Development and Application of Pure Rotational CARS for Reactive Flows

Abstract: The thesis deals with the further development of pure rotational coherent anti-Stokes Raman spectroscopy (RCARS) for improving the capabilities of gas phase thermometry. The main effort has been to make the technique more robust when employed under a wide range of temperatures and operational conditions. A primary aim has been to investigate the impact of collisional broadening on N2 RCARS thermometry, especially in an environment in which N2 is perturbed by H2. Since an interaction of this sort is species-specific and temperature-dependent, it plays a very critical role in RCARS thermometry. It was found that in a sequence of implementation, validation and application, thermometric accuracy could be improved by the implementation of N2-H2 line-broadening coefficients. Investigation of these topics involved exploring a novel technique of time-resolved picosecond RCARS for direct measurements of S-branch N2-N2/N2-H2 Raman linewidths. The N2 and O2 Herman-Wallis factors, used to quantify vibration-rotation interaction and breakdown of the rigid rotor model were also investigated. This correction affects the line-intensities, and also has an impact on RCARS thermometry. Conclusions regarding the sensitivity related to this factor could be achieved by employing different expressions available in the specialized literature. A theoretical code for N2O concerned with thermometric accuracy in a set of temperature-calibrated cell experiments was developed and was validated. This work expands the list of RCARS molecules previously developed, and points to interesting possibilities such as that of improving the thermometric precision. The technique was also applied to flame diagnostics. Temperatures were mapped along the centerline of a one-dimensional flame provided on a McKenna burner, this serving as important input data for other related optical techniques. The homogeneity of this flame was investigated for two different operational shielding co-flows, those of N2 and of air. Measurements were also performed in a low-swirl turbulent premixed flame, for validating existing models of large eddy simulations. Probability density functions for a large range of temperatures (300 K to 1700 K) and relative O2 concentrations were provided. The simultaneous measurements of these quantities provided a better understanding than possessed earlier of air entrainment from the surroundings.