We present the results of an LDRD project, funded by the Nuclear Deterrence IA, to develop capabilities for quantitative assessment of pyrotechnic thermal output. The thermal battery igniter is used as an exemplar system. Experimental methodologies for thermal output evaluation are demonstrated here, which can help designers and engineers better specify pyrotechnic components , provide thermal output guidelines for new formulations, and generate new metrics for assessing component performance and margin given a known failure condition. A heat-transfer analysis confirms that the dominant mode of energy transfer from the pyrotechnic output plume to the heat pellet is conduction via deposition of hot titanium particles. A simple lumped-parameter model of titanium particle heat transfer and a detailed multi-phase model of deposition heat transfer are discussed. Pyrotechnic function, as defined by "go/no-go" standoff testing of a heat pellet, is correlated with experimentally measured igniter plume temperature, titanium metal particle temperature, and energy deposition. Three high-speed thermal diagnostics were developed for this task. A three-color imaging pyrometer, acquiring 100k images per second on three color channels, is deployed for measurement of titanium particle temperatures. Complimentary measurements of the overall igniter plume emission ("color") temperature were conducted using a transmission-grating spectrograph in line-imaging mode. Heat flux and energy deposition to a cold wall at the heat-pellet location were estimated using an eroding thermocouple probe, with a frequency response of ~5 kHz. Ultimate "go/no-go" function in the igniter/heat-pellet system was correlated with quantitative thermal metrics, in particular surface energy deposition and plume color temperature. Titanium metal-particle and plume color temperatures both experience an upper bound approximated by the 3245-K boiling point of TiO2. Average metal-particle temperatures remained nearly constant for all standoff distances at T = 2850 K, ± 300 K, while plume color temperature and heat flux decay with standoff—suggesting that heat-pellet failure results from a drop in metal-particle flux and not particle temperature. At 50% likelihood of heat-pellet failure, peak time-resolved plume color temperatures drop well below TiO2 boiling to ~2000 - 2200 K, near the TiO2 melting point. Estimates of peak heat flux decline from up to 1 GW/m2 for near-field standoffs to below 320 MW/m2 at 50% failure likelihood.
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.
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.
Demonstration of broadband nanosecond coherent anti-Stokes Raman scattering (CARS) using a burst-mode-pumped noncolinear optical parametric oscillator (NOPO) has been achieved at a pulse repetition rate of 40 kHz. The NOPO is pumped with the 355-nm output of a burst-mode Nd:YAG laser at 50 mJ/pulse for 45 pulses and produces an output centered near 607 nm, with a bandwidth of 370 cm −1 at energies of 5 mJ/pulse. A planar BOXCARS phase matching scheme uses the broadband NOPO output as the Stokes beam and the narrowband 532-nm burst-mode output for the two CARS pump beams for single-laser-shot nitrogen thermometry in near adiabatic H 2 /air flames at temperatures up to 2200 K.
Demonstration of broadband nanosecond output from a burst-mode-pumped noncolinear optical parametric oscillator (NOPO) has been achieved at 40 kHz. The NOPO is pumped by 355-nm output at 50 mJ/pulse for 45 pulses. A bandwidth of 540 cm-1 was achieved from the OPO with a conversion efficiency of 10% for 5 mJ/pulse. Higher bandwidths up to 750 cm-1 were readily achievable at reduced performance and beam quality. The broadband NOPO output was used for a planar BOXCARS phase matching scheme for N2 CARS measurements in a near adiabatic H2/air flame. Single-shot CARS measurements were taken for equivalence ratios of φ=0.52-0.86 for temperatures up to 2200 K.
A high-speed, two-color pyrometer was developed and employed to characterize the temperature of the ejecta from pyrotechnic igniters. The pyrometer used a single objective lens, beamsplitter, and two high-speed cameras to maximize the spatial and temporal resolutions. The pyrometer used the integrated intensity of under-resolved particles to maintain a large region of interest to capture more particles. The spectral response of the pyrometer was determined based on the response of each optical component and the total system was calibrated using a black body source to ensure accurate intensity ratios over the range of interest.
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.
A high-speed thermometry diagnostic based on spontaneous Raman scattering (SRS) was demonstrated using a pulse-burst laser at a 3-kHz data acquisition rate, with a pulse duration of 200 ns and wavelength of 532 nm. The technique was evaluated in a challenging free-piston shock tube environment operated at conditions up to 1653 K and 112 bar following an incident shock Mach number of 3.5 and a reflected shock Mach number of 2.2. The SRS thermometry resolved the temperature in post-incident and post-reflected shock flows and clearly showed cooling associated with driver expansion waves. A detailed spectral physics model inferred temperatures within 1% of the predicted post-shock temperatures, when SNR was greater than 2.0. This was a significant advancement of spontaneous Raman vibrational thermometry.
