Single-laser-shot femtosecond coherent anti-Stokes Raman scattering temperature and concentration measurements in reacting flows

The objective of this research is to develop and apply single-laser-shot femtosecond (fs) coherent anti-Stokes Raman scattering (CARS) as a laser diagnostic technique for temperature and concentration measurements in flames. The main potential advantage of this technique is that measurements can be performed at data rates of greater than 1 kHz. Advances in fs laser technology have led to commercially available fs laser systems with repetition rates of 1-10 kHz and pulse energies of a few millijoules (mJ). By developing single-laser-shot techniques with such laser systems, measurement data rates at least two orders of magnitude higher those associated with typical nanosecond Nd:YAG lasers become possible. Such high data rate measurements will provide new insight into transient and turbulent events, reduce testing time for experiments, and eventually aid in the design and evaluation of new combustion devices. A theoretical model and fitting routine have been developed to extract temperature and concentration measurements by comparing theoretical and experimental spectra. The electric fields are modeled by incorporating the experimentally measured spectrum of the pump, Stokes, and probe pulses and assuming a polynomial spectral phase. The resonant and nonresonant polarizations induced at the probe volume are modeled using the electric fields and Raman transition information. Finally, the CARS signal is modeled as the product of the chirped probe pulse electric field with the resonant and nonresonant polarizations. All fitting routines used to compare experimental and theoretical spectra use a genetic algorithm called differential evolution. Values for the laser parameters in the theoretical model are obtained by fitting a calibration spectrum recorded at a known temperature or concentration. These parameters are then held constant while fitting single-laser-shot spectra recorded in a test section. Fs CARS temperature measurements at 1 kHz have been performed in a heated gas cell, laminar near-adiabatic hydrogen-air flames, driven flames, and turbulent methane-air flames. For all flame temperature measurements, the precision was better than 2% of the average measured temperature. The accuracy of the measurements is strongly dependent on accurate models for the electric fields of the laser pulses, and is typically better than 3% of the absolute temperature over the temperature range from 300 to 2400 K. In driven flames the measured temperature is in good agreement with the expected temperature deviations. Polarization suppression of the nonresonant background has been demonstrated for single-laser-shot temperature measurements at 5 kHz in steady and unsteady flames. By suppressing the nonresonant background the evolution of the Raman coherence near zero probe time delay is more clearly revealed. The structure of the CPP fs CARS spectra with and without nonresonant background suppression is compared. The utility of polarization suppression of the nonresonant background for CPP fs CARS measurements is discussed. Flame temperature fluctuations over the range of 500 to 1800 K are measured in turbulent diffusion flames. Concentration measurements have been performed at 1 kHz using fs CARS in argon-nitrogen and carbon monoxide-nitrogen binary gas mixtures in a heated gas cell from 300 to 900 K at atmospheric pressure. The broadband pump and Stokes pulses excite Raman transitions in both the carbon monoxide and nitrogen. For measurements in argon-nitrogen or carbon monoxide-nitrogen gas mixtures, concentration measurements could be performed over the range of 5% - 90% nitrogen with typical measurement error being less than 2.0% in absolute concentration. The precision of the measurements was typically better than 1.5% in absolute concentration. Polarization suppression of the nonresonant background is also demonstrated for concentration measurements. Nonresonant background suppression clearly reveals the resonant signals from each gas species and concentration measurements could be performed over a slightly reduced concentration range with comparable results. (Abstract shortened by UMI.