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).
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.
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 Wd 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 Wd 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 Wd 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.