Recent advances in coherent anti-Stokes Raman scattering spectroscopy: Fundamental developments and applications in reacting flows

Overventilated, coflowing axisymmetric laminar diffusion flames of ethylene, propane, and butane were used to study the influence on soot of hydrogen addition to the fuel. The flame temperatures were measured by CARS along the flame axis as well as at off-axis radial locations. CARS spectra taken in heavily sooting regions exhibited poor fits due to C 2 absorbtion of part of the fundamental band of the nitrogen spectrum. It was found that C 2 absorbtion was confined to frequencies greater than 2313 cm −1 . We, therefore, implemented a strategy that fitted only the CARS spectra in the frequency range smaller than 2313 cm −1 . Measured soot concentrations and the flame temperature data with and without hydrogen and helium dilution were evaluated and the relative influences of dilution and direct chemical interaction on soot formation, as a result of hydrogen addition, are presented. It is shown that when hydrogen or helium is added to the fuel as a diluent in moderate quantities, the changes in the temperature field of the coflow diffusion flames are negligible. When allowance is made for the influence of dilution, addition of hydrogen to the fuel side of an ethylene diffusion flame reduces the soot formation. For propane and butane flames, hydrogen addition does not show any influence on soot formation apart from the dilution effect.

Influence of hydrogen addition to fuel on temperature field and soot formation in diffusion flames

Laser diagnostics and minor species detection in combustion using resonant four-wave mixing

Broadband picosecond coherent anti-Stokes Raman scattering (CARS) spectroscopy of nitrogen is demonstrated using 145-ps pump and probe beams and a 115-ps Stokes beam with a spectral bandwidth of 5 nm. This is, to our knowledge, the first demonstration of broadband CARS using subnanosecond lasers. The short temporal envelope of the laser pulses and the broadband spectral nature of the Stokes beam will enable nonresonant-background-free, single-shot, or time-dependent spectroscopy in high-pressure or hydrocarbon-rich environments. Successful correlation of room-temperature broadband picosecond N2 CARS with a theoretical spectrum is presented.

Broadband coherent anti-Stokes Raman scattering spectroscopy of nitrogen using a picosecond modeless dye laser

Spark ignited hydrogen/air mixtures: two dimensional detailed modeling and laser based diagnostics

The noise in single-shot coherent anti-Stokes Raman (CARS) spectroscopy that employs a broadband modeless dye laser (MDL) is examined and the results are compared with those of a conventional dye laser. The noise of the dye-laser, the nonresonant CARS, and the resonant N(2) CARS signals are determined. The use of a MDL is shown to result in substantially reduced CARS noise when the CARS signal is generated with a single-mode pump laser, but only a marginal reduction of noise is observed with a multimode pump source The noise measurements are compared with theoretical predictions that are based on models that assume modes of random amplitudes and phases in the multimode laser sources. The combination of a MDL and a single-mode pump laser is shown to increase the precision of single-shot N(2) CARS temperature measurements.

Noise in single-shot broadband coherent anti-Stokes Raman spectroscopy that employs a modeless dye laser.

Spatial averaging is a potential problem in the application of coherent anti-Stokes Raman spectroscopy (CARS) to combustion diagnostics when varying temperatures and composition are present at spatial scales smaller than the typical 1–2-mm spatial resolution of CARS. The observed CARS spectrum is then a mixture of the hot and cold components. We show that simulated, spatially averaged spectra, generated by the incoherent addition of intensities, can be significantly different from those obtained by the coherent addition of the electric field amplitudes of the component spectra. The analyses of these simulated, spatially averaged CARS spectra demonstrate that the use of theoretical CARS spectra, generated by the addition of intensities of the hot and cold components of a binary gas mixture, can lead to errors in the estimated flame temperatures.

Estimation of spatial averaging of temperatures from coherent anti-Stokes Raman spectroscopy

The total nonresonant susceptibilities of C3H8, CO2, and C2H4, measured by fitting the coherent anti-Stokes Raman spectra of binary mixtures of these gases and nitrogen with theoretical N2 CARS spectra are reported. These measurements are compared with the values in the literature obtained by CARS and other techniques. For propane and carbon dioxide, the effect of including their contribution to the broadening of nitrogen Raman lines on the inferred nonresonant susceptibilities is also examined.

T. Parameswaran

Papers

Noise in single-shot broadband coherent anti-Stokes Raman spectroscopy that employs a modeless dye laser.

Estimation of spatial averaging of temperatures from coherent anti-Stokes Raman spectroscopy

Measurements of the total third-order nonresonant susceptibilities of C(3)H(8), CO(2), and C(2)H(4), and their application to N(2) CARS thermometry.