High-Resolution fs/ps CARS for Quantitative Measurements of Temperature and Species in a Dual-Mode Scramjet

High-resolution quantitative measurements of thermodynamic properties provide key insight into the fundamental combustion processes occurring within a scramjet engine. Quantitative measurements are necessary for validating numerical simulations of the reacting flowfield, locating reaction zones, and elucidating flame stabilization mechanisms within a scramjet engine.

A novel counter-propagating (CoP) phase-matching configuration for hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS) is demonstrated to have ultra-high spatial resolution on the order of 10's of µm. This represents a significant improvement over traditional phase matching schemes, enabling precise measurements in regions with steep temperature gradients and minimizing the impact of spatial averaging. Utilizing the CoPCARS system, the temperature gradient resulting from a high-velocity microscale gas jet emanating from a hypodermic needle (51 µm inner diameter) was accurately quantified.

To quantify temperature and relative species concentration in the flow of a dual-mode scramjet combustor, a fs/ps CARS system is developed and implemented in the University of Virginia Supersonic Combustion Facility (UVaSCF). Temperature measurements using fs/ps CARS within the combustor are presented for various fueling configurations. Ongoing work to develop a CARS spectral model for quantifying local equivalence ratio is discussed, and preliminary comparisons between experimental spectra and the spectral model are presented.