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Atomic processes in the polar cap
Polar cap atomic processes stimulated by photons, electrons and protons, considering particle morphology
Atomic processes in plasmas.
Measuring atomic radiation and collision cross section coefficients of plasmas
Competing atomic processes in Ba and Sr injection critical velocity experiments
The critical ionization velocity effect requires a superthermal electron population to ionize through collisional impact. Such superthermal electrons can however lose energy to competing atomic processes, as well as to ionization, thus limiting the efficiency of the effect. Considering Ba and Sr magnetospheric injection experiments designed to test the CIV theory, it is found that in both cases roughly 60 percent of the superthermal electron energy is lost on exciting line radiation. Moreover, energy loss to background neutral oxygen places a strict limit on the injected cloud densities for which critical velocity effects are possible; a finding which explains the consistently negative results in radial injection experiments.
Atomic Processes in X-ray Photoioinzed Gas
It has long been known that photoionization and photoabsorption play a dominant role in determining the state of gas in nebulae surrounding hot stars and in active galaxies. Recent observations of X-ray spectra demonstrate that these processes are also dominant in highly ionized gas near compact objects, and also affect the transmission of X-rays from the majority of astronomical sources. This has led to new insights into the understanding of what is going on in these sources. It has also pointed out the need for accurate atomic cross sections for photoionization and absorption, notably for processes involving inner shells. The xstar code can be used for calculating the heating, ionization and reprocessing of X-rays by gas in a range of ionization states and temperatures. It has recently been updated to include an improved treatment of inner shell transitions in iron. I will review the capabilities of xstar, the atomic data, and illustrate some applications to recent X-ray spectral observations.
Atomic Processes in X-ray Photoionized Gas
It has long been known that photoionization and photoabsorption play a dominant role in determining the state of gas in nebulae surrounding hot stars and in active galaxies. Recent observations of X-ray spectra demonstrate that these processes are also dominant in highly ionized gas near compact objects, and also affect the transmission of X-rays from the majority of astronomical sources. This has led to new insights into the understanding of what is going on in these sources. It has also pointed out the need for a better atomic cross sections for photoionization and absorption, notably for processes involving inner shells. In this talk I will discuss these issues, what is known and where more work is needed.
Atomic processes in astrophysics.
Studies of particle excitation processes in the solar corona are reviewed, covering electron affinities, negative ion experiments using two-photon laser and drift tube techniques, geometrical hindrance in molecular ion formation, negative ion-molecule reactions, and negative ions in the ionosphere and astrophysics. Aspects of forbidden atomic transitions are also discussed, including the calculation of transition probabilities, the Fe II problem, and magnetic quadrupole radiation in a relativistic approximation.
Atomic processes in a low-density hydrogen- helium plasma
H-He partially ionized plasma bremsstrahlung, radiative and dielectronic recombination, discrete levels excitation and collisional ionization
Calculations of atomic processes in Fe XVII
A synoptic study has been made of the neon-like ion Fe XVII in order to improve knowledge of its line production in high-temperature, low-density plasmas. The following were computed: (1) distorted wave, electron collision strengths from the ground level to all 88 levels with one n = 3 - or n = 4 electron; (2) a complete cascade matrix; (3) the contribution of resonances to the n = 3 level population rates; and (4) the resulting X-ray and EUV line emission coefficients as functions of temperature between T = 10 to the 5.6 and T = 10 to the 8.4 K. Results are presented for typical population rates, line emission coefficients fitted with a useful formula for spectrum calculations, and the dielectronic recombination rate.
Rapid Fabrication of Lightweight SiC Optics using Reactive Atom Plasma (RAP) Processing
Reactive Atom Plasma (RAP) processing is a non-contact, plasma-based processing technology that can be used to generate damage-free optical surfaces. We have developed tools and processes using RAP that allow us to shape extremely lightweight mirror Surfaces made from extremely hard-to-machine materials (e.g. SiC). We will describe our latest results using RAP in combination with other technologies to produce finished lightweight SiC mirrors and also discuss applications for RAP in the rapid fabrication of mirror segments for reflective and grazing incidence telescopes.
Fundamental Processes of Atomization in Fluid-Fluid Flows
This report outlines the major results of the grant "Fundamental Processes of Atomization in Fluid-Fluid Flows." These include: 1) the demonstration that atomization in liquid/liquid shear flow is driven by a viscous shear instability that triggers the formation of a long thin sheet; 2) discovery of a new mode of interfacial instability for oscillatory two-layer systems whereby a mode that originates within the less viscous liquid phase causes interfacial deformation as the oscillation proceeds; 3) the demonstration that rivulet formation from gravity front occurs because the local front shape specified by gravity and surface tension changes from a nose to a wedge geometry, thus triggering a large increase in viscous resistance; and 4) extension of the studies on nonlinear wave evolution on falling films and in stratified flow, particularly the evolution towards large-amplitude solitary waves that tend to generate drops.
