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A comprehensive numerical study is performed to investigate gas flows inside the inlet of the atmosphere-breathing electric propulsion system using the direct simulation Monte Carlo method. The effects of inlet geometries and gas-surface interaction (GSI) accommodation coefficients on gas pressure and flow rate are analyzed in depth, and a comprehensive assessment of the compression and collection performances of inlets is conducted. Inlet geometries have noticeable effects on gas pressures inside inlets, and the gas pressures of the two concave inlets are the highest. Among the four inlets considered here, the rounded concave inlet performs the best in terms of compression and collection performances. Relative to the simple slope inlet, the rounded concave inlet achieves a rate of increase of over 120% in gas pressure and more than 110% in gas flow rate. GSI accommodation coefficients play a crucial role in both the pressure and mass flux inside the inlet, and the drop of the GSI accommodation coefficient from 1 to 0.2 brings about a considerable increase in the pressure and mass flux, achieving an increase of a factor of 8 and 5, respectively. Therefore, smoothing the inlet surface is an effective means of improving the compression and collection performances of inlets. [ABSTRACT FROM AUTHOR]

Based on the COMSOL Multiphysics simulation software, this study carried out modeling and numerical simulation for the evaporation process of liquid metal lithium in the vacuum free molecular flow state. The motion of lithium atoms in the evaporation process was analyzed through a succession of studies. Based on the available experimental values of the saturated vapor pressure of liquid metal lithium, the relationship between saturated vapor pressure and temperature of liquid lithium in the range of 600 K–900 K was obtained. A two-dimensional symmetric model (3.5 mm×20 mm) was established to simulate the transient evaporation process of liquid lithium at wall temperatures of 750 K, 780 K, 800 K, 810 K, 825 K, and 850 K, respectively. The effects of temperature, the evaporation coefficient, back pressure, and length-to-diameter ratio on the evaporation process were studied; the variation trends and reasons of the molecular flux and the pressure during the evaporation process were analyzed. At the same time, the evaporation process under variable wall temperature conditions was simulated. This research made the evaporation process of liquid lithium in vacuum molecular flow clearer, and provided theoretical support for the space reactor and nuclear fusion related fields. [ABSTRACT FROM AUTHOR]

This paper utilizes a low-power Hall thruster to investigate its flow field’s neutral flow dynamics. Specifically, under a free molecular flow with a high Knudsen number (Kn>10), we employ the flow conductance theory to investigate the interplay between the internal structural parameters of the thruster flow field and its conductance, which is directly reflected in the neutral gas distribution.Hence, first, we increase the number and diameter of the orifices in the buffer chamber so that the baffle effectively increases the flow field conductance, and thus the number density (nn) of the neutral gas in the discharge channel increases. This phenomenon reduces the ionization mean free path (λi) and thus increases the utilization rate (ηi) of the neutral gas. Then, we alter the baffle orifices distribution to an unequally spaced distribution and stagger the outlet orifices, resulting in a highly homogeneity neutral gas distribution in the circumferential direction, ultimately increasing the effective outlet area and reducing the backflow probability of the neutral gas molecules. This operation reduces the low-frequency oscillation amplitude and thus enhances the thruster’s operation stability. Finally, we explore the influence of the entire operating conditions, especially temperature and mass flow rate variations, on the universality of the simulation results. Overall, this work provides a new concept for optimizing the neutral flow field dynamics of a Hall thruster.

The growing interest in low earth orbit (LEO) applications demands for accurate modeling of orbital aerodynamics. But classical analytical models of aerodynamic coefficients in free molecule flow, such as the Sentman's model, Schamberg's model and Schaaf-Chambre model, were built upon over simplistic gas-surface interaction models, which degrade the fidelity of aerodynamic prediction. This work presents a new analytical model of orbital aerodynamic coefficients based on the state-of-the-art Cercignani–Lampis–Lord (CLL) gas-surface interaction model, where lobular quasi-specular scattering pattern and separate accommodation degree for different velocity components can be well captured. A key component of the new model is a rigorous function approximation solution of the reflected normal momentum flux based on the CLL model which is derived for the first time and is validated within 1% for any hypothermal flow and surface accommodation conditions. Closed-form analytical solutions of aerodynamic coefficients for simple convex geometries are obtained and exhibit high accuracy (within 0.1%) in typical LEO scenarios. The new analytical model surpasses the classical models in some important aspects, such as overcoming the diffuse scattering hypothesis constraint, considering the variation of normal momentum exchange with the surface incidence angle and being applicable in any hypothermal flow situation. In virtue of the advanced CLL model and feasibility of coupling with the panel method technique, the new analytical model is promising to provide more accurate predictions on the orbital aerodynamic coefficients for LEO applications. [ABSTRACT FROM AUTHOR]

