Quantitative Multi-Scalar Raman Scattering Diagnostics in High Pressure Flames
We are performing a combined experimental and theoretical effort to develop a spectral calibration database
for multi-scalar diagnostics using spontaneous Raman scattering (SRS) in high-pressure flames.
SRS is perhaps the only optical diagnostic technique that can provide single-shot spatially-resolved
multi-scalar measurements of species concentration and temperature in turbulent flames.
However, such measurements in the past have used a one-of-a-kind experimental setup,
and a setup-dependent calibration procedure to empirically account for spectral interferences or
cross-talk amongst the major species of interest. Such calibration procedures, being non-transferable,
are prohibitively expensive to duplicate. Figure 1 shows the combined optical
and mechanical system used to acquire the spectral calibration database.
A goal of this effort is to provide a SRS calibration database using transferable standards
that can be widely implemented by other researchers for both atmospheric pressure and high-pressure
(< 30 atm) SRS studies. A secondary goal of this effort is to provide quantitative multi-scalar
diagnostics in high-pressure environments for the purposes of validating computational combustion codes.
In order to provide quantitative measurements of major species concentration and temperature
in high-pressure flames using SRS, the spectral cross-talk between different molecular species
has to be compensated through the use of a temperature-dependent calibration matrix.
We implement the spectral calibration using a high-pressure gaseous burner (HPGB)
combustion facility (Figure 2) that utilizes a specially designed
premixed array burner mounted inside a 60 atm pressure vessel providing the optical access for
the SRS measurement. By collecting SRS spectra (400 nm to 700 nm, 0.3 nm resolution)
from a variety of high-pressure (10 atm to 30 atm) hydrogen-fueled flames operating
over a wide range of equivalence ratios (0.15 to 5.0), the pixel-integrated spectral response
from the SRS process is then correlated to the molecular species densities calculated from
chemical equilibrium. The
chemical equilibrium calculations
are based on precisely measured (0.5% accuracy) fuel-oxidizer flow rates at an assigned
temperature and pressure. Since the temperature is spectroscopically measured with an accuracy
of better than 10 K, we do not require the assumption of adiabatic equilibrium.
In addition to the experimental SRS calibration process, we also theoretically model
the Raman scattering in order to quantitatively analyze the spectral interferences.
The model showed excellent agreement with data. The spectral modeling also aids the development
of the calibration matrix functional form, and ultimately, improves the calibration accuracy.
Initially, we use spectrally simple hydrogen-air flames to provide hot combustion products
such as N2, O2, H2O and H2 over a wide temperature range. We then progressively move to more
spectrally complicated carbon-containing fuels to include the SRS signals from CO and CO2.
We then define the appropriate wavelength integration regions for the relevant species of interest.
Using a technique that we previously developed for the analysis of Raman spectra at high-pressures,
we determined accurate thermodynamic temperatures and Raman signal intensities for each molecular
scattering species in fuel-lean and fuel-rich regions, which was then used to generate the calibration
matrix database for all major species and their associated spectral cross-talk effects as a function of
temperature. Using this calibration matrix, we then demonstrate, for the first time, quantitative
multi-scalar measurements of species concentration and temperature in premixed H2-air flames
at 10 atm using spontaneous Raman scattering.
This project was performed with the help of a very talented team of experts in their field:
Dr. Jun Kojima (OAI, Senior Scientist,
formerly an NRC Postdoctoral Research Associate),
Mr. Gregg Calhoun (QSS, Electrical Operations Engineer), Mr. Ray Lotenero (Akima, Electronics/Laser Technician),
and Mr. Gary Lorenz (GRC Mechanical Technician).
Dr. Quang-Viet Nguyen