Daniel, Kyle; Willhardt, Colton; Glumac, Nick; Chen, Damon; Guildenbecher, Daniel
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.
This work presents measurements of liquid drop deformation and breakup time behind approximately conical shock waves and evaluates the predictive capabilities of low-order models and correlations developed using planar shock experiments. A conical shock was approximated by firing a bullet at Mach 4.5 past a vertical column of water drops with a mean initial diameter of 192 µm. The time-resolved drop position and maximum transverse dimension were characterized using backlit stereo images taken at 500 kHz. The gas density and velocity fields experienced by the drops were estimated using a Reynolds-averaged Navier-Stokes simulation of the bullet. Classical correlations predict drop breakup times and deformation in error by a factor of 3 or more. The Taylor analogy breakup (TAB) model predicts deformed drop diameters that agree within the confidence bounds of the ensemble-averaged experimental values using a dimensionless constant C2 = 2 compared to the accepted value C2 = 2/3. Results demonstrate existing correlations are inadequate for predicting the drop response to the three-dimensional relaxation of the flowfield downstream of a conical-like shock and suggest the TAB model results represent a path toward improved predictions.
We report 100-kHz burst-mode planar laser-induced fluorescence imaging of the nitricoxide molecule in the free-piston facility at Sandia National Laboratories. Cylinder wakestartup transients are visualized in high-temperature (T ~ 3000 K) post-shock flow with the facility in the shock-tube mode of operation. In the reflected shock-tunnel mode, NO PLIF visualization of a 4.6-MJ/kg, 3 km/s startup flow over a cylinder is presented, and free-stream molecular-tagging velocimetry exploiting the long fluorescence lifetime of free-stream NO is demonstrated.
Accurately measuring aero-optical properties of non-equilibrium gases is critical for characterizing compressible flow dynamics and plasmas. At thermochemical non-equilibrium conditions, excited molecules begin to dissociate, causing optical distortion and non-constant Gladstone-Dale behavior. These regions typically occur behind a strong shock at high temperatures and pressures. Currently, no experimental data exists in the literature due to the small number of facilities capable of reaching such conditions and a lack of diagnostic techniques that can measure index of refraction across large, nearly-discrete gradients. In this work, a quadrature fringe imaging interferometer is applied at the Sandia free-piston high temperature shock tube for high temperature and pressure Gladstone-Dale measurements. This diagnostic resolves high-gradient density changes using a narrowband analog quadrature and broadband reference fringes. Initial simulations for target conditions show large deviations from constant Gladstone-Dale coefficient models and good matches with high temperature and pressure Gladstone-Dale models above 5000 K. Experimental results at 7653 K and 7.87 bar indicate that the index of refraction approaches high temperature and pressure theory, but significant flow bifurcation effects are noted in reflected shock.
Here we examine models for particle curtain dispersion using drag based formalisms and their connection to streamwise pressure difference closures. Focusing on drag models, we specifically demonstrate that scaling arguments developed in DeMauro et. al. [1] using early time drag modeling can be extended to include late time particle curtain dispersion behavior by weighting the dynamic portion of the drag relative velocity e.g. (Formula Presented) by the inverse of the particle volume fraction to the ¼th power. The additional parameter e.g. α introduced in this scaling is related to the model drag parameters by employing an early-time latetime matching argument. Comparison with the scaled measurements of DeMauro et. al. suggest that the proposed modification is an effective formalism. Next, the connection between drag-based models and streamwise pressure difference-based expressions is explored by formulating simple analytical models that verify an empirical (Daniel and Wagner [2]) upstream-downstream expression. Though simple, these models provide physics-based approached describing shock particle curtain interaction behavior.
High-enthalpy hypersonic flight represents an application space of significant concern within the current national-security landscape. The hypersonic environment is characterized by high-speed compressible fluid mechanics and complex reacting flow physics, which may present both thermal and chemical nonequilibrium effects. We report on the results of a three-year LDRD effort, funded by the Engineering Sciences Research Foundation (ESRF) investment area, which has been focused on the development and deployment of new high-speed thermochemical diagnostics capabilities for measurements in the high-enthalpy hypersonic environment posed by Sandia's free-piston shock tunnel. The project has additionally sponsored model development efforts, which have added thermal nonequilibrium modeling capabilities to Sandia codes for subsequent design of many of our shock-tunnel experiments. We have cultivated high-speed, chemically specific, laser-diagnostic approaches that are uniquely co-located with Sandia's high-enthalpy hypersonic test facilities. These tools include picosecond and nanosecond coherent anti-Stokes Raman scattering at 100-kHz rates for time-resolved thermometry, including thermal nonequilibrium conditions, and 100-kHz planar laser-induced fluorescence of nitric oxide for chemically specific imaging and velocimetry. Key results from this LDRD project have been documented in a number of journal submissions and conference proceedings, which are cited here. The body of this report is, therefore, concise and summarizes the key results of the project. The reader is directed toward these reference materials and appendices for more detailed discussions of the project results and findings.