Accurate knowledge of post-detonation fireball temperatures is important for understanding device performance and for validation of numerical models. Such measurements are difficult to make even under controlled laboratory conditions. In this work temperature measurements were performed in the fireball of a commercial detonator (RP-80, Teledyne RISI). The explosion and fragments were contained in a plastic enclosure with glass windows for optical access. A hybrid femtosecond-picosecond (fs-ps) rotational coherent anti-Stokes Raman scattering (CARS) instrument was used to perform gas-phase thermometry along a one-dimensional measurement volume in a single laser shot. The 13-mm-thick windows on the explosive-containment housing introduced significant nonlinear chirp on the fs lasers pulses, which reduced the Raman excitation bandwidth and did not allow for efficient excitation of high-J Raman transitions populated at flame temperatures. To overcome this, distinct pump and Stokes pulses were used in conjunction with spectral focusing, achieved by varying the relative timing between the pump and Stokes pulses to preferentially excite Raman transitions relevant to flame thermometry. Light scattering from particulate matter and solid fragments was a significant challenge and was mitigated using a new polarization scheme to isolate the CARS signal. Fireball temperatures were measured 35-40 mm above the detonator, 12-25 mm radially outward from the detonator centerline, and at 18 and 28 μs after initiation. At these locations and times, significant mixing between the detonation products and ambient air had occurred thus increasing the nitrogen-based CARS thermometry signal. Initial measurements show a distribution of fireball temperatures in the range 300-2000 K with higher temperatures occurring 28 μs after detonation.
Three-beam rotational coherent anti-Stokes Raman scattering (CARS) measurements performed in highly scattering environments are susceptible to contamination by two-beam CARS signals generated by the pump–probe and Stokes–probe interactions at the measurement volume. If this occurs, differences in the Raman excitation bandwidth between the two-beam and three-beam CARS signals can add significant errors to the spectral analysis. This interference, to the best of our knowledge, has not been acknowledged in previous three-beam rotational CARS experiments, but may introduce measurement errors up to 25% depending on the temperature, amount of scattering, and differences between the two-beam and three-beam Raman excitation bandwidths. In this work, the presence of two-beam CARS signal contamination was experimentally verified using a femtosecond–picosecond rotational CARS instrument in two scattering environments: (1) a fireball generated by a laboratory-scale explosion that contained particulate matter, metal fragments, and soot, and (2) a flow of air and small liquid droplets. A polarization scheme is presented to overcome this interference. By rotating the pump and Stokes polarizations +55◦ and −55◦ from the probe, respectively, the two-beam and three-beam CARS signals are orthogonally polarized and can be separated using a polarization analyzer. Using this polarization arrangement, the Raman-resonant three-beam CARS signal amplitude is reduced by a factor of 2.3 compared to the case where all polarizations are parallel. This method is successfully demonstrated in both scattering environments. A theoretical model is presented, and the temperature measurement error is studied for different experimental conditions. The criteria for when this interference may be present are discussed.
Accurate knowledge of post-detonation fireball temperatures is important for understanding device performance and for validation of numerical models. Such measurements are difficult to make even under controlled laboratory conditions. Here, temperature measurements were performed in the fireball of a commercial detonator (RP-80, Teledyne RISI). The explosion and fragments were contained in a plastic enclosure with glass windows for optical access. A hybrid femtosecond-picosecond (fs-ps) rotational coherent anti-Stokes Raman scattering (CARS) instrument was used to perform gas-phase thermometry along a one-dimensional measurement volume in a single laser shot. The 13-mm-thick windows on the explosive-containment housing introduced significant nonlinear chirp on the fs lasers pulses, which reduced the Raman excitation bandwidth and did not allow for efficient excitation of high-J Raman transitions populated at flame temperatures. To overcome this, distinct pump and Stokes pulses were used in conjunction with spectral focusing, achieved by varying the relative timing between the pump and Stokes pulses to preferentially excite Raman transitions relevant to flame thermometry. Light scattering from particulate matter and solid fragments was a significant challenge and was mitigated using a new polarization scheme to isolate the CARS signal. Fireball temperatures were measured 35–40 mm above the detonator, 12–25 mm radially outward from the detonator centerline, and at 18 and 28 µs after initiation. At these locations and times, significant mixing between the detonation products and ambient air had occurred thus increasing the nitrogen-based CARS thermometry signal. Initial measurements show a distribution of fireball temperatures in the range 300–2000 K with higher temperatures occurring 28 µs after detonation.
