Preface 1.General Considerations 2.Basic Processes in Atomization 3.Drop Size Distributions of Sprays 4.Atomizers 5.Flow in Atomizers 6.Atomizer Performance 7.External Spray Charcteristics 8.Drop Evaporation 9.Drop Sizing Methods Index

Atomization and Sprays

Many-particle charged-particle plasma simulations using spatial meshes for the electromagnetic field solutions, particle-in-cell (PIC) merged with Monte Carlo collision (MCC) calculations, are coming into wide use for application to partially ionized gases. The author emphasizes the development of PIC computer experiments since the 1950s starting with one-dimensional (1-D) charged-sheet models, the addition of the mesh, and fast direct Poisson equation solvers for 2-D and 3-D. Details are provided for adding the collisions between the charged particles and neutral atoms. The result is many-particle simulations with many of the features met in low-temperature collision plasmas; for example, with applications to plasma-assisted materials processing, but also related to warmer plasmas at the edges of magnetized fusion plasmas. >

Particle-in-Cell Charged Particle Simulations, plus Monte Carlo Collisions with Neutral Atoms, PIC-MCC

An efficient ghost-cell immersed boundary method (GCIBM) for simulating turbulent flows in complex geometries is presented. A boundary condition is enforced through a ghost cell method. The reconstruction procedure allows systematic development of numerical schemes for treating the immersed boundary while preserving the overall second-order accuracy of the base solver. Both Dirichlet and Neumann boundary conditions can be treated. The current ghost cell treatment is both suitable for staggered and non-staggered Cartesian grids. The accuracy of the current method is validated using flow past a circular cylinder and large eddy simulation of turbulent flow over a wavy surface. Numerical results are compared with experimental data and boundary-fitted grid results. The method is further extended to an existing ocean model (MITGCM) to simulate geophysical flow over a three-dimensional bump. The method is easily implemented as evidenced by our use of several existing codes.

A Ghost-Cell Immersed Boundary Method for Flow in Complex Geometry

This contribution presents a short review of electric propulsion (EP) technologies for satellites and spacecraft. Electric thrusters, also termed ion or plasma thrusters, deliver a low thrust level compared to their chemical counterparts, but they offer significant advantages for in-space propulsion as energy is uncoupled to the propellant, therefore allowing for large energy densities. Although the development of EP goes back to the 1960s, the technology potential has just begun to be fully exploited because of the increase in the available power aboard spacecraft, as demonstrated by the very recent appearance of all-electric communication satellites. This article first describes the fundamentals of EP: momentum conservation and the ideal rocket equation, specific impulse and thrust, figures of merit and a comparison with chemical propulsion. Subsequently, the influence of the power source type and characteristics on the mission profile is discussed. Plasma thrusters are classically grouped into three categories according to the thrust generation process: electrothermal, electrostatic and electromagnetic devices. The three groups, along with the associated plasma discharge and energy transfer mechanisms, are presented via a discussion of long-standing technologies like arcjet thrusters, magnetoplasmadynamic thrusters, pulsed plasma thrusters and ion engines, as well as Hall thrusters and variants. More advanced concepts and new approaches for performance improvement are discussed afterwards: magnetic shielding and wall-less configurations, negative ion thrusters and plasma acceleration with a magnetic nozzle. Finally, various alternative propellant options are analyzed and possible research paths for the near future are examined.

http://iopscience.iop.org/article/10.1088/0963-0252/25/3/033002/pdf

Electric propulsion for satellites and spacecraft: established technologies and novel approaches

The production of a liquid spray can be summarized as the succession of the following three steps; the liquid flow ejection, the primary breakup mechanism and the secondary breakup mechanism. The intermediate step—the primary breakup mechanism—covers the early liquid flow deformation down to the production of the first isolated liquid fragments. This step is very important and requires to be fully understood since it constitutes the link between the flow issuing from the atomizer and the final spray. This paper reviews the experimental investigations dedicated to this early atomization step. Several situations are considered: cylindrical liquid jets, flat liquid sheets, air-assisted cylindrical liquid jets and air-assisted flat liquid sheets. Each fluid stream adopts several atomization regimes according to the operating conditions. These regimes as well as the significant parameters they depend on are listed. The main instability mechanisms, which control primary breakup processes, are rather well described. This review points out the internal geometrical nozzle characteristics and internal flow details that influence the atomization mechanisms. The contributions of these characteristics, which require further investigations to be fully identified and quantified, are believed to be the main reason of experimental discrepancies and explain a lack of universal primary breakup regime categorizations.