We present the fabrication of three-dimensional inlets with gradually decreasing widths and depths and with nanopillars on the slope, all defined in just one lithography step. In addition, as an application, we show how these micro- and nanostructures can be used for micro- and nanofluidics and lab-on-a-chip devices to facilitate the flow and analyze single molecules of DNA. For the fabrication of 3D inlets in a single layer process, dose-modulated electron beam lithography was used, producing depths between 750 nm and 50 nm along a 30 μm long inlet, which is additionally structured with nanometer-scale pillars randomly distributed on top, as a result of incomplete exposure and underdevelopment of the resist. The fabrication conditions affect the slope of the inlet, the nanopillar density and coverage. The key parameters are the dose used for the electron beam exposure and the development conditions, like the developer’s dilution, stirring and development time. The 3D inlets with nanostructured pillars were integrated into fluidic devices, acting as a transition between micro and nanofluidic structures for pre-stretching and unfolding DNA molecules, avoiding the intrusion of folded molecules and clogging the analysis channel. After patterning these structures in silicon, they can be replicated in polymer by UV nanoimprinting. We show here how the inlets with pillars slow down the molecules before they enter the nanochannels, resulting in a 3-fold decrease in speed, which would translate to an improvement in the resolution for DNA optical mapping. [ABSTRACT FROM AUTHOR]

The accuracy and sensitivity of plasma generation predicted by several different chemical kinetics models is investigated in the context of weakly ionized hypersonic flowfields around the Radio Attenuation Measurement (RAM-C) vehicle. A computational fluid dynamics analysis is used to examine 13 independent trajectory points along the RAM-C II flight, and an assessment of the chemistry models is made by comparing results to available flight measurements. The limitations of making such comparisons with the raw flight data are established in detail, including the inherent shortcomings associated with interpolating the flight data to assess a single trajectory point. Two separate geometries are evaluated in this study, as the initial RAM-C geometry was altered during flight after its nose cap was pyrotechnically ejected. The blunter post-ejection geometry generates more electrons in the stagnation region. In general, good agreement is found between each chemistry model and flight data from both the electrostatic probe and reflectometer stations above 56 km. An expected sizable gap exists between the simulations and reflectometer data at lower altitudes. The impact of forward reaction rates, equilibrium constants, and number of species varies considerably based on altitude, velocity, and position along the body. [ABSTRACT FROM AUTHOR]

Atmosphere-Breathing Electric Propulsion (ABEP) is a promising propulsion concept for Very Low Earth Orbit (VLEO). Spacecraft (S/C) operating at lower altitudes has higher coverage and resolution. However, the rarefied gas causes drag on the S/Cs, so the propulsion is needed to overcome the drag. The ABEP system captures the rarefied ambient gas as the propellant, and collecting the gas efficiently by intake is significant for the ABEP application. Given the flow regime of ambient gas is free molecular flow, the Direct Simulation Monte Carlo (DSMC) method is used for numerical investigation in this paper. The intake schemes of three geometric configurations under three reflection models are proposed and simulated. The operating characteristics of different schemes are analyzed and the performance parameters are obtained. Based on the analysis results, an optimized intake scheme is proposed, achieving the compression ratio of 183.8 and capture efficiency of 74%. • Intake device: A passive mechanical, simple and feasible, low cost and high payload. • Collects and uses planet's atmosphere as propellant at very Low Earth Orbit (VLEO). • An optimized intake scheme is put forward, achieving the compression ratio of 183.8 and capture efficiency of 74%. • The DSMC method is used to analyze the performance of intake device, accurately and quickly. • The intake optimized can improve the performance of ABEP system. [ABSTRACT FROM AUTHOR]

