Researchers from the Combustion Branch at the NASA Glenn Research Center and from the Ohio Aerospace Institute (OAI) have developed a novel optical technique for improved temperature measurements in high-pressure combustion environments. This new thermometry technique utilizes low-spectral-resolution pure-rotational spontaneous Raman scattering of nitrogen (N2) and oxygen (O2) to improve the accuracy of single-shot temperature measurements in turbulent combustion. This approach is especially useful in cases where the vibrational Raman signals are inadequate for a conventional temperature analysis.
Rotational Raman spectra at low resolution (35 cm-1): calculated pure nitrogen spectra at different temperatures.
According to spectral simulations (see the preceding graph) of low-resolution, pure rotational Raman scattering based on the theory from references 1 and 2, it is clear that the shape of the rotational N2 spectrum becomes wider with an increase in temperature. This increase in width results from the fact that more rotational states with higher energies (longer wavelength at the Stokes side, shorter wavelength at the anti-Stokes side) are populated at higher temperatures as a result of the Boltzmann distribution. Thus, a measurement of the envelope bandwidth of the rotational spectrum can provide temperature information. The use of the low-resolution rotational bands permits an enhancement in the overall signal amplitude (because of a larger Raman cross section of rotational versus vibrational lines, and higher optical throughput resulting from the use of a wider optical slit), thereby decreasing the statistical uncertainty through a significant increase in the signal-to-noise ratio.
However, a challenge needs to be met for this strategy to be successful: the effects of spectral interferences from the rotational O2 Raman spectrum must be compensated for. Since the rotational frequency of O2 is very close to that of N2, the rotational spectral band of N2 and O2 appears only as a single band combining the two spectra. This O2 interference is unavoidable since any practical air-breathing combustion system essentially contains nitrogen and oxygen as the oxidizer. From the preceding graph, it is known that the overlapping causes a “narrowing” effect, in which the spectrum of the N2/O2 mixture shows a narrower envelope than that of the pure N2 at the same temperature. To conveniently and effectively compensate for this spectral band narrowing because of the O2 overlapping, we developed an algorithm based on the theoretically predicted spectral blending of the two individual species, adjusted by the relative concentrations of N2 and O2 obtained from the simultaneous vibrational Raman spectra (see the following graph).
Calculated correlations between the rotational-envelope bandwidth and the temperature of N2/O2 mixtures at 5 atm.
Long description of figure 2.
An experimental verification of our method is given in the graph on the next page. For this test, a typical Raman-scattering spectroscopy apparatus (refs. 3 to 6) was used in a high-pressure gaseous calibration burner (refs. 7 to 9), which provided a steady flame that can be predicted using the assumption of chemical equilibrium. In the following graph, the flame temperatures derived from the present method are compared with the calculated adiabatic temperatures. An excellent agreement between the two over a range of equivalence ratios shows that the present flame thermometry is promising.
Averaged flame temperatures determined by Raman thermometry in high-pressure (10-atm) lean hydrogen/air flames.
By using this new frequency-domain rotational Raman technique together with conventional vibrational scattering, we have demonstrated quantitative, spatially resolved, single-shot multi-scalar measurements of temperature and fuel/oxidizer concentrations in a high-pressure turbulent methane-air flame. The technology described has been disclosed as a NASA New Invention and Technology (ref. 10). For a description of the application of this technique in flames, see reference 11.
Find out more about Glenn’s combustion diagnostics research: http://www.grc.nasa.gov/WWW/combustion/zDiag.htm
Glenn contact: Dr. Quang-Viet Nguyen, 216-433-3574, Quang-Viet.Nguyen-1@nasa.govLast updated: December 17, 2007
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