Development of CO2-O2 CARS thermometry and concentration measurements for applied flame diagnostics

Abstract

In this M.Sc. thesis project a model is developed and validated that simulates the coherent anti-Stokes Raman scattering (CARS) signal of CO₂ and O₂ in the spectral region between 1250cm^-1 and 1680cm^-1. The aim is to perform temperature and concentration measurements in a typical hydrocarbon-air combustion flame. The project makes use of a two-beam time resolved CARS setup, with ultraboadband generated light from the pump/Stokes beam to excite the Raman transitions in this spectral window. In the 1250-1680cm^-1 region, CARS signatures of both CO₂ and O₂ are visible making it possible to perform thermometry on both molecular species, while also offering the option of evaluating relative CO₂-O₂/O₂-CO₂ concentrations. The appearance of the oxygen ro-vibrational (O-, S- and Q-branch) spectrum at low temperatures, along with a strong manifestation of the CO₂ Fermi influenced Q-branch (with red-shifted peaks below 1300cm^-1 and blue shifted peaks above 1350cm^-1) at higher temperatures, makes it possible to perform thermometry in low and high temperature combustion environments on both the reactant and product side. A total of 256 vibrational levels for CO₂ are taken into account for the model to simulate the CO₂ Fermi polyad. From these 256 vibrational levels, 181 Raman transitions are possible that fulfill the criteria of Δv₁=1, Δv₂=0, Δv₃=0, Δl=0 and ΔJ=0 . Three different experiments are performed including an M-flame, a V-flame and two experiments conducted in air for model validation. The temperature analysis using CO₂ provided satisfactory results regarding temperature assessments. Depending on the experiment, standard deviations below 2.3% and mean temperatures to within 1% of the temperatures corresponding to the expected values. The O₂ analysis showed a good correspondence to the CO₂ temperature values, differing by 43-76K. The O₂ analysis showed low standard deviations for the air temperature assessment (3.27%) and reliably predicted the ambient temperature with a difference lower than 2K. The M-flame experiments showed the least correspondence to actual values. These unsatisfactory results can for one part be attributed to the high signal-to-noise ratios (SNR) and for one part due to the flickering of the flame. In terms of concentration assessment the model closely evaluates O₂-CO₂ concentrations in the ambient air and from an exhaled human breath, while the flame assessments had a mediocre correspondence to the predicted ones from \textit{chem1d}. All in all the project shows that CO₂ temperature and concentration measurements in this CARS spectral region is feasible. It offers promise from a combustion perspective due to the possibility of performing simultaneous (O₂) rotational and (CO₂/O₂) vibrational thermometry, which makes it possible to perform measurements on both the product and reactant side of the flame front, while including spectral signatures of possibly three major molecular combustion species: CO₂, O₂ and H₂. Further improvements to the model and the application of the technique, make more complex combustion studies possible and help achieve the goal to develop a ultrafast, multiplex, state-of-the-art laser diagnostic tool for gas phase combustion measurements.