This paper presents a simple model for slightly charged gas expanding into a vacuum from a planar exit. The number density, bulk velocity, temperature, and potential at the exit are given. The electric field force is assumed weaker than the convection term and is neglected in the analysis. As such, the quasi-neutral condition is naturally adopted and the potential field is computed with the Boltzmann relation. At far field, the exit degenerates as a point source, and simplified analytical formulas for flow and electric fields are obtained. The results are generic and offer insights on many existing models in the literature. They can be used to quickly approximate the flowfield and potential distributions without numerical simulations. They can also be used to initialize a simulation. Based on these results, more advanced models may be further developed. [ABSTRACT FROM AUTHOR]

Abstract Thanks to the excellent barrier property and fabrication accessibility, Parylene has been actively used in the microelectromechanical system. An ultra-thin Parylene film with thickness smaller than 100 nm is usually required to precisely tune the surface property of substrate or protect the functional unit. The commercially available regular Parylene deposition is a dimer mass determined chemical vapor deposition process with a high output (i.e. a low deposition precision in term of thickness control), around 1.6 μm/g (the ratio of film thickness to the loaded dimer mass) for the machine in the author’s lab. Therefore, it is hard to controllably and reliably prepare a Parylene film with thickness smaller than 100 nm, which requires a dimer mass less than 62.5 mg. This paper reported a method to prepare ultra-thin Parylene films with the nominal thickness down to 1 nm. A home-made deposition chamber was put inside and connected with the regular machine chamber through a microfabricated orifice with feature size smaller than 1 mm. According to the free molecular flow theory, the pressure inside the deposition chamber can be predictably and controllably reduced, thereby an ultra-low output of Parylene deposition, as low as 0.08 nm/g, was successfully obtained. The deposition precision was increased by 4 orders of magnitude compared to that of a direct Parylene deposition. This highly controllable and reliable ultra-thin Parylene deposition technique will find promising applications in flexible electronics and biomedical microdevices.

This paper presents a simple model for slightly charged gas expanding into a vacuum from a planar exit. The number density, bulk velocity, temperature, and potential at the exit are given. The electric field force is assumed weaker than the convection term and is neglected in the analysis. As such, the quasi-neutral condition is naturally adopted and the potential field is computed with the Boltzmann relation. At far field, the exit degenerates as a point source, and simplified analytical formulas for flow and electric fields are obtained. The results are generic and offer insights on many existing models in the literature. They can be used to quickly approximate the flowfield and potential distributions without numerical simulations. They can also be used to initialize a simulation. Based on these results, more advanced models may be further developed.

We present a novel analysis of gas damping in capacitive MEMS transducers that is based on a simple analytical model, assisted by Monte-Carlo simulations performed in Molflow+ to obtain an estimate for the geometry dependent gas diffusion time. This combination provides results with minimal computational expense and through freely available software, as well as insight into how the gas damping depends on the transducer geometry in the molecular flow regime. The results can be used to predict damping for arbitrary gas mixtures. The analysis was verified by experimental results for both air and helium atmospheres and matches these data to within 15% over a wide range of pressures.