A high-speed Raman thermometry diagnostic was evaluated in lean H2-air flames at a data acquisition rate of 5 kHz. Bursts of nanosecond pulses were generated at a 10 kHz burst rate with energy of E ≈ 13 J/burst at λ = 532 nm. The pulses had a duration of ≈ 200 ns and were used to interrogate a stabilized flat flame burner. Spectra were collected using an electron multiplying charge-coupled device (EMCCD) detector. Raman spectra were integrated over the full burst to map adiabatic flame temperature versus equivalence ratio. The measured spectra resolved vibrational band features to infer temperature. A detailed spectral fitting model was used in the burst-integrated and burst-mode spectra. Two pulses were used for each burst-mode measurement resulting in a 5 kHz rate up to flame temperatures of ≈ 2100 K. The measurement precision in burst mode was 23 K and 62 K at flame temperatures of 1160 K and 2080 K, respectively. The measurement accuracy was benchmarked against the spectrally fitted full-burst spectra, chemical equilibrium calculations and previous coherent anti-Stokes Raman scattering (CARS) measurements. In summary, the measurement precision and accuracy were within 3% of the measured and adiabatic equilibrium temperatures, respectively.
The temperature inside fireballs produced by detonations is an important quantity of interest for the validation of models. However, such measurements are very difficult to make due to the large pressure and temperature gradients and the harsh environment. In this abstract we will report on one-dimensional rotational coherent anti-Stokes Raman scattering (1D RCARS) measurements performed in such fireballs. CARS measurements were performed at 18 and 28 µs after detonation of a commercial detonator, and the measured temperatures are in the range 300–1600 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.
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.
Knowledge of soot particle sizes is important for understanding soot formation and heat transfer in combustion environments. Soot primary particle sizes can be estimated by measuring the decay of time-resolved laser-induced incandescence (TiRe-LII) signals. Existing methods for making planar TiRe-LII measurements require either multiple cameras or time-gate sweeping with multiple laser pulses, making these techniques difficult to apply in turbulent or unsteady combustion environments. Here, we report a technique for planar soot particle sizing using a single high-sensitivity, ultra-high-speed 10 MHz camera with a 50 ns gate and no intensifier. With this method, we demonstrate measurements of background flame luminosity, prompt LII, and TiRe-LII decay signals for particle sizing in a single laser shot. The particle sizing technique is first validated in a laminar non-premixed ethylene flame. Then, the method is applied to measurements in a turbulent ethylene jet flame.
We present spatial profiles of temperature and soot-volume-fraction statistics from a sooting, 2-m base diameter turbulent pool fire, burning a 10%-toluene/90%-methanol fuel mixture. Dual-pump coherent anti-Stokes Raman scattering and laser-induced incandescence are utilized for simultaneous point measurements of temperature and soot. The research fuel-blend used here results in a lower soot loading than real transportation fuels, but allows us to apply high-fidelity laser diagnostics for spatially resolved measurements in a fully turbulent, buoyant fire of meter-scale base size. Profiles of mean and rms fluctuations are radially resolved across the fire plume, both within the hydrocarbon-rich vapor-dome region near fuel pool, and higher within the actively burning region of the fire. The spatial evolution of the soot and temperature probability density functions is discussed. Soot fluctuations display significant intermittency across the full extent of the fire plume for the research fuel blend used. Simultaneous, spatially overlapped temperature/soot measurements permit us to obtain estimates of joint statistics that are presented as spatially resolved conditional averages across the fire plume, and in terms of a joint pdf obtained by including measurements from multiple spatial locations. Within the actively burning region of the fire, soot is observed to occupy a limited temperature range between ∼1000 and 2000 K, with peak soot concentration occurring at 1600–1700 K across the full radial extent of the fire plume, despite marked changes in the local temperature pdf across the same spatial extent. A wider range of soot temperatures is observed in the fuel vapor-dome region low in the pool fire, with detectable cold soot persisting into conditionally averaged statistics. The results yield insight into soot temperature across a wide spatial extent of a fully turbulent pool fire of meaningful size, which are valuable for development of soot radiative-emission models and for validation of fire fluid-dynamics codes.
Single-laser-shot femtosecond rotational coherent anti-Stokes Raman scattering (fs-RCARS) temperature measurements are performed across a 3- mm line in a turbulent, sooting ethylene jet flame to characterize temperature gradients. A 60-fs pulse is used to excite many rotational Raman transitions, and a 160-ps pulse is used to probe the Raman coherence. The spatial resolution of the measurements is 670 μm in the direction of beam propagation, 200 μm in the direction along the 1D line, and 50 μm in the transverse direction. Measurements have been performed at multiple locations in the jet flame, and the measured temperature are similar to previously recorded point measurements.
AIAA SciTech Forum - 55th AIAA Aerospace Sciences Meeting
Retter, Jonathan E.; Elliott, Gregory S.; Kearney, S.P.
