Spatially and Temporally Resolved Temperature and Species Concentration Measurements in High-Pressure Combustors using fs-CARS

The objectives of the proposed Phase-II research effort is (1) to develop single-shot femtosecond (fs) laser-based coherent anti-Stokes Raman scattering (CARS) spectroscopy for providing quantitative high-speed (1-10 kHz) temperature and species concentration measurements in unsteady reacting flows, (2) to explore the use of a single-wavelength, broadband fs laser-beam for CARS spectroscopy thereby eliminating the problems associated with crossing multiple laser beams in turbulent media, and (3) to investigate the group velocity dispersion of the fs laser pulse as a function of temperature and pressure through multiple gas-phase media that are relevant to combustion processes.  The initial frequency-spread dephasing rate of the Raman coherence induced by the ultrafast (~85 fs) Stokes and pump beams will be used to measure gas-phase temperature for high-pressure combustion.  Single-shot CARS thermometry will be performed by obtaining time-resolved CARS spectra from a chirped probe pulse.  A theoretical model will be developed to interpret and extract high-speed, single-shot temperature from experimental measurements.  The use of shaped ultrashort pulses for selective detection of combustion species will be also performed during the Phase-II research activities.  The Phase-II research efforts will identify the technologies for successfully transitioning this technique from laboratory to high-pressure gas-turbine test rigs and augmentors at Wright-Patterson Air Force Base (WPAFB) and similar research facilities in industry and academia. BENEFIT: The proposed research effort will provide new diagnostic capabilities that will enable the Air Force and aircraft engine manufacturers to address the challenges associated with combustion instability. These tools are critical for increasing the acceptance rate and long-term health of engines for the war fighter.  The proposed research will also help to validate numerical models of instability phenomena affecting modern high-pressure combustor and augmentor performance, and lead to improved control strategies ensuring rapid and stable combustion during critical phases of the flight envelope.  Such experimental and numerical tools will also prove extremely valuable in the analysis of military and commercial gas-turbine combustors, as well as internal combustion engines and stationary power generation systems.