Flame Temperatures from Vibrational Raman Scattering
Raman scattering signatures are functionally dependent upon temperature, and are therefore useful as diagnostic probes. Various Raman scattering techniques for the measurement of temperature are outlined here. Those methods based upon rotational molecular structure are then briefly discussed in order to compare and contrast them with the ones based upon vibrational structure. For flame gases, the elevated temperatures and the multicomponent, variable composition make the vibrational scattering techniques appear to be more useful than those based upon pure rotational scattering. Temperature measurements based upon vibrational Raman scattering are described next, with an emphasis on the vibrational techniques developed in this laboratory. These techniques are based upon the spectral structure of the fundamental Stokes vibrational band series, which consists of the ground state band (initial → final molecular vibrational levels: v = 0 → v = 1) and the upper state or “hot” bands (1 → 2, 2 → 3, etc.).
KeywordsVibrational Level State Band Rotational Level Band Peak Ground State Band
Unable to display preview. Download preview PDF.
- 2.M. Lapp, in Advances in Raman Spectroscopy, Vol. 1, ed. by J. P. Mathieu (Heyden and Son, Ltd., London, 1973) Chap. 31.Google Scholar
- 3.M. Lapp, C. M. Penney, and R. L. St. Peters, Project SQUID Technical Report GE-1-PU, Office of Naval Research (1973).Google Scholar
- 5.M. Lapp, C. M. Penney, and J. A. Asher, “Application of Light-Scattering Techniques for Measurements of Density, Temperature, and Velocity in Gasdynamics,” Aerospace Research Laboratories Report No. ARL 73–0045 (1973).Google Scholar
- 6.M. Lapp, C. M. Penney, and J. A. Asher, “Application of Light-Scattering Techniques for Measurements of Density, Temperature, and Velocity in Gasdynamics,” Aerospace Research Laboratories Report No. ARL 73–0045 (1973), pg. 109.Google Scholar
- 7.A possible exception to our conclusion that rotational Raman scattering is unsuitable for flame work is the interferonstric “comb” method of Dr. Barrett discussed earlier in these Proceedings. Here, a technique based upon use of a Fabry-Perot interferometer is described that has the capability of distinguishing rotational Raman signatures for dissimilar species. Utilization of this method on a high-temperature multi-component gas mixture for diagnostics of many species with roughly similar rotational constants (i.e., roughly similar rotational line spacings) will, no doubt, be difficult, but hopefully, will be possible. (See also “note added in proof” at the end of Section III, C.)Google Scholar
- 8.Note that only alternate “strong” lines are shown in Figure 6A, for purposes of clarity. Because of nuclear spin degeneracy, the nitrogen rotational lines alternate in intensity between “strong” and “weak” sets of lines, for which the statistical weights are 6 for Q-branch lines corresponding to even values of the rotational quantum number J, or 3 for lines corresponding to odd values of J.Google Scholar
- 9.The spectral position of the ground state band peaks varies with temperature, and is considered properly in computer-performed Q-branch profile fits. Here, however, when we view the profiles in Figure 11 over the range of 1300°K to 1700°K, the variation of peak position is relatively small. This is discussed further in Section III. C.Google Scholar
- 12.M. Lapp, L. M. Goldman, and C. M. Penney, General Electric Report No. 71-C-267 (1971).Google Scholar
- 13.W. E. Kaskan, Sixth Symposium (International) on Combustion (Reinhold, New York, 1957), p. 134.Google Scholar
- 14.For nitrogen data, the steady stoichiometric hydrogen-air flame utilized flow rates of 37.5 cc/sec H2 and 88.8 cc/sec air, for which 65% of the product gases was nitrogen. Since the image of the monochromator entrance slits at the flame scattering position was about 5 mm high, an estimate was made of the temperature variation along this zone. This was found to be about 16°K. The reproducibility of the thermocouple data was about ± 1/2°K.Google Scholar
- 15.For hydrogen data, a somewhat unsteady fuel-rich (four-times-stoichiometric) hydrogen-air flame was used (79.3 cc/sec H2 and 47.0 cc/sec air) for which about 51% of the product gases was hydrogen. The variation of temperature with position along the slit image was much more severe than was the case for the stoichiometric flame, being roughly 110°K over a 5 mm vertical zone. Furthermore, the reproducibility was significantly poorer, being roughly ± 3°K. This flame, colored red from the emission of water vapor vibration-rotation bands [See A. G. Gaydon, Spectroscopy of Flames (Chapman and Hall, London, 1957), pp. 79, 90, and 241.] was subject to significantly more diffusion by the ambient atmosphere than the stoichiometric flame, which undoubtedly contributed to its less reproducible characteristics. It had, however, the virtue of a high hydrogen content.Google Scholar