A two-beam, one-dimensional hybrid fs/ps rotational CARS scheme was applied to a coaxial dielectric barrier discharge burner to spatially resolve and simultaneously measure temperature, relative oxygen concentration, and relative hydrogen concentration. At higher applied voltages, the 1 L/min hydrogen burner produces a collapsed flame with a curved reaction zone to the surrounding quiescent air, extending roughly 5 mm above the burner surface, making this a perfect candidate for single-shot realizations of flame properties with a vertical line CARS imaging technique. Time-delayed probing of the impulsively created Raman coherence allowed for improved dynamic range in regions of high temperature gradients, but also introduced the reliance on collisional modeling. Temperature measurements proved robust with probe delay, but the higher detection limit of oxygen at longer delays encouraged the use of isolated oxygen line calibrations to Hencken burner data in place of collisional modeling. A spatial resolution of 140 μm in the axis normal to the burner surface was adequate for mapping out flame properties along the reaction zone.
Ultrafast pure-rotational CARS is applied to an aluminized ammonium-perchlorate propellant flame. Background-free spectra were acquired in this challenging high-temperature, particle-laden environment and successfully fit for temperature and oxygen/nitrogen ratio using a simple theoretical model.
We apply ultrafast pure-rotational coherent anti-Stokes Raman scattering (CARS) for temperature and relative oxygen concentration measurements in the plume emanating from a burning, aluminized ammonium-perchlorate propellant strand. Combustion of these metal-based propellants is a particularly hostile environment for laserbased diagnostics, with intense background luminosity and scattering from hot metal particles as large as several hundred micrometers in diameter. CARS spectra that were previously obtained using nanosecond pulsed lasers in an aluminum-particle-seeded flame are examined and are determined to be severely impacted by nonresonant background, presumably as a result of the plasma formed by particulate-enhanced laser-induced breakdown. Introduction of femtosecond/picosecond (fs/ps) laser pulses improves CARS detection by providing time-gated elimination of strong nonresonant background interference. Single-laser-shot fs/ps CARS spectra were acquired from the burning propellant plume, with picosecond probe-pulse delays of 0 and 16 ps from the femtosecond pump and Stokes pulses. At zero delay, nonresonant background overwhelms the Raman-resonant spectroscopic features. Time-delayed probing results in the acquisition of background-free spectra that were successfully fit for temperature and relative oxygen content. Temperature probability densities and temperature/oxygen correlations were constructed from ensembles of several thousand single-laser-shot measurements with the CARS measurement volume positioned within 3 mm or less of the burning propellant surface. The results show that ultrafast CARS is a potentially enabling technology for probing harsh, particle-laden flame environments.
We report the application of ultrafast rotational coherent anti-Stokes Raman scattering (CARS) for temperature and relative oxygen concentration measurements in the plume emanating from a burning aluminized ammonium perchlorate propellant strand. Combustion of these metal-based propellants is a particularly hostile environment for laserbased diagnostics, with intense background luminosity, scattering and beam obstruction from hot metal particles that can be as large as several hundred microns in diameter. CARS spectra that were previously obtained using nanosecond pulsed lasers in an aluminumparticle- seeded flame are examined and are determined to be severely impacted by nonresonant background, presumably as a result of the plasma formed by particulateenhanced laser-induced breakdown. Introduction of fs/ps laser pulses enables CARS detection at reduced pulse energies, decreasing the likelihood of breakdown, while simultaneously providing time-gated elimination of any nonresonant background interference. Temperature probability densities and temperature/oxygen correlations were constructed from ensembles of several thousand single-laser-shot measurements from the fs/ps rotational CARS measurement volume positioned within 3 mm or less of the burning propellant surface. Preliminary results in canonical flames are presented using a hybrid fs/ps vibrational CARS system to demonstrate our progress towards acquiring vibrational CARS measurements for more accurate temperatures in the very high temperature propellant burns.
While historically there have been many demonstrations of phase-conjugate holography to correct for noise created by phase distortions, the technique has not been applied to modern digital inline holographic measurements. Here, we present the theory and initial experiments exploring the impact of using phase-conjugate digital inline holography (PCDIH) to reduce noise produced by phase distortions through index-of-refraction gradients, such as would be experienced in a combustion environment. We demonstrate the ability to measure 3D object locations using PCDIH and correct for some types of disturbances. To quantify the technique, a plasma generator is used to produce strong indexof- refraction changes in room air. Object position errors in these measurements from both traditional DIH and PCDIH are compared. Preliminary results suggest that the use of PCDIH could potentially reduce positional error by approximately half when measuring through phase disturbances.
We report the application of ultrafast rotational coherent anti-Stokes Raman scattering (CARS) for temperature and relative oxygen concentration measurements in the plume emanating from a burning aluminized ammonium perchlorate propellant strand. Combustion of these metal-based propellants is a particularly hostile environment for laserbased diagnostics, with intense background luminosity, scattering and beam obstruction from hot metal particles that can be as large as several hundred microns in diameter. CARS spectra that were previously obtained using nanosecond pulsed lasers in an aluminumparticle- seeded flame are examined and are determined to be severely impacted by nonresonant background, presumably as a result of the plasma formed by particulateenhanced laser-induced breakdown. Introduction of fs/ps laser pulses enables CARS detection at reduced pulse energies, decreasing the likelihood of breakdown, while simultaneously providing time-gated elimination of any nonresonant background interference. Temperature probability densities and temperature/oxygen correlations were constructed from ensembles of several thousand single-laser-shot measurements from the fs/ps rotational CARS measurement volume positioned within 3 mm or less of the burning propellant surface. Preliminary results in canonical flames are presented using a hybrid fs/ps vibrational CARS system to demonstrate our progress towards acquiring vibrational CARS measurements for more accurate temperatures in the very high temperature propellant burns.
