OCR Text |
Show no Interior angle greater than 5 degrees was used to slow the flow until laminar conditions were achieved. A ceramic honeycomb was used as a flow straightener and a flame holder. A nitrogen sheath was used around the 11.4 em diameter burner to reduce wall effects and conductive heat loss. A sal1l>le probe and a tel1l>erature probe have been mounted on a traverser system capable of millimeter accuracy in the axial and radial coordinates. A modular data acquisition system linked to and driven by a microcomputer monitors the various flows, feed rates, thermocouple temperatures, and time and position of the sample probe as well as contrOlling the sa~Ie probe movement. CARS Table and Optical Extension A schematic of the CARS instrument used in this study is shown in Figure 4. Its main features are similar to other such instruments in current use (Eckbreth et al. 1984, Goss et al. 1983). However, one recent modification has been the addition of a second dye (or Stokes) laser to allow the measurement of several gas species simuttaneously. The spectrometer used for detection of the resulting CARS signal has also been modified such that two portions of the Raman spectrum are imaged onto the same multichannel detector face. This allows the simultaneous recording of all major combustion species whose spectra appear in the range between 1350 cm-1 and 2350 cm-1 (e.g. C02, ~, H2 S(5) line, C2, CO, N2). The laser beams used to generate the CARS signal originate on a stationary optical table located in the optical laboratory of the Combustions and Reactions Laboratory at Brigham Young University. The laminar flame reactor within which the CARS measurements were taken was located in a test bay of that same laboratory. The distance the laser beams had to travel between the table and the reactor was about 23 meters. Therefore, the laser beams were piped from the optical table to the reactor through a series of right angle prisms. Optical breadboards were positioned next to the windows of the reactor and were supported by an aluminum and steel structure that was cantilevered off the wall of the reactor test bay (See Figure 3). The structure was designed to allow optical access to any of the reactor sections. The optical breadboards were designed to be moved over a 15 cm range by a crank and chain mechanism which allowed for the accurate positioning of the breadboards to within about 0.25 mm of the desired location. Transporting the laser beams to the distant reactor presented a problem in that the laser beams diverged significantly over the 23 meter path length. A laser beam has a particular Rayleigh range, or distance over which the beam can be transported without significant deterioration. The Rayleigh range is proportional to the beam diameter squared and inversely proportional to the beam's wavelength. Thus, small diameter laser beams of a given wavelength diverge more than large diameter beams of the same wavelength over an equivalent path length. The laser beams were originally transported to the reactor in a USED CARS phase matching geometry. USED CARS utilizes multiple regions of one large toroidal shaped pump beam to mix with concentrically located smaller diameter dye beams (Marko and Rimai 5 |