High‐speed drag in a free molecular flow is still poorly understood despite playing an important role in a variety of physical situations, including meteor ablation in the upper atmosphere, the orbits of satellites, and the dynamics of cosmic dust grains. To measure drag at high speeds, small aluminum spheres 0.1–2.1 μm in radius were shot at 1–10 km/s into air, N2, Ar, and CO2 using an electrostatic dust accelerator. The measured drag coefficient in air is Γ = 1.29 ± 0.13, with similar values for the other gases. The measurement constrains the heating coefficient to Λ = 0.58 ± 0.37 in air assuming the gas molecules reflect diffusely from the particles' surfaces. The drag appears to be independent of the molecular structures of the four gases tested but has a slight dependence on molecular mass. The drag is also higher than frequently assumed, which has implications for the modeling of high‐speed objects in free molecular flows. Plain Language Summary: When an object is moving in a free molecular flow, the average distance that a gas molecule travels before colliding with another molecule is much larger than the object's size. The object's slowdown then depends on individual molecules interacting with its surface. High‐speed gas‐surface interactions are relevant to meteor entry into planetary atmospheres, satellite drag, and the dynamics of interstellar dust but are still not fully understood. To measure high‐speed drag, we use a dust accelerator to shoot spherical aluminum particles into a chamber holding rarefied air, nitrogen, argon, or carbon dioxide. The particles are timed as they fly through the chamber, allowing us to calculate the drag coefficient, which measures the amount of momentum transfer between the particles and the gas. We measured a drag coefficient of about 1.3 in all four gases, which is higher than usually assumed for meteors and spacecraft. Assuming that gas molecules bounce off of a particle's surface diffusely, we estimate that in air about 58% of the kinetic energy of the incoming gas molecules goes into heating each particle. The heating and drag appear to be independent of the molecular structure of the gas, although a slight dependence on molecular mass is observed. Key Points: The free molecular drag coefficient of micron‐sized aluminum particles was measured to be about 1.3 in air, N2, Ar, and CO2The measured drag corresponds to an energy accommodation or heating coefficient of about 0.6 in air assuming a diffuse reflection modelThe results are applicable to modeling basic drag physics at high speeds relevant to meteors, satellites, and cosmic dust [ABSTRACT FROM AUTHOR]

Highlights • Heat and mass transfer of an oscillatory rarefied gas flow in a square cavity is investigated. • Effects of frequency and speed of the oscillating lid are studied. • The hot to cold heat transfer could be dominant for a highly rarefied oscillatory flow. • The convective heat transfer could be dramatically enhanced by lid oscillation. • The average Nusselt number on the lid is reported in all the flow regimes. Abstract Although effective cooling of micro-electro-mechanical systems (MEMS) with oscillatory components is essential for reliable device operation, the role of oscillation on heat transfer remains poorly understood. In this work, heat and mass transfer of the oscillatory gas flow inside a square cavity is computationally studied by solving the Boltzmann model equation, i.e. the Shakhov model. The oscillation frequency of the lid and rarefaction and nonlinearity of the flow field are systematically investigated. Our results show that, when the oscillation frequency of the lid increases, the usual cold-to-hot heat transfer pattern for highly rarefied flow changes to hot-to-cold, which contradicts the well-known anti-Fourier (i.e. cold-to-hot) heat transfer in a non-oscillatory lid-driven cavity. In addition, the thermal convection will be dramatically enhanced by lid oscillation, which may play a dominant role in the heat transfer. Meanwhile, the average Nusselt number varies non-monotonically with the oscillation frequency, with the maximum occurring at the anti-resonance frequency. Finally, the average Nusselt number on the lid at various oscillation frequencies is found to reduce when the gas becomes more rarefied. These findings may be useful for the thermal design of MEMS. [ABSTRACT FROM AUTHOR]

One of the main problems that affects Ruhrstahl-Heraeus (RH) degasser circulation rate is the obstruction of the gas-injection nozzles. This study aims to elucidate the effects of obstructions in quantity and location using a physical model that depicts 16 injection nozzles on the up-leg distributed symmetrically in 2 rings (8 nozzles/ring). We simulated six, symmetric and nonsymmetric, obstruction conditions to gas flow. The gas-flow rate was varied from 30 to 130 NL/min. The results for symmetric obstructions when the gas-flow rate is kept constant, indicates that the circulation rate in the equipment does not significantly change. However, when these obstructions are non-symmetric, the circulation rate varies drastically, even when the gas-flow rate is kept constant. The analysis of the experimental data generated from this study allowed to determine an equation for the circulation rate that takes into consideration the number and distribution of blocked nozzles. [ABSTRACT FROM AUTHOR]

Inspired by natural photosynthesis in an organized assembly, compact H2-evolving molecular devices, which tether sensitizer and catalyst modules in one single molecule, present an opportunity to overcome the diffusion limit required for multi-component molecular systems, and increase intramolecular electron transfer rates from the photoactivated unit to the catalytic center to improve H2-evolving efficiency. Thereinto absolutely noble-metal free H2-evolving molecular devices are of particular interest because they don't contain precious and scarce noble-metal based components. This Frontier article focuses specifically on the recent advances in the design, synthesis, and photocatalytic properties of all-abundant-element molecular devices for photoinduced H2 generation via intramolecular processes. Some challenges and suggestions for future directions in this field are also illustrated. [ABSTRACT FROM AUTHOR]

