Simulations and Measurements of Gas-Droplet Flows in Supersonic Jets Expanding into Vacuum

Simulations and measurements of gas-droplet multiphase flows in application to supersonic expansions into vacuum have been considered and compared with each other. The experiments involved exposing a control surface to a supersonic plume from two different nozzles and measuring the size distribution of droplets at various locations. The simulations are based on the direct simulation Monte Carlo modeling of vapor-phase flow with a one-way coupling of droplet momentum and energy. The droplet trajectories are computed for the experimental conditions for droplets originating at the throat and lip of two different nozzles. The maximum droplet radius reaching the control surface and the variation of droplet size with angle predicted by the trajectory computations agree well with the measurements using Optical Microscopy

Scaling law for direct current field emission-driven microscale gas breakdown

The effects of field emission on direct current breakdown in microscale gaps filled with an ambient neutral gas are studied numerically and analytically. Fundamental numerical experiments using the particle-in-cell/Monte Carlo collisions method are used to systematically quantify microscale ionization and space-charge enhancement of field emission. The numerical experiments are then used to validate a scaling law for the modified Paschen curve that bridges field emission-driven breakdown with the macroscale Paschen law. Analytical expressions are derived for the increase in cathode electric field, total steady state current density, and the ion-enhancement coefficient including a new breakdown criterion. It also includes the effect of all key parameters such as pressure, operating gas, and field-enhancement factor providing a better predictive capability than existing microscale breakdown models. The field-enhancement factor is shown to be the most sensitive parameter with its increase leading to a significant drop in the threshold breakdown electric field and also to a gradual merging with the Paschen law. The proposed scaling law is also shown to agree well with two independent sets of experimental data for microscale breakdown in air. The ability to accurately describe not just the breakdown voltage but the entire pre-breakdown process for given operating conditions makes the proposed model a suitable candidate for the design and analysis of electrostatic microscale devices

Direct simulation Monte Carlo modeling of e-beam metal deposition

Three-dimensional direct simulation Monte Carlo (DSMC) method is applied here to model the electron-beam physical vapor deposition of copperthin films. Various molecular models for copper-copper interactions have been considered and a suitable molecular model has been determined based on comparisons of dimensional mass fluxes obtained from simulations and previous experiments. The variable hard sphere model that is determined for atomic copper vapor can be used in DSMC simulations for design and analysis of vacuum deposition systems, allowing for accurate prediction of growth rates, uniformity, and microstructure

Molecular Models for DSMC Simulations of Metal Vapor Deposition

The direct simulation Monte Carlo (DSMC) method is applied here to model the electron‐beam (e‐beam) physical vapor deposition of copper thin films. A suitable molecular model for copper‐copper interactions have been determined based on comparisons with experiments for a 2D slit source. The model for atomic copper vapor is then used in axi‐symmetric DSMC simulations for analysis of a typical e‐beam metal deposition system with a cup crucible. The dimensional and non‐dimensional mass fluxes obtained are compared for two different deposition configurations with non‐uniformity as high as 40% predicted from the simulations

Visualizing Non-Equilibrium Flow Simulations using 3-D Velocity Distribution Functions

Scientific visualization techniques have been used to probe and understand better the physics of non-equilibrium flows. A visualization methodology for nonequilibrium flow simulations using 3-D velocity distribution functions (VDFs) is illustrated in application to various non-equilibrium flow problems. A one-dimensional normal shock wave problem is considered for two different upstream Mach numbers corresponding to weak and strong non-equilibrium flow conditions. The iso-surfaces of 3-D VDFs inside the shock wave obtainedusing various solution techniques including the ES-BGK method, DSMC technique, Mott-Smith solution, and the Navier-Stokes (NS) distribution functions using Chapman-Enskog theory are compared and contrasted. The visualization technique is extended to two-dimensional hypersonic flow at M-19 past a flat plate with sharp leading edge by comparing the isosurfaces of 3-D NS VDFs obtained at three different locations in the flowfield. The visualization of 3-D VDFs is shown to provide valuable information about the degree and direction of non-equilibrium for both 1-D and 2-D flows

Lyo Calculator – the Calculator of Primary Freeze-Drying

Freeze-drying (lyophilization) is an important process of a pharmaceutical solution state transformation for storage. Due to complexities of the process and costs related to experiments, numerical simulations of freeze-drying become more useful and cost-efficient for research and development of the process and the hardware. Many numerical models have been created to model separate steps of the drying process. However, these models are not available for any user. This work presents an open-source model of primary drying in a vial. Pseudo steady- state heat and mass transfer model was used to compute vapor pressure, drying time, product temperature, and percent of fluid dried as functions of time. To verify the numerical model, results were compared to experimental data of a mannitol (5%) solution and a numerical model created by M.J. Pikal. Results show accuracy within 20% at low chamber pressures and shelf temperatures. Simulations and analysis showed that the tool can be successfully used as a basic approximation of drying results for a single vial and constant chamber pressure and shelf temperature

Microspike Based Chemical/Electric Thruster Concept for Versatile Nanosat Propulsion

Small spacecraft that can be categorized as microsats, nanosats, and picosats has a strong potential for wide applications in communication, scientific experiments, and space exploration. In order to combine the advantages of both electric and chemical propulsion thrusters, a dual-mode microspike based thruster concept is proposed. For a fixed input power, it can operate in either a high-Isp mode or a high-thrust mode depending on the propulsive maneuver requirements. The hybrid thruster consists of a plug-annular cold or heated gas thruster in the chemical mode and a field emission thruster housed within the plug operating in the electric mode using a metallic propellant. The direct simulation Monte Carlo (DSMC) technique and the particle-in-cell technique are used to model the two different modes to estimate performance parameters of the thruster. The DSMC simulations show that the microspike nozzle can provide an improved specific impulse (Isp) at low Reynolds numbers when compared to a straight orifice or converging-diverging nozzle. The PIC simulations for the field emission thruster are shown to compute the current density, ion beam density, ion beam velocities and, hence, specific impulse and thrust accurately for conditions corresponding to earlier published experiment