Fundamental Processes of Atomization in Fluid-Fluid Flows
This paper discusses our proposed experimental and theoretical study of atomization in gas-liquid and liquid-liquid flows. While atomization is a very important process in these flows, the fundamental mechanism is not understood and there is no predictive theory. Previous photographic studies in (turbulent) gas-liquid flows have shown that liquid is atomized when it is removed by the gas flow from the crest of large solitary or roll waves. Our preliminary studies in liquid-liquid laminar flows exhibit the same mechanism. The two-liquid system is easier to study than gas-liquid systems because the time scales are much slower, the length scales much larger, and there is no turbulence. The proposed work is intended to obtain information about the mechanism of formation, rate of occurrence and the evolving shape of solitary waves; and quantitative aspects of the detailed events of the liquid removal process that can be used to verify a general predictive theory.
Electron-atom /molecule/ collision processes
Electron-atom (molecule) collision processes at low and intermediate energies, from near threshold to a few hundred electron volts, are discussed. Attention is given to experimental techniques and procedures, electron impact cross sections, impact excitation and electron-atom scattering in laser fields. Specific examples are presented that illustrate various experimental techniques and interpretations of observations.
Modeling of Turbulence Effects on Liquid Jet Atomization and Breakup
Recent experimental investigations and physical modeling studies have indicated that turbulence behaviors within a liquid jet have considerable effects on the atomization process. This study aims to model the turbulence effect in the atomization process of a cylindrical liquid jet. Two widely used models, the Kelvin-Helmholtz (KH) instability of Reitz (blob model) and the Taylor-Analogy-Breakup (TAB) secondary droplet breakup by O Rourke et al, are further extended to include turbulence effects. In the primary breakup model, the level of the turbulence effect on the liquid breakup depends on the characteristic scales and the initial flow conditions. For the secondary breakup, an additional turbulence force acted on parent drops is modeled and integrated into the TAB governing equation. The drop size formed from this breakup regime is estimated based on the energy balance before and after the breakup occurrence. This paper describes theoretical development of the current models, called "T-blob" and "T-TAB", for primary and secondary breakup respectivety. Several assessment studies are also presented in this paper.
Metal powder production by gas atomization
The confined liquid, gas-atomization process was investigated. Results from a two-dimensional water model showed the importance of atomization pressure, as well as delivery tube and atomizer design. The atomization process at the tip of the delivery tube was photographed. Results from the atomization of a modified 7075 aluminum alloy yielded up to 60 wt pct. powders that were finer than 45 microns in diameter. Two different atomizer designs were evaluated. The amount of fine powders produced was correlated to a calculated gas-power term. An optimal gas-power value existed for maximized fine powder production. Atomization at gas-power greater than or less than this optimal value produced coarser powders.
Numerical analysis of SSME preburner injector atomization and combustion processes
The coaxial spray injection and combustion flowfields of a Space Shuttle Main Engine preburner injector element have been analyzed using a three-phase numerical code. The processes of atomization, evaporation, secondary droplet breakup, and multispecies chemistry, as well as turbulent diffusion, are included. The model produced realistic pictures of the complex internal flowfield, including liquid jet length, spray shape, flame-zone size and characteristics, and predicted temperatures that seem to be in agreement with test data envelopes. It predicted an external group combustion type of flame. Salient combustion and mixing features are discussed and sources of uncertainty are pointed out for future studies.
Modeling of Turbulence Effect on Liquid Jet Atomization
Recent studies indicate that turbulence behaviors within a liquid jet have considerable effect on the atomization process. Such turbulent flow phenomena are encountered in most practical applications of common liquid spray devices. This research aims to model the effects of turbulence occurring inside a cylindrical liquid jet to its atomization process. The two widely used atomization models Kelvin-Helmholtz (KH) instability of Reitz and the Taylor analogy breakup (TAB) of O'Rourke and Amsden portraying primary liquid jet disintegration and secondary droplet breakup, respectively, are examined. Additional terms are formulated and appropriately implemented into these two models to account for the turbulence effect. Results for the flow conditions examined in this study indicate that the turbulence terms are significant in comparison with other terms in the models. In the primary breakup regime, the turbulent liquid jet tends to break up into large drops while its intact core is slightly shorter than those without turbulence. In contrast, the secondary droplet breakup with the inside liquid turbulence consideration produces smaller drops. Computational results indicate that the proposed models provide predictions that agree reasonably well with available measured data.
Development of an Atomization Methodology for Spray Combustion
In liquid rocket propulsion, the knowledge and the understanding of liquid-gas interfacial phenomena are very important. This is important for predicting the onset of cavitation occurring in swirl injection elements used in STME, as well as atomization processes in shear-induced injectors (co-axial) and impinging injector elements. From the fact that all the physical processes including droplet size distribution, droplet dispersion, mixing and combustion are controlled by atomization processes, it is expected that the successful incorporation of the volume of fraction (VOF) will greatly enhance the analytical capability of predicting spray combustion processes in liquid-fueled engines. In this paper, a methodology is developed to define and track interfaces between two fluids in non-orthogonal, body-fitted grids using a single fractional volume of fluid (VOF) variable to describe the distribution of the liquid phase in a gas-liquid flow field. This method was implemented in a mature CFD code MAST (Multiphase All-Speed Transient) utilizing the general PISO-C algorithm. For the preliminary study on the analysis of spray combustion and tracking of the interface between two phases, we will report on the progress of the simulation of the instability on the liquid column; the surface wave instability and the droplet breakup from the liquid surface.