Vibrational measurement, optical data

Figure 3. Schematic of turbulent combustor geometry and optical data acquisition system for vibrational Raman-scattering temperature measurements using SAS intensity ratios. Also shown are sketches of the expected Raman contours viewed by each of the photomultiplier detectors, the temperature calibration curve, and several expected pdf s of temperature at different flame radial positions. The actual SAS temperature calibration curve was calculated theoretically to within a constant factor. This constant, which accounted for the optical and electronic system sensitivities, was determined experimentally by means of SAS measurements made on a premixed laminar flame of known temperature. Measurements of Ne concentration were made also with this apparatus, based on the integrated Stokes vibrational Q-branch intensities. These signals were related to gas densities by calibration against ambient air signals.

The bias observed between experimental measurements and Kieffer s model predictions is due to the relative paucity of experimental data concerning cutoff frequencies of acoustic branches, and also to the assumption that the frequencies of the lower optical branches are constant with K and equivalent to those detected by Raman and IR spectra (corresponding only to vibrational modes at K = 0). Indeed, several of these vibrational modes, and often the most important ones, are inactive under Raman and IR radiation (Gramaccioli, personal communication). The limits of the Kieffer model and other hybrid models with respect to nonempirical computational procedures based on the equation of motion of the Born-Von Karman approach have been discussed by Ghose et al. (1992). 

Concentration profiting uses on- or in-line measurements of optical properties, typically not done for the whole volume, but along lines such as the channel cross-section (see e.g. [20]). Concentrations are accessible by photometric, electric or fluorescence measurements. Furthermore, vibrational analysis such as IR and Raman spectroscopy can be used for the same task [80,81]. Concentration profiling can also be achieved simply by gray-scale or comparable image analysis for quantitative data extraction from microscopy images of colored flows [20, 37, 68],... 

The Raman spectra are quicker and easier to determine than the infrared absorption spectra because ordinary optical equipment can be used, but frequently they are more difficult to interpret. The quantum restrictions in the two phenomena, particularly for symmetrical molecules, are not always the same, because the Raman spectrum involves an intermediate excited state of the molecule. For this reason, it is desirable to have the data of both Raman and infrared absorption spectra in order to determine completely the rotational and rotational-vibrational energy levels in the molecule. The Raman spectrum can be obtained in some solutions where direct absorption measurements are impossible because the solvent is opaque in the infrared. Aqueous solutions offer a good example of such a case. 

---