Due to the beaming effect of inlet tube and reflection effect of turbine cover in the turbo molecular pump, the positional and angular distribution of incident gas molecules (IGM) on the turbine blades would no longer be uniformly distributed. Based on the test particle Monte Caro method, a new self-defining procedure is proposed and it can be more accurate to describe the status of IGM into turbine blades. The results show that the influence of rotor speed on the transmission probability is indistinctive. Meanwhile, the incident density between the tip and root of the blades is distinct. The positional beaming effect and the angular beaming effect are enhanced with the lengthening of the tube and less molecule diverge towards the tube wall. The positive transmission probability of first-stage turbine blades using our new method is different from those obeying the cosine distribution under the same number of incident molecules. The calculation results of new method verified by another procedure are reasonable and accurate. [ABSTRACT FROM AUTHOR]

In this paper, an experimental study on the flow patterns of two-phase air-light oil flow is performed in a 20 mm diameter pipe with a length of 6 m in different orientations with pipe angles in the range of -45 to +45. The flow regimes are captured by a high speed camera. In the experiments, the air with the viscosity of 0.019 mPa.s and the density of 1.2 Kg/m3 and a light oil with the viscosity of 2.6 mPa.s and the density of 840 Kg/m3 are used. During the experiments, different flow patterns are observed such as bubbly, slug, smooth stratified, wavy stratified, and annular flows. Flow regimes in different pipe inclination angles are inspected in two-phase airlight oil flow and flow pattern maps are proposed for every pipe inclination angle. In addition, a comprehensive study on major forces acted on dispersed phase are presented theoretically to perform a thorough discussion on effects of pipe inclination angle on transition boundaries between flow patterns in two-phase air-light oil flow. It is inferred that non-stratified flows are dominant flow patterns in the upward flows and stratified flows are dominant flow patterns in the downward flows. [ABSTRACT FROM AUTHOR]

Rarefication effects on jet impingement loads are studied by comparing recent new formulas at the collisionless flow limit and numerical simulations. The jet exit size is finite, and can be either planar or round. In the simulations, the jets have different degrees of rarefication, with a Knudsen (Kn) number ranging from 0 to infinity; i.e., the jet flows can be continuum, collisional, or collisionless. The comparison results indicate that (1) the new surface load formulas are accurate at the collisionless flow limit; (2) in general, the formulas offer upper limits for the peak loads; (3) however, it is improper to assert that local loads always decrease. The new formulas can offer fast estimations of impingement loads. This may be quite helpful for applications in space engineering by significantly reducing the amount of simulations and experiment costs. Those expressions explicitly include non-dimensional parameters, and their contribution and influence on the loads can be studied in a systematic manner (e.g., with a swift parameter study). [ABSTRACT FROM AUTHOR]

Recent work on studying rarefied background and jet flow interactions is reported. A new gaskinetic method is developed to investigate two closely related problems. The first problem is how a collisionless background flow can affect a highly rarefied jet flow. The rarefied jet and background flow conditions are assumed available and described with seven parameters. Gaskinetic theories are applied and formulas are obtained for the mixture properties. Simulations are performed to validate these expressions, and excellent agreement is obtained. The second problem is to recover the collisionless background and jet flow parameters with limited measurements. A group of linearized equations are derived for the flowfield properties. The solving process includes initial estimations on the seven parameters, followed with iterations. Numerical tests are performed and the results indicate the procedure is accurate and efficient. The new method and expressions can reduce the amount of experimental work and numerical simulations to analyze facility effects. Parameter studies with particle simulations may require several months; however, the new methods may require minutes. These methods can be used to quantify and predict jet performance, vacuum chamber designs and optimization. Applications may be for many societies using vacuum conditions. [ABSTRACT FROM AUTHOR]

