Detonation of explosive devices produces extremely hazardous fragments and hot, luminous fireballs. Prior experimental investigations of these post-detonation environments have primarily considered devices containing hundreds of grams of explosives. While relevant to many applications, such large- scale testing also significantly restricts experimental diagnostics and provides limited data for model validation. As an alternative, the current work proposes experiments and simulations of the fragmentation and fireballs from commercial detonators with less than a gram of high explosive. As demonstrated here, reduced experimental hazards and increased optical access significantly expand the viability of advanced imaging and laser diagnostics. Notable developments include the first known validation of MHz-rate optical fragment tracking and the first ever Coherent Anti-Stokes Raman Scattering (CARS) measures of post-detonation fireball temperatures. While certainly not replacing the need for full-scale verification testing, this work demonstrates new opportunities to accelerate developments of diagnostics and predictive models of post-detonation environments.
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 simple linear configuration for multi-line femtosecond laser electronic excitation tagging (FLEET) velocimetry is used for the first time, to the best of our knowledge, to image an overexpanded unsteady supersonic jet. The FLEET lines are spaced 0.5-1.0mmapart, and up to six lines can be used simultaneously to visualize the flowfield. These lines are created using periodic masks, despite the mask blocking 25%-30%of the 10 mJ incident beam.Maps of mean singlecomponent velocity in the direction along the principal flow axis, and turbulence intensity in that same direction, are created using multi-line FLEET, and computed velocities agree well with those obtained from single-line (traditional) FLEET. Compared to traditional FLEET, multi-line FLEET offers increased simultaneous spatial coverage and the ability to produce spatial correlations in the streamwise direction. This FLEET permutation is especially well suited for short-duration test facilities.
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.
Three ultra-high-speed, 10 MHz, cameras imaged the time-resolved decay of laser-induced incandescence (LII) from soot in a turbulent non-premixed ethylene jet flame. Cameras were equipped with a stereoscope allowing each CMOS array to capture two separate views of the flame. The resulting six views were reconstructed into a volumetric soot decay series using commercially available DaVis tomographic software by LaVision. Primary soot particle sizes were estimated from the decay time history on a per voxel basis by comparing measured signals to an LII model. Experimentally quantified soot particle sizes agree with existing predictions and previous measurements.
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.
Femtosecond Laser Electronic Excitation Tagging (FLEET) is used to measure velocity flowfields in the wake of a sharp 7◦ half-angle cone in nitrogen at Mach 8, over freestream Reynolds numbers from 4.3∗106 /m to 13.8∗106 /m. Flow tagging reveals expected wake features such as the separation shear layer and two-dimensional velocity components. Frequency-tripled FLEET has a longer lifetime and is more energy efficient by tenfold compared to 800 nm FLEET. Additionally, FLEET lines written with 267 nm are three times longer and 25% thinner than that written with 800 nm at a 1 µs delay. Two gated detection systems are compared. While the PIMAX 3 ICCD offers variable gating and fewer imaging artifacts than a LaVision IRO coupled to a Photron SA-Z, its slow readout speed renders it ineffective for capturing hypersonic velocity fluctuations. FLEET can be detected to 25 µs following excitation within 10 mm downstream of the model base, but delays greater than 4 µs have deteriorated signal-to-noise and line fit uncertainties greater than 10%. In a hypersonic nitrogen flow, exposures of just several hundred nanoseconds are long enough to produce saturated signals and/or increase the line thickness, thereby adding to measurement uncertainty. Velocity calculated between the first two delays offer the lowest uncertainty (less than 3% of the mean velocity).
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.
