Development of pure-rotational coherent Raman spectroscopy on hydrogen for applied flame thermometry

Abstract

Work is presented on a novel hybrid fs/ps two beam CARS application for spatiotemporal gas phase hydrogen thermometry, capable of sampling at a rate of 1 kHz. The aim of the research is to assess the feasibility of directly probing hydrogen for accurate temperature measurements in a combustion environment. Typically, nitrogen is used in applied CARS thermometry because of the inert nature of the gas and the availability on both the products- and reactants side of the flame. Nitrogen is well understood in CARS applications and is therefore used as a benchmark for the results obtained through hydrogen thermometry. No hydrogen was available in the lab, so a rich premixed methane-air flame was used in a regular Bunsen burner. At high equivalence ratio’s sufficient amounts of hydrogen are formed as an intermediate species to be probed in the flame. Quantum-mechanicalmodels were used to predict the molecular responses of the corresponding molecules, after which they are compared to the experimental data. The feasibility of hydrogen thermometry was assessed through a series of three different experiments: using the standard output pulse of the regenerative amplifier at 35 fs for spatiotemporal measurements, using the 35 fs pulse for point measurements, and using a frequency broadened 10 fs pulse produced through filamentation for point measurements. Nitrogen and hydrogen spectra were recorded simultaneously to ensure proper data comparison. The 1 kHz acquisition rate enabled the evaluation of temporal correlation in the temperature data. The spatiotemporal CARS measurements clearly depicted flame dynamics at 1 Hz and 12Hz, but the signal to noise ratio in the hydrogen signal was too weak to properly assess the data. The point measurements using the 35 fs pulse yielded similar average temperatures, but the precision in the hydrogen temperatures was poor (25%) due to the fact that only the first two S-branch transition in hydrogen were excited. In the experiments using the 10 fs pulse, the first four S-branch transitions in hydrogen were excited. This greatly improved the precision for the temperature measurements (2%), surpassing that of the nitrogen thermometry (4%). The nitrogen and hydrogen temperatures also showed great temporal correlation (0.85-0.9). However, there was a very consistent temperature deficit in the hydrogen temperatures of 550K, most like caused by a wavelength dependence in the optics (which are optimized for narrowband applications) which severely affects the higher Raman shifted hydrogen signal. Lastly, an experiment was done using the 10 fs pulse to probe O-branch transitions in hydrogen, and a serious discrepancy between the CARS and CSRS spectra were found, again likely caused by the optics in the setup. These results approximated nitrogen temperatures much better, within 7% at 2000 K. Overall, the results were promising, but not yet conclusive and more research is needed.