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# Low-Resolution Raman-Spectroscopy Combustion Thermometry

- Created on Saturday, 01 November 2008

### This method offers advantages over related prior Ramanspectroscopy- based methods.

A method of optical thermometry,
now undergoing development, involves
low-resolution measurement of the spectrum
of spontaneous Raman scattering
(SRS) from N_{2} and O_{2} molecules. The
method is especially suitable for measuring
temperatures in high-pressure combustion
environments that contain N_{2},
O_{2}, or N_{2}/O_{2} 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
N
_{2}and O_{2}, in contradistinction to higher-resolution measurement of the vibrational Raman spectrum of N_{2}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
N_{2} 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 N_{2}
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 N_{2} and
O_{2} overlap and (2) almost any practical
combustion system contains N_{2} and O_{2}.
The net effect of the superposition of
the N_{2} and O_{2} rotational Raman spectra
is to produce a taller, narrower version
of the spectrum of pure N_{2}, the amount
of narrowing depending on the relative
proportions of N_{2} and O_{2}.

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
N_{2} and O_{2} are calculated theoretically
over a range of temperature at a certain
pressure to obtain the conversion formulas
for N_{2} and pure O_{2}. Then a blended
conversion formula is obtained as a
weighted average, wherein the weighting
factors are determined by the relative
proportions of N_{2} and O_{2} as measured
or calculated by independent
means. (The independent means could
be measurements of vibrational Raman
spectra of N_{2} and O_{2} 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.*

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