A direct optical absorption diagnostic has been developed for rotationally resolved measurements of diatomic carbon (a3Πu→d3Πg (0,0) and (1,1)) at a repetition rate of 525 kHz and resolution of 7.3 pm over the range of 511.1–514.1 nm. The diagnostic utilizes a high-power pulsed continuum laser light source dispersed in a spectrograph and imaged by a high-speed camera. Measurements have been performed that capture the temporal evolution of C2 number density and temperature of a cloud of carbon black sublimating in shock heated gas at high temperature and pressure (T = 5500 K, P = 3.7 atm). A simultaneous light absorption measurement of the condensed phase enables comparisons of the gaseous C2 number density to the condensed phase mass. The continuum laser absorption diagnostic is applied to C2 in this work but shows promise in being a simple “drop-in” system for absorption spectroscopy in the visible range at rapid repetition rates and high dispersion, filling an important gap for laser diagnostic systems.

This manuscript presents a quantum-cascade-laser-absorption-spectroscopy (QCLAS) diagnostic for the partial pressure and internal temperatures (rotational and vibrational) of nitric oxide (NO) in hypersonic flows. Two quantum-cascade lasers (QCLs) were used to measure four transitions of NO near 1887 cm ^-1 and 1930 cm ^-1 at 25 or 100 kHz using scanned-wavelength direct absorption. Tests were performed in the Purdue High-Pressure Shock Tube (HPST) using an NO–Ar mixture to confirm the accuracy of the diagnostic. The diagnostic was then applied to characterize the Hypersonic Shock Tunnel (HST) at Sandia National Laboratories. In the HST, two flow cutters were used to direct the measurement line-of-sight through the quasi-uniform core flow exiting the nozzle, thereby avoiding measurement complications associated with the thick boundary layers at the nozzle exit. In the HST, tests were performed with air velocities of 3, 4, and 5 km/s where the rotational and vibrational temperature of NO varied from 150 to 850 K and the partial pressure of NO was near 20 Pa. Additionally, dry bottled air and humid room air were used as test gases to quantify the impact of water contamination on the vibrational non-equilibrium of NO. Comparisons with two CFD predictions using unique rate constants for vibrational relaxation are also presented. The vibrational non-equilibrium of NO was more pronounced for 3 km/s tests, and water had a negligible impact on the thermal non-equilibrium of NO. Lastly, the measured rotational temperature of NO agreed well with CFD predictions, the measured partial pressure of NO was consistently above CFD predictions, and the vibrational temperature had moderate agreement with CFD predictions for 4 and 5 km/s tests, and poor agreement for 3 km/s tests.

Surface mass loss rates due to sublimation and oxidation at temperatures of 3000–7000 K have been measured in a shock tube for graphite and carbon black (CB) particles. Diagnostics are presented for measuring surface mass loss rates by diffuse backlit illumination extinction imaging and thermal emission. The surface mass loss rate is found by regression fitting extinction and emission signals with an independent spherical primary particle assumption. Measured graphite sublimation and oxidation rates are reported to be an order of magnitude greater than CB sublimation and oxidation rates. It is speculated that the difference between CB and graphite surface mass loss rates is largely due to the primary particle assumption of the presented technique which misrepresents the effective surface area of an aggregate particle where primary particles overlap and shield inner particles. Measured sublimation rates are compared to sublimation models in the literature, and it is seen graphite shows fair agreement with the models while CB underestimates, likely a result of the particle shielding affect not being considered in the sublimation model.

A micro-electromechanical system, tunable vertical cavity surface emitting laser (MEMS-VCSEL) scanning 7350-7900 cm-1 at 100 kHz repetition rate was employed in the Multiphase Shock Tube at Sandia National Laboratories to optically measure pressure via spectroscopic line broadening. Water vapor absorption transitions in the near-infrared were targeted to simultaneously measure temperature and pressure of shock-heated air. These elevated temperatures and pressures are representative of post-detonation conditions. Results showed fitted temperatures matched values calculated using ideal shock relations and fitted pressures agreed closely with pressure transducer measurements. Future work involves measurements in CO2 to better represent a post detonation gas mixture.

This study presents measurements on the combustion of nominally 20 micron diameter particles burning in air at elevated temperature (3600 +/- 150K) and pressures (8 – 63 atm) in Sandia’s High-Temperature Shock Tube (HST) facility. Measurements were made using a suite of advanced diagnostics that coupled laser absorption spectroscopy (LAS) measurements of the temperature and column density of aluminum monoxide (AlO) with either digital in-line holography (DIH) or nitric oxide (NO) laser absorption. DIH measurements were made within the AlO LAS laser probe volume and provided a measure of the diameter and number of particles. This enabled a comparison of the particle density in the AlO measurement volume between cases. In a separate case, the NO laser absorption probe was overlapped with the AlO laser absorption diagnostic, enabling simultaneous measures of the bath gas and AlO temperatures. Results show that the energy addition by a combusting particle field with densities less than ~0.1 particle/mm^3 will not increase the bath temperature. Further, measurements indicate that AlO production is decreased at elevated pressure conditions. This suggests a shift to heterogeneous surface combustion or a large increase in the ignition delay with increasing pressure.

