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.
The design, construction, and testing of a high-magnification, long working-distance plenoptic camera is reported. A plenoptic camera uses a microlens array to enable resolution of the spatial and angular information of the incoming light field. Instantaneous images can be numerically refocused and perspective shifted in post-processing to enable threedimensional (3D) resolution of a scene. Prior to this work, most applications of plenoptic imaging were limited to relatively low magnifications (1× or less) or small working distances. Here, a unique system is developed with enables 5× magnification at a working distance of over a quarter meter. Experimental results demonstrate ~25 µm spatial resolution with 3D imaging capabilities. This technology is demonstrated for 3D imaging of the shock structure in a underexpanded, Mach 3.3 free air jet.
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.
Laser diagnostics are essential for time-resolved studies of solid rocket propellant combustion and small explosive detonations. Digital in-line holography (DIH) is a powerful tool for three-dimensional particle tracking in multiphase flows. By combining DIH with complementary diagnostics, particle temperatures and soot/smoke properties can be identified.
This work details the development of an algorithm to determine 3D position and in plane size and shape of particles by exploiting the perspective shift capabilities of a plenoptic camera combined with stereo-matching methods. This algorithm is validated using an experimental data set previously examined in a refocusing based particle location study in which a static particle field is translated to provide known depth displacements at varied magnification and object distances. Examination of these results indicates increased accuracy and precision is achieved compared to a previous refocusing based method at significantly reduced computational costs. The perspective shift method is further applied to fragment localization and sizing in a lab scale fragmenting explosive.