Development of spatiotemporal 1D-CARS for the investigation of hydrogen flame propagation

Abstract

This doctoral thesis presents the development and application of spatiotemporal coherent anti-Stokes Raman scattering (CARS) imaging for quantitative measurements of local structures in laminar and turbulent hydrogen flames. The study of spatial-thermochemical scalars in these flames is of paramount importance in advancing the next generation of sustainable combustion systems. The complexities of these environments require high-fidelity scalar measurements, demanding optical diagnostic tools with exceptional spatial and temporal resolution. Ideally, such instruments must be capable of delivering in-situ nonintrusive measurements of temperature, as well as species mole fraction in dynamical environments occurring across narrow spatial gradients of typically ~100’s μm. Recent developments in ultrafast —femtosecond (fs) and picosecond (ps)— laser technology have enabled the advancement of time-resolved CARS spectroscopy achieving independently, heightened repetition rates (from Hz to kHz), multidimensional imaging (1D and 2D), and multiplex spectroscopic measurements targeting major combustion species. Despite these advancements, there remains a gap in the availability of CARS instruments capable of seamlessly combining all these functionalities.
The primary goal of this research is to combine these advancements in a unique design, that allows for spatiotemporal 1D-CARS measurements of temperature and concentration of N2, H2, O2, and H2O across the flame front of hydrogen flames. Using a single ultrafast regenerative amplifier laser system (Astrella, coherent) combined with a Second harmonic bandwidth compressor (Light conversion), twobeam fs/ps pure-rotational CARS imaging is achieved across a one-dimensional field-of-view of ~1.5 mm at 1 kHz repetition rate in flames. To detect these signals, a novel polarization-sensitive coherent imaging spectrometer equipped with a highspeed sCMOS camera was designed, achieving an excellent spectral and imaging resolution (<20 μm). The performance of the instrument is demonstrated for cinematographic 1D-CARS gas-phase thermometry (300-2200K) across an unstable premixed methane/air flame-front, achieving a single-shot precision <1% and an accuracy of <3%.
Furthermore, the thesis introduces a groundbreaking strategy for deducing water vapor concentration from the pure-rotational N2 CARS signal in the time domain, overcoming the challenge posed by weak H2O pure-rotational Raman spectra. This is achieved by exploiting the sensitivity of the N2 rotational Raman coherence to energy transfer during inelastic collisions between N2-N2 and N2-H2O. The developed technique allows simultaneous measurements of temperature, as well as O2, H2, and water vapor concentrations in a laminar H2/air diffusion flame. These experimental findings are complemented by numerical investigations, underscoring the potential to extend the technique to measure water vapor in more complex ternary collisional systems found in hydrocarbon flames. The further application of these developments in the canonical H3 flame reveals the capability to directly measure molecular transport processes affecting flame structure using spatiotemporal 1D-CARS. A dual-probe CARS approach combined with a polarization-sensitive coherent imaging spectrometer enables simultaneous acquisition of molecular N2 coherence at short and long probe delays, providing one-dimensional thermometry and concentration measurements for H2, O2, and H2O in a single laser shot.