Understanding the optical properties of air is essential for the validation and characterization of plasmas and hypersonic flows. Beyond 6000 K, the dissociation of nitrogen and oxygen molecules, along with other reactions, alters the equilibrium composition of air, causing a temperature and pressure dependence in the Gladstone–Dale coefficient. Due to measurement complexities, there is currently very little experimental data to validate model predictions under these conditions. In this work, a unique quadrature fringe imaging interferometer technique is applied to high temperature and pressure measurements of air in the Sandia free-piston high enthalpy shock tube. The diagnostic method combines a narrowband and broadband source to capture large, nearly-discrete changes in the index of refraction by calibrating to interference pattern changes. For the experiments, the reflected shock front is used to generate temperatures between 6000 and 7800 K at pressures up to 300 psi (20 bars). Results behind the shock front exhibit complex flow bifurcation and tail shock feature before equilibrium conditions are reached. Measurements in these flows show close agreement with theoretical predictions of the nonconstant Gladstone–Dale coefficient at high temperatures and high pressures, providing new validation data for chemical equilibrium gas models.

Shock-particle interactions at elevated Mach numbers are studied using the Sandia free-piston High-Temperature Shock Tube (HST). Like previous studies in a lower-strength facility, the particle curtain was comprised of 100-micron glass at an initial volume fraction of approximately 20%. Shock-particle interactions were investigated using high-speed imaging, where the incident shock Mach number ranged from 3 – 4.2. The corresponding post-shock velocities were 760 – 1170 m/s, nearly doubling the range available in previous experiments. The spread of the particle was curtain compared to that in the literature. The scaling law of DeMauro et al. (2019) effectively collapsed the curtain spread over experiments ranging approximately one order of magnitude of post-shock velocities.

Spatially-resolved optical emission spectra of nitric oxide are measured in a hypervelocity flow along the stagnation streamline of a circular cylinder at 4.5 and 4.9 km/s. Multi-temperature spectral fitting of NO is performed to determine the degree of non-equilibrium present using two different spectral modeling codes for multiple points between the shock front and cylinder surface to visualize spatial trends. The contribution of the NO beta, epsilon, and delta electronic bands as well as the affect of non-Boltzmann distribution of the excited electronic states is assessed in the spectral fits. A comparison is shown to results from a multi-vibrational temperature computational fluid dynamics model.

A broadband coherent anti-Stokes Raman scattering (CARS) system using a burst-mode laser and a burst-mode-pumped noncolinear optical parametric oscillator (NOPO) has been constructed for single-shot N2 thermometry measurements at 50 kHz in Sandia’s Hypersonic Shock Tunnel (HST). Simultaneous vibrational and rotational temperature measurements of N2 have been made in the freestream flow throughout the startup transient and steady run time of the shock tunnel at total enthalpies of 4.6 and 9.0 MJ/kg. The CARS spectra indicate nonequilibrium exists between the vibrational and rotational modes of N2 due to the rapid expansion of the test gas through the nozzle. Inferred rotational and vibrational temperatures were compared to modeling results from the Sandia Parallel Aerodynamics Reentry Code (SPARC), which was shown to overpredict nonequilibrium in the HST freestream.

The design, validation, and application of a quantum-cascade-laser-absorption-spectroscopy diagnostic for measuring gas temperature, pressure, and nitric oxide (NO) in high-temperature air are presented. A distributed-feedback quantum-cascade laser (QCL) centered near 1976c m -1 was used to scan across two transitions of NO in its ground electronic state (X 2 Π 1/2). A measurement rate of 500 kHz was achieved using a single QCL by: (1) performing current modulation through a bias-tee, and (2) targeting closely spaced transitions with a large difference in lower-state energy. The diagnostic was validated in a mixture of 95% argon and 5% NO, which was shock-heated to ≈2000 to 3700 K. The average mean percent differences between laser-absorption-spectroscopy (LAS) measurements and predictions from shock-jump relations for temperature, pressure, and NO mole fraction were 3.1%, 4.1%, and 6.5%, respectively. The diagnostic was then applied to characterize shock-heated air at high temperatures (up to ≈5500K) and high pressures (up to 12 atm) behind either incident or reflected shocks. The LAS measurements were compared to theoretical predictions from shock-jump relations, pressure sensors mounted in the wall of the shock tube, and equilibrium values of the NO mole fraction. The average mean percent differences between LAS measurements and their aforementioned reference values were 3.2%, 10.8%, and 10.4% for temperature, pressure, and NO mole fraction, respectively. Last, a comparison between a measured NO mole fraction time history and a time-stepped homogeneous reactor simulation performed using two different chemical kinetics mechanisms is presented.