Search for: All records

Hybrid fs/ps coherent anti-Stokes Raman scattering employing coherent control is presented for selective excitation of combustion-relevant species. Femtosecond pulse shaping is accomplished experimentally using a 4-f pulse shaper with a spatial light modulator at the Fourier plane. A feedback-controlled genetic algorithm for adaptive pulse shaping is used to optimize for the selective excitation of CO2 (near 1388 cm−1) and O2 (near 1556 cm−1) rovibrational transitions in a non-reacting flow. 0D fs/ps CARS measurements acquired in a near-adiabatic flame demonstrate the use of coherent control in a combustion environment. 

A hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS) system was developed for quantitative measurements of temperature in a laboratory-scale supersonic jet facility. Measurements were recorded at low pressures and densities relevant for supersonic and hypersonic environments, with special interest in exploring the feasibility of deploying this technique in the 20-inch Mach 6 and 31-inch Mach 10 wind tunnels located at NASA Langley Research Center. Modifications to the existing supersonic jet facility were made to simulate a test section with a width of 31 inches, so that the size of the test section is relevant for either wind tunnel. The CARS system was designed such that a similar beam geometry can be used in the laboratory to acquire point-based fs/ps CARS measurements along two axes of translation. Rotational Raman transitions of O2 and N2 were targeted. 

Vibrational and rotational temperatures in various detonation conditions involving diluted hydrogen-oxygen mixtures were studied in a microscale detonation tube using hybrid fem- tosecond/picosecond Coherent anti-Stokes Raman scattering (hybrid fs/ps CARS). Measured temperatures at various locations behind the shockwave were compared to Chapman-Jouguet conditions as predicted by equilibrium calculations. Simultaneous shadowgraphy was also employed to establish timing between the detonation wave and laser beams. Comparison between vibrational nitrogen and oxygen thermometry were made for detonations in the same gas mixture. Oxygen rotational temperature was measured and compared to vibrational temperature measured in a similar gas mixture. 

Hybrid fs/ps coherent anti-Stokes Raman scattering (CARS) is employed to investigate the vibrational temperature evolution of N2 in lean methane flames exposed to pulsed microwave irradiation. Vibrational temperature during and post microwave illumination by a 2 μs, 30 kW peak power, 3.05 GHz pulse is monitored in flames diluted with N2, N2 and CO2 , and N2 and Ar. Electric field strengths inside the microwave cavity are monitored directly using electric field probes. Temperature increases up to 140 K were observed in flames with additional Ar and CO2 dilution, whereas temperature increases by 80 K were observed in mixtures diluted with only N2 . The microwave energy deposition to excited states begins to thermalize over scales of 100 μs, however, equilibrium is not reached before excited combustion products convect out of the probe volume on the order of several 1 ms. Understanding the impact of varying bath gases on microwave interaction, magnitude of temperature rise and thermalization timescales is critical for the development and validation of new kinetic models for applications exhibiting significant degrees of thermal non-equilibrium, such as high-speed reentry flows and plasma-assisted combustion.