The interaction of a Mach 1.67 shock wave with a dense particle curtain is quantified using flash radiography. These new data provide a view of particle transport inside a compressible, dense gas–solid flow of high optical opacity. The curtain, composed of 115-µm glass spheres, initially spans 87 % of the test section width and has a streamwise thickness of about 2 mm. Radiograph intensities are converted to particle volume fraction distributions using the Beer–Lambert law. The mass in the particle curtain, as determined from the X-ray data, is in reasonable agreement with that given from a simpler method using a load cell and particle imaging. Following shock impingement, the curtain propagates downstream and the peak volume fraction decreases from about 23 to about 4 % over a time of 340 µs. The propagation occurs asymmetrically, with the downstream side of the particle curtain experiencing a greater volume fraction gradient than the upstream side, attributable to the dependence of particle drag on volume fraction. Bulk particle transport is quantified from the time-dependent center of mass of the curtain. The bulk acceleration of the curtain is shown to be greater than that predicted for a single 115-µm particle in a Mach 1.67 shock-induced flow.
We present a detailed set of measurements from a piloted, sooting, turbulent C 2 H 4 - fueled diffusion flame. Hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (CARS) is used to monitor temperature and oxygen, while laser-induced incandescence (LII) is applied for imaging of the soot volume fraction in the challenging jet-flame environment at Reynolds number, Re = 20,000. Single-laser shot results are used to map the mean and rms statistics, as well as probability densities. LII data from the soot-growth region of the flame are used to benchmark the soot source term for one-dimensional turbulence (ODT) modeling of this turbulent flame. The ODT code is then used to predict temperature and oxygen fluctuations higher in the soot oxidation region higher in the flame.
We report measurements of temperature and O2/N2 mole-fraction ratio in the vicinity of a burning and decomposing carbon-epoxy composite aircraft material samples exposed to uniform heat fluxes of 48 and 69 kW/m2. Controlled laboratory experiments were conducted with the samples suspended above a cone-type heater and enclosed in an optically accessible chimney. Noninvasive coherent anti-Stokes Raman scattering (CARS) measurements we performed on a single-laser-shot basis. The CARS data were performed with both a traditional point measurement system and with a one-dimensional line imaging scheme that provides single-shot temperature and O2/N2 profiles to reveal the quantitative structure of the temperature and oxygen concentration profiles over the duration of the 30-40 minute duration events. The measured near-surface temperature and oxygen transport are an important factor for exothermic chemistry and oxidation of char materials and the carbon fibers themselves in a fire scenario. These unique laser-diagnostic experiments provide new information on physical/chemical processes in a well-controlled environment which may be useful for the development of heat-and mass-transfer models for the composite fire scenario.
We present the results of an LDRD project to develop diagnostics to perform fundamental measurements of material properties during shock compression of condensed phase materials at micron spatial scales and picosecond time scales. The report is structured into three main chapters, which each focus on a different diagnostic devel opment effort. Direct picosecond laser drive is used to introduce shock waves into thin films of energetic and inert materials. The resulting laser - driven shock properties are probed via Ultrafast Time Domain Interferometry (UTDI), which can additionally be used to generate shock Hugoniot data in tabletop experiments. Stimulated Raman scattering (SRS) is developed as a temperature diagnostic. A transient absorption spectroscopy setup has been developed to probe shock - induced changes during shock compressio n. UTDI results are presented under dynamic, direct - laser - drive conditions and shock Hugoniots are estimated for inert polystyrene samples and for the explosive hexanitroazobenzene, with results from both Sandia and Lawrence Livermore presented here. SRS a nd transient absorption diagnostics are demonstrated on static thin - film samples, and paths forward to dynamic experiments are presented.
A simple spectral focusing scheme for bandwidth optimization of gas-phase rotational coherent anti- Stokes Raman scattering (CARS) spectra is presented. The method is useful when femtosecond pump/Stokes preparation of the Raman coherence is utilized. The approach is of practical utility when working with laser pulses that are not strictly transform limited or when windows or other sources of pulse chirp may be present in the experiment. A delay between the femtosecond preparation pulses is introduced to shift the maximumRaman preparation away from zero frequency and toward the Stokes or anti-Stokes side of the spectrum with no loss in total preparation bandwidth. Shifts of 100 cm-1or more are attainable and allow for enhanced detection of high-energy (150-300 cm-1) rotational Raman transitions at near-transform-limited optimum sensitivity. A simple theoretical treatment for the case of identical pump and Stokes pulses with linear frequency chirp is presented. The approach is then demonstrated experimentally for typical levels of transform-limited laser performance obtained in our laboratory with nonresonant CARS in argon and Raman-resonant spectra from a lean H2/air flat flame.
