Publications

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Measurements of gas-phase temperature and pressure in hypersonic flows are important for understanding gas-phase fluctuations which can drive dynamic loading on model surfaces and to study fundamental compressible flow turbulence. To achieve this capability, femtosecond coherent anti-Stokes Raman scattering (fs CARS) is applied in Sandia National Laboratories’ cold-flow hypersonic wind tunnel facility. Measurements were performed for tunnel freestream temperatures of 42–58 K and pressures of 1.5–2.2 Torr. The CARS measurement volume was translated in the flow direction during a 30-second tunnel run using a single computer-controlled translation stage. After broadband femtosecond laser excitation, the rotational Raman coherence was probed twice, once at an early time where the collisional environment has not affected the Raman coherence, and another at a later time after the collisional environment has led to significant dephasing of the Raman coherent. The gas-phase temperature was obtained primarily from the early-probe CARS spectra, while the gas-phase pressure was obtained primarily from the late-probe CARS spectra. Challenges in implementing fs CARS in this facility such as changes in the nonresonant spectrum at different measurement location are discussed.

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We report pure-rotational N2-N2, N2-air, and O2-air S-branch linewidths for temperatures of 80-200 K by measuring the time-dependent decay of rotational Raman coherences in an isentropic free-jet expansion from a sonic nozzle. We recorded pure-rotational hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS) spectra along the axial centerline of the underexpanded jet, within the barrel shock region upstream of the Mach disk. The dephasing of the pure-rotational Raman coherence was monitored using probe-time-delay scans at different axial positions in the jet, corresponding to varying local temperatures and pressures. The local temperature was obtained by fitting CARS spectra acquired at zero probe time delay, where the impact of collisions was minimal. The measured decay of each available Raman transition was fit to a dephasing constant and corrected for the local pressure, which was obtained from the CARS-measured static temperature and thermodynamic relationships for isentropic expansion from the known stagnation state. Nitrogen self-broadened transitions decayed more rapidly than those broadened in air for all temperatures, corresponding to higher Raman linewidths. In general, the measured S-branch linewidths deviated significantly in absolute and relative magnitudes from those predicted by extrapolating the modified exponential gap model to low temperatures. The temperature dependence of the Raman linewidth for each measured rotational state of nitrogen (J ≤ 10) and oxygen (N ≤ 11) was fit to a temperature-dependent power law over the measurable temperature domain (80-200 K) and extrapolated to both higher rotational states and room temperature. The measured and modeled low-temperature linewidth data provided here will aid low temperature gas-phase pressure measurements with fs/ps CARS.

A hybrid femtosecond/picosecond CARS instrument probed the Q-branch of molecular hydrogen in the multiphase plume of an aluminized solid propellant burn. A single 50 fs regenerative amplifier pumped an OPA and etalon, providing the Stokes and probe pulses respectively. The spectra were recorded at 1 kHz and fit to synthetic spectra to infer the gas rotational temperature. Recorded spectra required dynamic background corrections due to the intense emission of the propellant plume. Two different days of propellant burns were studied, with the lessons learned from nonresonant background issues with the first test applied to the second. For the second attempt, three burns were examined, with mean temperatures differing only by 30 K with a combined mean of 2574 K.

We demonstrate simultaneous monitoring of temperature and pressure using a hybrid femtosecond/picosecond pure-rotational CARS technique in a one-dimensional line-imaging configuration. The method employs two detection channels and two 60-ps-duration probe laser beams with independently adjustable time delays from the broadband 35-fs pump/Stokes pulse. Simultaneous temperature and pressure monitoring is demonstrated along the centerline of a canonical underexpanded compressible air jet flow emanating from a choked, sonic nozzle. Temperature is measured almost independently of pressure by analyzing CARS spectra obtained with a probe pulse near zero time delay for nearly collision-free acquisition. Pressure is obtained from spectra acquired with long probe time delays to sample the impact of gas-phase collisions. The CARS measurements were obtained in both time-averaged and single-laser-shot mode with 67 µm spatial resolution along the jet axis along a nominally 6-mm line. The measurements span a temperature and pressure range of T = 70-300 K and P = 0.05-1.2 atm.

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We have investigated the utility of femtosecond/picosecond (fs/ps) coherent anti-Stokes Raman scattering(CARS)for simultaneous measurement of temperature, pressure, and velocity in hypersonic flows. Experiments were conducted in underexpanded jets of air and molecular nitrogen to assess CARS diagnostic performance in terms of signal level scaling, measurement precision, and dynamic range. Pure-rotational CARS of the Raman S branch was applied for simultaneous measurement of temperature and pressure. Thermometry was performed by fitting CARS spectra acquired under nearly collision-free conditions by introducing a picosecond CARS probe pulse at zero delay from the femtosecond pump. Pressure could be subsequently obtained by from a second CARS spectral acquisition with a picosecond probe introduced at time delay to sample molecular collisions. CARS velocimetry was attempted by monitoring the Doppler shift of the N₂ vibrational, Q-branch spectrum, with both direct spectral resolution and optical heterodyne detection schemes. Doppler shifts from the sub-I-km/s air jet flow proved too small to measure with this approach, prompting us to turn to femtosecond laser electronic excitation tagging (FLEET) for reliable single-laser-shot velocimetry and CARS temperature/pressure measurement. Scaling of the CARS signal level to very low pressure and temperature conditions expected in the Sandia hypersonic wind tunnel (HWI) was performed. CARS measurements of temperature in HWT appear to be very feasible, while prospects for HWT pressure measurements are reasonable.

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One-dimensional hybrid fs/ps CARS imaging provides single-laser-shot measurements of temperature, oxygen, and hydrogen in a plasma-assisted hydrogen diffusion flame. The coaxial dielectric-barrier-discharge burner collapses the Re ∼50 hydrogen diffusion flame to within ∼5 mm of the burner surface at an applied AC potential of 8.75 kV at 18 kHz, coinciding nicely with the full spatial extent of the 1D CARS measurements. Translating the burner through the measurement volume allowed for measurements at numerous radial locations in increments of 1 mm with a resolution of 140 µm × 30 µm × 600 µm, sufficient to resolve spatial gradients in this unsteady flame. Longer probe delays, required for improved dynamic range in regions of high temperature fluctuations, proved difficult to model as a result of a nontrivial decay in the O2 Raman coherence arising from complexities associated with the triplet ground electronic state of the O2 molecule. Oxygen linewidths were treated empirically using the observed O2 coherence decay in spectra acquired from the product gases of lean, near-adiabatic H2/air flames stabilized on a Hencken flat-flame burner. While still leading to errors up to 10% at worst, the empirically determined Raman linewidth factors eliminated any systematic error in the O2/N2 measurements with probe delay. Temperature measurements in the Hencken Burner flames proved to be insensitive to probe pulse delay, providing robust thermometry. Demonstration of this technique in both the canonical Hencken burner flames and a new DBD burner validates its effectiveness in producing multiple spatially resolved measurements in combustion environments. Measurements in the DBD burner revealed an unsteady, counterflow flattened flame structure near the fuel orifice which became unsteady as the reaction zone curves towards the surface for larger radial positions. Fluctuations in the fuel concentration were largest at the source, as the large, plasma-generated, unsteady external toroidal vortex that dominates the transport in this flame provides enhanced ventilation of the flame surface in close proximity to the fuel tube.

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