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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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NASA NTRS1983

Ring and plasma - The enigmae of Enceladus

The E ring associated with the Kronian moon Enceladus has a lifetime of only a few thousand years against sputtering by slow corotating O ions. The existence of the ring implies the necessity for a continuous supply of matter. Possible particle source mechanisms on Enceladus include meteoroidal impact ejection and geysering. Estimates of ejection rates of particulate debris following small meteoroid impact are on the order of 3 x 10 to the -18th g/(sq cm sec), more than an order of magnitude too small to sustain the ring. A geyser source would need to generate a droplet supply at a rate of approximately 10 to the -16th g/(sq cm sec) in order to account for a stable ring. Enceladus and the ring particles also directly supply both plasma and vapor to space via sputtering. The absence of a 60 eV plasma at the Voyager 2 Enceladus L-shell crossing, such as might have been expected from sputtering, cannot be explained by absorption and moderation of plasma ions by ring particles, because the ring is too diffuse. Evidently, the effective sputtering yield in the vicinity of Enceladus is on the order of, or smaller than, 0.4, about an order of magnitude less than te calculated value. Small scale surface roughness may account for some of this discrepancy.

Haff, P. K.↗

NASA NTRS1994

Soft x ray optics by pulsed laser deposition

A series of molybdenum thin film depositions by PLD (Pulsed Laser Deposition) have been carried out, seeking appropriate conditions for multilayer fabrication. Green (532 nm) and UV (355 nm) light pulses, in a wide range of fluences, were used. Relatively large fluences (in comparison with Si) are required to cause evaporation of molybdenum. The optical penetration depths and reflectivities for Mo at these two wavelengths are comparable, which means that results should be, and do appear to be similar for equal fluences. For all fluences above threshold used, a large number of incandescent particles is ejected by the target (either a standard Mo sputtering target or a Mo sheet were tried), together with the plasma plume. Most of these particles are clearly seen to bounce off the substrate. The films were observed with light microscopy using Nomarski and darkfield techniques. There is no evidence of large debris. Smooth films plus micron-sized droplets are usually seen. The concentration of these droplets embedded in the film appears not to vary strongly with the laser fluence employed. Additional characterization with SEM and XRD is under way.

Fernandez, Felix E.↗

NASA NTRS2013

In Situ Solid Particle Generator

Particle seeding is a key diagnostic component of filter testing and flow imaging techniques. Typical particle generators rely on pressurized air or gas sources to propel the particles into the flow field. Other techniques involve liquid droplet atomizers. These conventional techniques have drawbacks that include challenging access to the flow field, flow and pressure disturbances to the investigated flow, and they are prohibitive in high-temperature, non-standard, extreme, and closed-system flow conditions and environments. In this concept, the particles are supplied directly within a flow environment. A particle sample cartridge containing the particles is positioned somewhere inside the flow field. The particles are ejected into the flow by mechanical brush/wiper feeding and sieving that takes place within the cartridge chamber. Some aspects of this concept are based on established material handling techniques, but they have not been used previously in the current configuration, in combination with flow seeding concepts, and in the current operational mode. Unlike other particle generation methods, this concept has control over the particle size range ejected, breaks up agglomerates, and is gravity-independent. This makes this device useful for testing in microgravity environments.

Agui, Juan H.↗

DOE OSTI2021

Modelling the complex evaporated gas flow and its impact on particle spattering during laser powder bed fusion

The additive manufacturing (AM) of metals is becoming an increasingly important production process with the potential to replace traditional techniques such as casting. Laser Powder Bed Fusion (LPBF) is used in many applications to print metal parts from powder. The metal powder is heated locally with sufficient laser radiation that the liquid melt easily reaches its boiling temperature, which leads to a metallic vapour jet that can entrain both powder bed particles and molten droplets. The small size of laser-matter interaction site makes a detailed experimental analysis of the process challenging. Synchrotron X-ray imaging experiments are one of the few methods which can capture the dynamic melting and solidification processes. Comparing such experiments with computer simulations of the process is an important approach in order to better understand the manufacturing process and to analyse the influence of process parameters on the evaporated gas jet and the subsequent impact on particle ejection, leading to potentially reduced AM component quality. The melting and solidification of the metal powder is simulated using an Eulerian multiphase approach based on a control volume discretization of powder bed and substrate and a volume of liquid separation from melt and gas phase. The gas phase modelled as an ideal gas reaches velocities up to 100 m/s. Lagrangian particle tracking in the simulation demonstrates that the velocity fields calculated by the Eulerian multi-phase approach in combination with a standard drag-force model lead to particle accelerations in good agreement with those measured experimentally. In order to avoid numerical laborious Lagrangian calculations, a direct method to compare an Eulerian multiphase simulation with synchrotron X-ray experiments was introduced and validated. This approach is used to analyse the influence of process parameters including laser power and laser speed on the maximal acceleration of particles from the melt pool area. While the particle acceleration increases linearly with line energy in the conduction mode, a linear decrease of the acceleration with increasing line energy can be found in the transition mode before the accel-eration increases again with line energy in the keyhole mode.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Atmospheric clearing of solid-particle debris using femtosecond filaments