A hybrid fs/ps pure-rotational coherent anti-Stokes Raman scattering (CARS) scheme is systematically evaluated over a wide range of flame conditions in the product gases of two canonical flat-flame burners. Near-transform-limited, broadband femtosecond pump and Stokes pulses impulsively prepare a rotational Raman coherence, which is later probed using a high-energy, frequency-narrow picosecond beam generated by the second-harmonic bandwidth compression scheme that has recently been demonstrated for rotational CARS generation in H2/air flat flames. The measured spectra are free of collision effects and nonresonant background and can be obtained on a single-shot basis at 1kHz. The technique is evaluated for temperature/oxygen measurements in near-adiabatic H2/air flames stabilized on the Hencken burner for equivalence ratios of ϕ=0.20-1.20. Thermometry is demonstrated in hydrocarbon/air products for ϕ=0.75-3.14 in premixed C2H4/air flat flames on the McKenna burner. Reliable spectral fitting is demonstrated for both shot-averaged and single-laser-shot data using a simple phenomenological model. Measurement accuracy is benchmarked by comparison to adiabatic-equilibrium calculations for the H2/air flames, and by comparison with nanosecond CARS measurements for the C2H4/air flames. Quantitative accuracy comparable to nanosecond rotational CARS measurements is observed, while the observed precision in both the temperature and oxygen data is extraordinarily high, exceeding nanosecond CARS, and on par with the best published thermometric precision by femtosecond vibrational CARS in flames, and rotational femtosecond CARS at low temperature. Threshold levels of signal-to-noise ratio to achieve 1-2% precision in temperature and O2/N2 ratio are identified. The results show that pure-rotational fs/ps CARS is a robust and quantitative tool when applied across a wide range of flame conditions spanning lean H2/air combustion to fuel-rich sooting hydrocarbon flames.
We present the results of a two year early career LDRD project, which has focused on the development of ultrafast diagnostics to measure temperature, pressure and chemical change during the shock initiation of energetic materials. We compare two single-shot versions of femtosecond rotational CARS to measure nitrogen temperature: chirped-probe-pulse and ps/fs hybrid CARS thermometry. The applicability of measurements to the combustion of energetic materials will be discussed. We have also demonstrated laser shock and particle velocity measurements in thin film explosives using stretched femtosecond laser pulses. We will discuss preliminary results from Al and PETN thin films. Agreement between our results and previous work will be discussed.
Currently there is a substantial lack of data for interactions of shock waves with particle fields having volume fractions residing between the dilute and granular regimes, which creates one of the largest sources of uncertainty in the simulation of energetic material detonation. To close this gap, a novel Multiphase Shock Tube has been constructed to drive a planar shock wave into a dense gas-solid field of particles. A nearly spatially isotropic field of particles is generated in the test section by a gravity-fed method that results in a spanwise curtain of spherical 100-micron particles having a volume fraction of about 19%. Interactions with incident shock Mach numbers of 1.66, 1.92, and 2.02 were achieved. High-speed schlieren imaging simultaneous with high-frequency wall pressure measurements are used to reveal the complex wave structure associated with the interaction. Following incident shock impingement, transmitted and reflected shocks are observed, which lead to differences in particle drag across the streamwise dimension of the curtain. Shortly thereafter, the particle field begins to propagate downstream and spread. For all three Mach numbers tested, the energy and momentum fluxes in the induced flow far downstream are reduced about 30-40% by the presence of the particle field. X-Ray diagnostics have been developed to penetrate the opacity of the flow, revealing the concentrations throughout the particle field as it expands and spreads downstream with time. Furthermore, an X-Ray particle tracking velocimetry diagnostic has been demonstrated to be feasible for this flow, which can be used to follow the trajectory of tracer particles seeded into the curtain. Additional experiments on single spherical particles accelerated behind an incident shock wave have shown that elevated particle drag coefficients can be attributed to increased compressibility rather than flow unsteadiness, clarifying confusing results from the historical database of shock tube experiments. The development of the Multiphase Shock Tube and associated diagnostic capabilities offers experimental capability to a previously inaccessible regime, which can provide unprecedented data concerning particle dynamics of dense gas-solid flows.