On the experimental investigation on primary atomization of liquid streams

This paper summarizes and compares the results of systematic research programs at two independent laboratories regarding the injection of cryogenic liquids at subcritical and supercritical pressures, with application to liquid rocket engines. Both single jets and coaxial jets have been studied. Cold flow studies provided valuable information without introducing the complexities of combustion. Initial studies utilized a single jet of cryogenic nitrogen injected into a quiescent room temperature nitrogen environment with pressures below and above the thermodynamic critical pressure of the nitrogen. Later, the work was extended to investigate the effects of a co-flowing gas. Parallel to this work, combustion studies with cryogenic propellants were introduced to understand high pressure coaxial injection phenomena with the influence of chemical reaction. Shadowgraphy and spontaneous Raman scattering were used to measure quantities such as growth rates, core lengths, turbulent length scales, fractal d...

Injection of fluids into supercritical environments

Understanding the complex environment of rocket chambers involves a good knowledge of injection phenomena and gives the designer the ability to employ time and cost saving modeling tools to design a higher performance engine. This project looked at injection processes in the supercritical regime using cryogenic nitrogen. Experimental data taken by 2-D Raman imaging allowed the comparison of density and divergence angels with computational models. These parameters provide much information about the jet development and mixing with the surrounding gas. The process used to derive divergence angles from Raman images proves difficult to compare directly with other techniques.

Raman Measurements of Cryogenic Injection at Supercritical Pressure

The performance of a nominal 200 W Hall effect thruster fueled by iodine vapor was evaluated. The system included a laboratory propellant feed system, a flight-model Hall thruster, and breadboard power processing unit. Operation of the Hall thruster with iodine vapor was stable on both short and long time scales, enabling measurements of thruster performance across a broad range of operation conditions. Performance was found to be comparable with xenon. At 200 W, thrust is 13 mN, anode specific impulse is 1500 s, and anode efficiency is 48%. Plume-current measurements indicate a profile typical of xenon Hall thrusters, while E B probe measurements indicate the presence of ionic dimers.

Performance Evaluation of an Iodine-Vapor Hall Thruster

Understanding the Complex environment of the rocket chamber involves good knowledge of the injection phenomena. Understanding the injection phenomena allows the rocket designer to employ time and cost saving modeling tools to design a higher performance rocket engine. The rocket engine performance is highly dependent on the injection processes within the chamber. This project looked at injection processes in the supercritical regime of the injected fluid, cryogenic nitrogen, in order to better understand realistic conditions in the rocket engines of today. The investigation considered test conditions from 4.0 to 6.0 MPa at two different injection velocities and temperatures. Experimental data taken by Raman imaging and Shadowgraphy were compared to computational models for these various test conditions. The test data allows comparisons of density, length scales and jet spreading angles. The results validate the computational models fairly well and agree with classical theory.This agreement allows further use of the models to predict injection behavior under these operating conditions. Also, further comparisons were made between the various test cases using the computational models to consider the effects of pressure, velocity and temperature variations on the cryogenic jet.

Characterization of Cryogenic Injection at Supercritical Pressure

Fundamental mechanisms of liquid jet breakup are identified and quantified. The quality of the atomization of liquids is an important parameter of many technological processes and is, e.g. for fuels and propellants critical in defining engine performance. This investigation takes a look at the jet behavior for a single injector element to determine the influence of the injection conditions on a round liquid jet. The study focuses on the atomization of a liquid forming a classical spray. To adjust the relative velocity between the liquid jet and the gaseous ambient a wind tunnel-like coaxial flow configuration was used. This made it possible to distinguish between effects of aerodynamic forces, chamber pressure and jet velocity, which determine the liquid Reynolds number and thereby the internal jet turbulence. Shadowgraphy and a novel image-processing approach was used to determine the jet surface characteristics: wavelength and amplitude. The absolute injection velocity of the jet seems to affect the structures the most with an increasing velocity causing the wavelengths to be smaller. An increase in chamber pressure seemed to have little influence on the jet with no relative velocity between the gas and liquid jet, but increased the amplitude and drop formation frequency at other testing conditions with relative motion. The wave amplitude trends provide information about the likelihood of drop formation but are limited in maximum size due to this breakup phenomenon of the jet. The study of the direction of the relative velocity demonstrated that injector performance cannot simply be described by scalar geometrical and operational injection parameters (e.g., We , Re or Oh), but has to include the injection direction of the atomizing fluids in relation to each other and to the ambient (e.g., combustion chamber). The undisturbed jet length and the spread angle were investigated, and a correlation for the droplet separation position was proposed. The data led to an extended classification of liquid jet breakup regimes. Large wave instabilities were experimentally analyzed and compared with linear stability theory.

Richard Branam

Papers

Injection of fluids into supercritical environments

Raman Measurements of Cryogenic Injection at Supercritical Pressure

Performance Evaluation of an Iodine-Vapor Hall Thruster

Characterization of Cryogenic Injection at Supercritical Pressure

Atomization characteristics on the surface of a round liquid jet