Through nonlinear self-focusing, femtosecond pulses can propagate several kilometers beyond diffraction limits, forming an ionization channel in air known as a laser filaments. It has been demonstrated that in the wake of the filament, aerosols can be effectively cleared to improve the transmission of subsequent laser pulses or secondary light sources, pertinent to applications in atmospheric sensing. However, the current understanding of aerosol clearing is founded on interactions with droplets to simulate fogs and clouds and thus does not extend to solid particles or atmospheric debris. Using optical trapping, we isolate both graphite and silica microparticles and directly measure the subsequent displacement caused by the filament using time-resolved shadowgraphy. The shock wave from the filament is demonstrated to propel particles away from the filament, directly contributing to atmospheric debris clearing. Particles exposed to the laser light in either the intense filament core or the surrounding energy reservoir are axially displaced along the beam path. Furthermore, it is found that the optomechanical properties of the particle largely influence the axial displacement induced by laser exposure through mechanisms such as radiation pressure, mass ejection from ablation or optical damage, and particle deagglomeration.

42 ENGINEERING↗

DOE OSTI2021

Droplet and Particle Methods to Investigate Turbulent Particle Laden Jets

The SARS-CoV-2 pandemic has heightened the interest in particle-laden turbulent jets generated by breathing, talking, coughing and sneezing, and how these can contribute to disease transmission. We present quantitative measurement methods for such flows, while exploring and offering improvements for common shortcomings. We generate jets consisting of either liquid droplets or solid particles in an isothermal, quiescent and electrically isopotential experimental chamber that was constructed to control the effects of ambient forcing on jet behavior. For liquid droplets, we find promise in surface deposition analysis based on fluorescent tracer use. For particles, we explore the performance of commercially available adhesive sampling strips and develop conductive grounded carbon tape based sampling strips. We explore ways in which the smallest of thermal gradients or electrostatic charge issues can affect particle dispersion, and suggest practical methods to address these issues. The developed methods are applied to study the simultaneous deposition of (Formula presented.) 25, 50 and 200 μm solid particles from a particle laden turbulent jet with a mean velocity of 33.2 m/s. The deposition location as a function of particle size was compared to results from a simple numerical RANS model, and illustrates ways in which imprecise initial or boundary conditions can lead to a notable deviation from experimental results. The differences in deposition pattern seen in experimental and numerical results despite a carefully controlled environment and characterized particle ejection indicate the need for a more stringent numerical model validation, especially when studying fate and transport of mid-range (neither purely aerosol or ballistic) sized particles.

42 ENGINEERING↗

DOE OSTI2021

Direct numerical simulation of turbulent dispersion of evaporative aerosol clouds produced by an intense expiratory event

Airborne particles are a major route for transmission of COVID-19 and many other infectious diseases. When a person talks, sings, coughs, or sneezes, nasal and throat secretions are spewed into the air. After a short initial fragmentation stage, the expelled material is mostly composed of spherical particles of different sizes. While the dynamics of the largest droplets are dominated by gravitational effects, the smaller aerosol particles, mostly transported by means of hydrodynamic drag, form clouds that can remain afloat for long times. In subsaturated air environments, the dependence of pathogen-laden particle dispersion on their size is complicated due to evaporation of the aqueous fraction. Particle dynamics can significantly change when ambient conditions favor rapid evaporation rates that result in a transition from buoyancyto-drag dominated dispersion regimes. To investigate the effect of particle size and evaporation on pathogen-laden cloud evolution, a direct numerical simulation of a mild cough was coupled with an evaporative Lagrangian particle advection model. The results suggest that while the dispersion of cough particles in the tails of the size distribution are unlikely to be disrupted by evaporative effects, preferential aerosol diameters (30-40 μm) may exhibit significant increases in the residence time and horizontal range under typical ambient conditions. Using estimations of the viral concentration in the spewed fluid and the number of ejected particles in a typical respiratory event, we obtained a map of viral load per volume of air at the end of the cough and the number of virus copies per inhalation in the emitter vicinity.

60 APPLIED LIFE SCIENCES↗