Sputter deposition calculation adds a major cost to a complete electric propulsion plume simulation due to the significantly lower velocities of sputtered atoms compared to plume ions. Simulation time is reduced considerably by decoupling the sputtered atom transport from the full plume simulation and solving sputter deposition via the non-Maxwellian Radiosity View Factor model. In order to do this, the incident ion or sputtered atom particle data are compressed on the fly and used in the subsequent simulation to construct the velocity distribution functions of the outgoing sputtered atoms in an interpolatable form. This paper explores several different methods enabling this approach and demonstrates the benefit in terms of accuracy, speed, and memory usage. • Reduced the computation time of a conventional EP plume simulation by over 95 percent. • Achieved by separating the sputter deposition calculation from the plume simulation. • Used a particle merging algorithm and the Radiosity View Factor model. • Best to construct sputter atom VDF directly from incident ion particle data. • Convergence of the method with surface mesh elements and VDF resolution. [ABSTRACT FROM AUTHOR]

This paper utilizes a low-power Hall thruster to investigate its flow field's neutral flow dynamics. Specifically, under a free molecular flow with a high Knudsen number ( K n > 10), we employ the flow conductance theory to investigate the interplay between the internal structural parameters of the thruster flow field and its conductance, which is directly reflected in the neutral gas distribution. Hence, first, we increase the number and diameter of the orifices in the buffer chamber so that the baffle effectively increases the flow field conductance, and thus the number density (n n) of the neutral gas in the discharge channel increases. This phenomenon reduces the ionization mean free path (λ i) and thus increases the utilization rate (η i) of the neutral gas. Then, we alter the baffle orifices distribution to an unequally spaced distribution and stagger the outlet orifices, resulting in a highly homogeneity neutral gas distribution in the circumferential direction, ultimately increasing the effective outlet area and reducing the backflow probability of the neutral gas molecules. This operation reduces the low-frequency oscillation amplitude and thus enhances the thruster's operation stability. Finally, we explore the influence of the entire operating conditions, especially temperature and mass flow rate variations, on the universality of the simulation results. Overall, this work provides a new concept for optimizing the neutral flow field dynamics of a Hall thruster. • Presenting a 200 W Hall thruster and its gas distributor to investigate the dynamics of neutral gas under free molecular flow. • We analyze the influence of neutral gas on the operating performance of thruster and identified an optimization focus. • Using COMSOL Multiphysics, change the number and distribution of orifices and the outlet orifices' distribution to improve the neutral gas number density and azimuthal homogeneity. • Besides, we investigate the effect of different temperatures and mass flow rates on the simulation results. [ABSTRACT FROM AUTHOR]

Based on the previous researches of the Gas-Kinetic Unified Algorithm (GKUA) for flows from highly rarefied free-molecule transition to continuum, a new implicit scheme of cell-centered finite volume method is presented for directly solving the unified Boltzmann model equation covering various flow regimes. In view of the difficulty in generating the single-block grid system with high quality for complex irregular bodies, a multi-block docking grid generation method is designed on the basis of data transmission between blocks, and the data structure is constructed for processing arbitrary connection relations between blocks with high efficiency and reliability. As a result, the gas-kinetic unified algorithm with the implicit scheme and multi-block docking grid has been firstly established and used to solve the reentry flow problems around the multi-bodies covering all flow regimes with the whole range of Knudsen numbers from 10 to 3.7E−6. The implicit and explicit schemes are applied to computing and analyzing the supersonic flows in near-continuum and continuum regimes around a circular cylinder with careful comparison each other. It is shown that the present algorithm and modelling possess much higher computational efficiency and faster converging properties. The flow problems including two and three side-by-side cylinders are simulated from highly rarefied to near-continuum flow regimes, and the present computed results are found in good agreement with the related DSMC simulation and theoretical analysis solutions, which verify the good accuracy and reliability of the present method. It is observed that the spacing of the multi-body is smaller, the cylindrical throat obstruction is greater with the flow field of single-body asymmetrical more obviously and the normal force coefficient bigger. While in the near-continuum transitional flow regime of near-space flying surroundings, the spacing of the multi-body increases to six times of the diameter of the single-body, the interference effects of the multi-bodies tend to be negligible. The computing practice has confirmed that it is feasible for the present method to compute the aerodynamics and reveal flow mechanism around complex multi-body vehicles covering all flow regimes from the gas-kinetic point of view of solving the unified Boltzmann model velocity distribution function equation. [ABSTRACT FROM AUTHOR]