A new reflected shock tunnel has been commissioned at Sandia capable of generating hypersonic environments at realistic flight enthalpies. The tunnel uses an existing free-piston driver and shock tube coupled to a conical nozzle to accelerate the flow to approximately Mach 9. The facility design process is outlined and compared to other ground test facilities. A representative flight enthalpy condition is designed using an in-house state-to-state solver and piston dynamics model and evaluated using quasi-1D modeling with the University of Queensland L1d code. This condition is demonstrated using canonical models and a calibration rake. A 25 cm core flow with 4.6 MJ/kg total enthalpy is achieved over an approximately 1 millisecond test time. Analysis shows that increasing piston mass should extend test time by a factor of 2-3.
Coherent anti-Stokes Raman scattering of the N2 molecule is performed at rates up to 100 kHz for thermometry in the Sandia free-piston, high-temperature shock-tube facility (HST) for reflected-shock conditions in excess of T = 4000 K at pressures up to P = 10 atm. A pulse-burst laser architecture delivers picosecond-duration pulses to provide both the CARS pump and probe photons, and to pump a solid-state optical parametric generator (OPG)/optical parametric amplifier (OPA) source, which provides frequency tunable Stokes pulses with a bandwidth of 100-120 cm-1 . Single-laser-shot and averaged CARS spectra obtained in both the incident (P = 1.1 atm, T = 2090 K) and reflected (P ~ 8-10.5 atm, T > 4000 K) shock regions of HST are presented. The results indicate that burst-mode CARS is capable of resolving impulsive, high-temperature events in HST.
This work presents a high-speed laser-absorption-spectroscopy diagnostic capable of measuring temperature, pressure, and nitric oxide (NO) mole fraction in shock-heated air at a measurement rate of 500 kHz. This diagnostic was demonstrated in the High-Temperature Shock Tube (HST) facility at Sandia National Laboratories. The diagnostic utilizes a quantum-cascade laser to measure the absorbance spectra of two rovibrational transitions near 5.06 µm in the fundamental vibration bands (v" = 0 and 1) of NO in its ground electronic state (X2 Π1/2 ). Gas properties were determined using scanned-wavelength direct absorption and a recently established fitting method that utilizes a modified form of the time-domain molecular free-induction-decay signal (m-FID). This diagnostic was applied to acquire measurements in shock-heated air in the HST at temperatures ranging from approximately 2500 to 5500 K and pressures of 3 to 12 atm behind both incident and reflected shocks. The measurements agree well with the temperature predicted by NASA CEA and the pressure measured simultaneously using PCB pressure sensors. The measurements presented demonstrate that this diagnostic is capable of resolving the formation of NO in shock-heated air and the associated temperature change at the conditions studied.
This work presents an experimental investigation of the deformation and breakup of water drops behind conical shock waves. A conical shock is generated by firing a bullet at Mach 4.5 past a vertical column of drops with a mean initial diameter of 192 µm. The time-resolved drop position and maximum transverse dimension are characterized using backlit stereo videos taken at 500 kHz. A Reynolds-Averaged Navier Stokes (RANS) simulation of the bullet is used to estimate the gas density and velocity fields experienced by the drops. Classical correlations for breakup times derived from planar-shock/drop interactions are evaluated. Predicted drop breakup times are found to be in error by a factor of three or more, indicating that existing correlations are inadequate for predicting the response to the three-dimensional relaxation of the velocity and thermodynamic properties downstream of the conical shock. Next, the Taylor Analogy Breakup (TAB) model, which solves a transient equation for drop deformation, is evaluated. TAB predictions for drop diameter calculated using a dimensionless constant of C2 = 2, as compared to the accepted value of C2 = 2/3, are found to agree within the confidence bounds of the ensemble averaged experimental values for all drops studied. These results suggest the three-dimensional relaxation effects behind conical shock waves alter the drop response in comparison to a step change across a planar shock, and that future models describing the interaction between a drop and a non-planar shock wave should account for flow field variations.
A high-speed temperature diagnostic based on spontaneous Raman scattering (SRS) was demonstrated using a pulse-burst laser. The technique was first benchmarked in near-adiabatic H2-air flames at a data-acquisition rate of 5 kHz using an integrated pulse energy of 1.0 J per realization. Both the measurement precision and accuracy in the flame were within 3% of adiabatic predictions. This technique was then evaluated in a challenging free-piston shock tube environment operated at a shock Mach number of 3.5. SRS thermometry resolved the temperature in post-incident and post-reflected shock flows at a repetition rate of 3 kHz and clearly showed cooling associated with driver expansion waves. Collectively, this Letter represents a major advancement for SRS in impulsive facilities, which had previously been limited to steady state regions or single-shot acquisition.
Measurements of bifurcated reflected shocks over a wide range of incident shock Mach numbers, 2.9 < Ms < 9.4, are carried out in Sandia’s high temperature shock tube. The size of the non-uniform flow region associated with the bifurcation is measured using high speed schlieren imaging. Measurements of the bifurcation height are compared to historical data from the literature. A correlation for the bifurcation height from Petersen et al. [1] is examined and found to over estimate the bifurcation height for Ms > 6. An improved correlation is introduced that can predict the bifurcation height over the range 2.15 < Ms < 9.4. The time required for the non-uniform flow region to pass over a stationary sensor is also examined. A non-dimensional time related to the induced velocity behind the shock and the distance to the endwall is introduced. This non-dimensional time collapses the data and yields a new correlation that predicts the temporal duration of the bifurcation.