Low-Resolution Raman-Spectroscopy Combustion Thermometry

Glenn Research Center

A method of optical thermometry, now undergoing development, involves low-resolution measurement of the spectrum of spontaneous Raman scattering (SRS) from N₂ and O₂ molecules. The method is especially suitable for measuring temperatures in high-pressure combustion environments that contain N₂, O₂, or N₂/O₂ mixtures (including air).

Figure 1. Rotational Raman Spectra of pure N₂ at three temperatures were calculated to show how W_d increases with temperature.

Methods based on SRS (in which scattered light is shifted in wavelength by amounts that depend on vibrational and rotational energy levels of laser-illuminated molecules) have been popular means of probing flames because they are almost the only methods that provide spatially and temporally resolved concentrations and temperatures of multiple molecular species in turbulent combustion. The present SRS-based method differs from prior SRS-based methods that have various drawbacks, a description of which would exceed the scope of this article. Two main differences between this and prior SRS-based methods are that

It involves analysis in the frequency (equivalently, wavelength) domain, in contradistinction to analysis in the intensity domain in prior methods; and
It involves low-resolution measurement of what amounts to predominantly the rotational Raman spectra of N₂ and O₂, in contradistinction to higher-resolution measurement of the vibrational Raman spectrum of N₂ only in prior methods.

Figure 2. The Conversion Formulas for Pure N₂ and O₂ are represented by the lower and upper boundaries, respectively, of the shaded region. For an N₂/O₂ mixture, the conversion formula lies between these boundaries.

Analysis in the frequency domain reduces the effects of uncertainties in the spectral-response calibration and permits greater signal-to-noise ratios by excluding the noise contributed by intensity or amplitude fluctuations. One advantage of utilizing the rotational Raman spectral bands is that they are much stronger than are the vibrational Raman spectral bands. In particular, in this method, one utilizes the rotational N₂ bands near the laser wavelength. The deliberate choice of lower resolution makes it acceptable to use wider spectrograph slits and thereby to collect more light to obtain greater signal-to-noise ratios. A further advantage of lower resolution is the independence of the spectra on pressure broadening effects.

According to theoretical simulations, the rotational Raman spectrum of N₂ widens with increasing temperature (see Figure 1). This is because at higher temperature, greater proportions of rotational states having higher energies become excited. Consequently, it should be possible to establish a relationship between the width W_d of the envelope of the rotational Raman spectrum and the temperature and to express this relationship as a conversion formula for determining the temperature from W_d of a measured spectrum; this is the basic principle of the present method. The method as described thus far would be simple, were it not for the facts that (1) the rotational Raman spectra of N₂ and O₂ overlap and (2) almost any practical combustion system contains N₂ and O₂. The net effect of the superposition of the N₂ and O₂ rotational Raman spectra is to produce a taller, narrower version of the spectrum of pure N₂, the amount of narrowing depending on the relative proportions of N₂ and O₂.

To account for this narrowing, it becomes necessary to generate and use a more comprehensive conversion formula, as illustrated in Figure 2. First, the envelopes of rotational SRS spectra of N₂ and O₂ are calculated theoretically over a range of temperature at a certain pressure to obtain the conversion formulas for N₂ and pure O₂. Then a blended conversion formula is obtained as a weighted average, wherein the weighting factors are determined by the relative proportions of N₂ and O₂ as measured or calculated by independent means. (The independent means could be measurements of vibrational Raman spectra of N₂ and O₂ or a chemical-equilibrium calculation.) Finally, the measured W_d is inserted into the blended conversion formula, yielding the temperature.

This work was done by Quang-Viet Nguyen of Glenn Research Center and Jun Kojima of Ohio Aerospace Institute.

Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-18100-1.