Development of High-speed Water Vapor Absorption Thermometry in Harsh Environments

This work aimed to improve near IR, direct absorption spectroscopy measurements of H2O vapor for thermometry purposes by focusing on spectroscopic light source systems and post-processing techniques. A survey was conducted of available near IR spectroscopic light source systems, and the 2-VCSEL system was chosen as the focus of this research. The 2-VCSEL system developed and constructed utilizes two, time-division multiplexed, injection-current tuned, vertical-cavity surface-emitting lasers (VCSELs) to measure high-resolution spectra at a rate of 10 kHz. Post-processing techniques were improved by developing a method to simultaneously fit baseline, temperature, and broadening variables when comparing measured spectra to simulated database spectra. This new post-processing also improved the consistency of output temperature data thereby enabling a study of the sources of error in H2O vapor absorption temperature measurements. Two of the largest sources of temperature error discussed are inaccuracies in the H2O line list database (e.g., line positions, linestrengths, and lower state energies), and inaccurate broadening models. A method for minimizing temperature error due to broadening inaccuracies is presented and tested. The 2-VCSEL system was used to take experimental data in a constant volume combustion chamber and in a single-cylinder research engine. The data was analyzed using the post-processing techniques and broadening models developed in this research in order to determine line-of-sight average temperature. The measured spectra were matched to database simulations with low residuals, and good agreement was found between optically measured and theoretically predicted temperatures.