Laser-induced incandescence measurements have recently been obtained from 10% and 30% toluene in methanol blended fuel pool fires of 2-m diameter. Calibration of the instrument was performed using an ethylene/air laminar diffusion flame produced by a Santoro-type burner which allowed the extraction of absolute soot-volume-fractions from these images. Performance of the optical probe was characterized using the laminar diffusion flame and corrections were implemented for signal dependence upon detector gain, flat field, and location within the probe laser sheet when processing the images. Probability density functions of the soot-volume fraction were constructed for the blended fuels used in this study and the mean values were determined to be 0.0077 and 0.028 ppm for the 10% and 30% blended fuels, respectively. Signal trapping was estimated for the two types of blended fuel and it was determined to be negligible for the ∼10% toluene/methanol blend and require ∼10% correction for the 30% toluene/methanol blend.
A novel multiphase shock tube has been constructed to test the interaction of a planar shock wave with a dense gas-solid field of particles. The particle field is generated by a gravity-fed method that results in a spanwise curtain of 100-micron particles producing a volume fraction of about 15%. Interactions with incident shock Mach numbers of 1.67 and 1.95 are reported. High-speed schlieren imaging is used to reveal the complex wave structure associated with the interaction. After the impingement of the incident shock, transmitted and reflected shocks are observed, which lead to differences in flow properties across the streamwise dimension of the curtain. Tens of microseconds after the onset of the interaction, the particle field begins to propagate downstream, and disperse. The spread of the particle field, as a function of its position, is seen to be nearly identical for both Mach numbers. Immediately downstream of the curtain, the peak pressures associated with the Mach 1.67 and 1.95 interactions are about 35% and 45% greater than tests without particles, respectively. For both Mach numbers tested, the energy and momentum fluxes in the induced flow far downstream are reduced by about 30-40% by the presence of the particle field.
A novel multiphase shock tube has recently been developed to study particle dynamics in gas-solid flows having particle volume fractions that reside between the dilute and granular regimes. The method for introducing particles into the tube involves the use of a gravity-fed contoured particle seeder, which is capable of producing dense fields of spatially isotropic particles. The facility is capable of producing planar shocks having a maximum shock Mach number of about 2.1 that propagate into air at initially ambient conditions. The primary purpose of this new facility is to provide high fidelity data of shock-particle interactions in flows having particle volume fractions of about 1 to 50%. To achieve this goal, the facility drives a planar shock into a spatially isotropic field, or curtain, of particles. Experiments are conducted for two configurations where the particle curtain is either parallel to the spanwise, or the streamwise direction. Arrays of high-frequency-response pressure transducers are placed near the particle curtain to measure the attenuation and shape change of the shock owing to its interaction with the dense gas particle field. In addition, simultaneous high-speed imaging is used to visualize the impact of the shock on the particle curtain and to measure the particle motion induced downstream of the shock.
As the capabilities of numerical simulations increase, decision makers are increasingly relying upon simulations rather than experiments to assess risks across a wide variety of accident scenarios including fires. There are still, however, many aspects of fires that are either not well understood or are difficult to treat from first principles due to the computational expense. For a simulation to be truly predictive and to provide decision makers with information which can be reliably used for risk assessment the remaining physical processes must be studied and suitable models developed for the effects of the physics. A set of experiments are outlined in this report which will provide soot volume fraction/temperature data and heat flux (intensity) data for the validation of models for the radiative transfer equation. In addition, a complete set of boundary condition measurements will be taken to allow full fire predictions for validation of the entire fire model. The experiments will be performed with a lightly-sooting liquid hydrocarbon fuel fire in the fully turbulent scale range (2 m diameter).
A novel multiphase shock tube to study particle dynamics in gas-solid flows has been constructed and tested. Currently, there is a gap in data for flows having particle volume fractions between the dusty and granular regimes. The primary purpose of this new facility is to fill that gap by providing high quality data of shock-particle interactions in flows having dense gas particle volume fractions. Towards this end, the facility aims to drive a shock into a spatially isotropic field, or curtain, of particles. Through bench-top experimentation, a method emerged for achieving this challenging task that involves the use of a gravity-fed contoured particle seeder. The seeding method is capable of producing fields of spatially isotropic particles having volume fractions of about 1 to 35%. The use of the seeder in combination with the shock tube allows for the testing of the impingement of a planar shock on a dense field of particles. The first experiments in the multiphase shock tube have been conducted and the facility is now operational.
We report an application of coherent anti-Stokes Raman scattering (CARS) to full-scale fire testing. A CARS instrument has been constructed at the newly commissioned FLAME (Fire Laboratory for Accreditation of Models and Experiments) facility at Sandia, where the CARS system has been used for thermometry in 2-m-diameter, turbulent pool fires. The details of a CARS instrument for probing the challenging pool-fire environment are presented, along with the construction of the unique new FLAME facility itself, which has been designed to accommodate optical and laser-based diagnostics to full-scale fire experimentation. Single-shot CARS spectra and best-fit temperatures from turbulent pool fires are presented, and an estimate of the pdf of the temperature fluctuations from the pool-fire environment is obtained.
