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.

There is a tremendous push to explore alternative fuels such as hydrogen or synthetic fuels to combat the effects of climate change. An important step in the development of alternative fuels is the characterisation of its combustion characteristics. There are various diagnostic tools which aid in the determination of flame properties. Optical diagnostics play a crucial role by providing a non-invasive form of measurement. In the current project the idea is to utilise the passive chemiluminescence from flames to determine flame properties. The focus is on the emission from the OH* radical present in flames. This radical is responsible for radiation in the ultraviolet region ranging from 280 nm to 350 nm. This radical was also chosen as it is a radical common to both hydrocarbon as well as hydrogen flames. The objective of the project was to develop an emission spectra model of the OH¤ radical and to test its feasibility in predicting flame temperature by comparing it with emission from a laminar premixed methane/air Bunsen flame. The underlying physics regarding the various aspects of the project are discussed in detail in the initial part of the report. In the latter part, the work done for the project is explained. An emission model was developed for the OH* radical in MATLAB. The model is capable of calculating the emission spectra at different temperatures and line shapes. Then a spectrometer was built to capture the radiation from the flames in the UV region. The obtained data was post processed to get the flame spectra. The flame spectra obtained from the flame was compared against the emission model to determine the best temperature fit. It was observed that the temperatures were much higher than the gas temperatures observed in methane/air flames. This is attributed to a non thermal population distribution in the excited molecules. There however exists a unique mapping to the equivalence ratio which can be utilised for equivalence ratio determination when properly calibrated. There are many possible directions uncovered to take the work further by exploring the applicability of the tool for other fuels and flame regimes.