Stokes beams, are focused to a common probe volume. When the frequency difference between the pump (<-^p) and Stokes (GJ$ ) beams corresponds to a Raman resonance, a strong coherent signal at the anti-Stokes frequency (2CJ_ - C J $ ) is generated. The signal strength is proportional to the square of the population difference between the lower and upper levels of the Raman resonance. The coherent nature of the CARS interaction produces a strong signal which has laser-like directionality. Because of the directionality and strength of the signal, CARS has inherently good discrimination against background radiation, and has been successfully applied in a variety of luminous flames. While CARS has been widely aoolied in sooting flames (e.g., Eckbreth, 1980; Farrow et al., 1985), applications in coal-particle-laden flows have been somewhat sparse (Taylor, 1983; Beiting, 1983). In this exoeriment CARS measurements were performed in a coal-seeded laminar flow reactor to investigate the applicability of CARS to coal-particle-1aden combustion media. CARS spectra of oxygen and nitrogen were successfully acquired. Gas phase temperatures were determined from theoretical fits of nitrogen soectra. Significant soectral interferences were observed in the oxygen spectra due to laser-induced particle breakdown at high laser powers. EXPERIMENTAL The pump and Stokes beams are generated by a Molectron MY34 Nd:YAG laser and a Quanta-Ray PDL dye laser, respectively. The 532 nm, frequency-doubled output of the Nd:YAG laser serves both as a CARS pump beam and as a pump for the dye laser. The frequency bandwidths of the Nd:YAG and dye laser radiation are 0.1 and 0.2 cm"*, respectively. The laser reDetition rate is 10 Hertz. The pump beam is split into two equal intensity beams, which, along with the Stokes beam, are focused to a common probe volume by a 238 mm focal length lens. The resulting probe volume is approximately 4 mm long, with a height and width of approximately 0.2 mm. The narrowband dye laser frequency is scanned across the Raman resonances of oxygen or nitrogen to generate the CARS signal, which is focused onto the entrance slit of a 1.5 m monochromator that is scanned synchronously with the dye laser. An RCA C31034 photmultiplier is used to detect the CARS signal, which is digitized and stored on a POP 11/24 •This work was suooorted by the U.S. Deoartment of Energy. &-0 |