Fires pose the dominant risk to the safety and security of nuclear weapons, nuclear transport containers, and DOE and DoD facilities. The thermal hazard from these fires primarily results from radiant emission from high-temperature flame soot. Therefore, it is necessary to understand the local transport and chemical phenomena that determine the distributions of soot concentration, optical properties, and temperature in order to develop and validate constitutive models for large-scale, high-fidelity fire simulations. This report summarizes the findings of a Laboratory Directed Research and Development (LDRD) project devoted to obtaining the critical experimental information needed to develop such constitutive models. A combination of laser diagnostics and extractive measurement techniques have been employed in both steady and pulsed laminar diffusion flames of methane, ethylene, and JP-8 surrogate burning in air. For methane and ethylene, both slot and coannular flame geometries were investigated, as well as normal and inverse diffusion flame geometries. For the JP-8 surrogate, coannular normal diffusion flames were investigated. Soot concentrations, polycyclic aromatic hydrocarbon (PAH) laser-induced fluorescence (LIF) signals, hydroxyl radical (OH) LIF, acetylene and water vapor concentrations, soot zone temperatures, and the velocity field were all successfully measured in both steady and unsteady versions of these various flames. In addition, measurements were made of the soot microstructure, soot dimensionless extinction coefficient (&), and the local radiant heat flux. Taken together, these measurements comprise a unique, extensive database for future development and validation of models of soot formation, transport, and radiation.
Simulation-based life-cycle-engineering and the ASCI program have resulted in models of unprecedented size and fidelity. The validation of these models requires high-resolution, multi-parameter diagnostics. Within the thermal-fluids disciplines, the need for detailed, high-fidelity measurements exceeds the limits of current engineering sciences capabilities and severely tests the state of the art. The focus of this LDRD is the development and application of filtered Rayleigh scattering (FRS) for high-resolution, nonintrusive measurement of gas-phase velocity and temperature. With FRS, the flow is laser-illuminated and Rayleigh scattering from naturally occurring sources is detected through a molecular filter. The filtered transmission may be interpreted to yield point or planar measurements of three-component velocities and/or thermodynamic state. Different experimental configurations may be employed to obtain compromises between spatial resolution, time resolution, and the quantity of simultaneously measured flow variables. In this report, we present the results of a three-year LDRD-funded effort to develop FRS combustion thermometry and Aerosciences velocity measurement systems. The working principles and details of our FRS opto-electronic system are presented in detail. For combustion thermometry we present 2-D, spatially correlated FRS results from nonsooting premixed and diffusion flames and from a sooting premixed flame. The FRS-measured temperatures are accurate to within {+-}50 K (3%) in a premixed CH4-air flame and within {+-}100 K for a vortex-strained diluted CH4-air diffusion flame where the FRS technique is severely tested by large variation in scattering cross section. In the diffusion flame work, FRS has been combined with Raman imaging of the CH4 fuel molecule to correct for the local light scattering properties of the combustion gases. To our knowledge, this is the first extension of FRS to nonpremixed combustion and the first use of joint FRS-Raman imaging. FRS has been applied to a sooting C2H4-air flame and combined with LII to assess the upper sooting limit where FRS may be utilized. The results from this sooting flame show FRS temperatures has potential for quantitative temperature imaging for soot volume fractions of order 0.1 ppm. FRS velocity measurements have been performed in a Mach 3.7 overexpanded nitrogen jet. The FRS results are in good agreement with expected velocities as predicted by inviscid analysis of the jet flowfield. We have constructed a second FRS opto-electronic system for measurements at Sandia's hypersonic wind tunnel. The details of this second FRS system are provided here. This facility is currently being used for velocity characterization of these production hypersonic facilities.
This paper describes the application of a filtered-Rayleigh-scattering (FRS) instrument for nonintrusive temperature imaging in a vortex-driven diffusion flame. The FRS technique provides quantitative, spatially correlated temperature data without the flow intrusion or time lag associated with physical probes. Use of a molecular iodine filter relaxes the requirement for clean, particulate-free flowfields and offers the potential for imaging near walls, test section windows and in sooty flames, all of which are precluded in conventional Rayleigh imaging, where background interference from these sources typically overwhelms the weak molecular scattering signal. For combustion applications, FRS allows for full-field temperature imaging without chemical seeding of the flowfield, which makes FRS an attractive alternative to other laser-based imaging methods such as planar laser-induced fluorescence (PLIF). In this work, the details of our FRS imaging system are presented and temperature measurements from an acoustically forced diffusion flame are provided. The local Rayleigh cross-section is corrected using Raman imaging measurements of the methane fuel molecule, which are then correlated to other major species using a laminar flamelet approach. To our knowledge, this is the first report of joint Raman/FRS imaging for nonpremixed combustion. Measurements are presented from flames driven at 7.5 Hz, where a single vortex stretches the flame, and at 90 Hz, where two consecutive vortices interact to cause a repeatable strain-induced flame-quenching event.