Investigating fs/ps CARS for Quantifying Pressure in Reacting Flow Environments

Femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS) was investigated for quantifying in-stream pressure fluctuations in reacting flows. Accurate quantification of pressure fluctuations can provide insight into coupling between flow kinetic energy and molecular internal energy for high-speed reacting flows. For the current work, ro-vibrational (Q-branch) fs/ps CARS was investigated to optimize pressure sensitivity.

This study addresses two primary challenges. First, improvements to the CARS spectral model were made to accommodate the effects of various collisional partners. Experimental results from literature over a broad range of gas temperatures were used to refine the Modified Exponential Gap (MEG) model. Second, the impact of experimental uncertainties including laser bandwidth and stability were investigated by exploring the fs/ps CARS response at controlled pressures. Experimental measurements were conducted in a static pressure cell containing controlled gas mixtures (N2, O2, CH4, and CO2). The pressure and species dependence of the fs/ps CARS response for various gas mixtures was investigated and reported. Various features in the measured fs/ps Q-branch CARS spectra for N2 and CO2 and N2 and CH4 are used to deduce an empirical pressure dependence. The recorded fs/ps Q-branch CARS spectra for N2 and O2 were compared to spectral simulations. A simple spectral analysis technique was introduced to mitigate the impact of excitation variability. This approach was validated across various N2/O2 mixtures and holds promise as a foundational method for pressure measurements in dynamic combustion environments.

This Master's thesis research was funded by the Air Force Office of Scientific Research (AFOSR)
Grant: #FA9550-21-1-0103 (PM Dr. Chiping Li).