Internal Gas Flows Research Articles

Wear resistance of the main friction pairs of serial and upgraded plate type vacuum pumps

Plate type vacuum pumps are used in various industries and agriculture due to their high specific efficiency and simple design. The disadvantage of this type of pump is a low inter-repair resource, which is caused by an increase in internal gas flows due to wear of the main friction pair “cast iron-textolite”. To modernize the design of a plate-type vacuum pump, the authors proposed using an anti-friction material PTFE, and as a result, replacing the main friction pair “cast iron-textolite” with friction pairs “cast iron-PTFE” and “textolite - PTFE”. Therefore, this article presents the results of a comparative experiment to determine the wear resistance of friction pairs of a serial and upgraded pump. Analysis of the experiment results showed that the “cast iron-PTFE” and “textolite-PTFE” friction pairs are more wear-resistant when changing the speed of relative movement and pressure in the contact zone than the “cast iron-textolite” friction pair, but they are more demanding to the presence of abrasive in the contact zone. The use of PTFE material is promising as a material for the surfaces of friction pairs of vacuum pumps. However, it requires protection measures and cleaning of the contact zone from wear products of the pump friction pairs themselves and the abrasive coming from the pumped medium.

Implementation of Linear Stability Theory on Hollow Cone-shaped Liquid Sheet

Surface instability of a swirling liquid sheet emanating from a centrifugal injector in presence of external and internal gas flows is studied in this paper. A three-dimensional flow for the liquid sheet and two-dimensional flows for external and internal gas flows are considered. The set of equations involved in this analysis differs from the earlier analyzes. In previous studies, a cylindrical liquid sheet has been considered to implement the linear theory but in this study, the linear stability theory is implemented on a cone-shaped liquid sheet for different cone angles. Actually more over than axial and tangential movements, the radial movements of liquid sheet and gas flows are considered in the present model. Due to complexity of the derived governing equations, semi-analytical and numerical methods were applied to solve them. The case study is oxidizer injector of rocket engines. Implementation of linear stability theory on a hollow cone-shaped liquid sheet better can predict instability phenomenon than the general linear stability analysis for this type of liquid sheets. The results show very close agreement with available experimental data.

Numerical simulation and design methodology of a novel asymmetric cylinder profile for sliding vane vacuum pumps

Large built-in volumetric ratio is the major obstacle to improve the extraction performance of vacuum pumps. In order to achieve larger built-in volumetric ratio of the sliding vane vacuum pump (SVVP), a novel kind of asymmetric cylinder profile, called quadratic spiral, was proposed in this study. A mathematical model of the asymmetric cylinder profile and its general design methodology were presented. Moreover, the whole geometric structures of asymmetric SVVPs with single-chamber, double-chamber, and three-chamber were designed according to the proposed design methodology. Also, the comparisons between asymmetric SVVPs and symmetric SVVPs were conducted in terms of the suction and discharge volumes, and built-in volumetric ratio. The results indicated that the larger built-in volumetric ratio was realized by reducing discharge volume with the suction volume keeping constant. Numerical simulations of internal gas flows of the asymmetric SVVPs and symmetric SVVPs were carried out to compare their working characteristics. The study results showed that the asymmetric SVVPs has better working performance in terms of variational trends of pressure, smaller pressure fluctuation and energy saving.

Influence of three-dimensional transverse micro-ridges on the Poiseuille number in a gaseous slip flow

Micro-ridges are regular micro-engineering structures protruding outward from the microchannel wall surface. The role of these micro-ridges in changing the friction factor (f) of a microchannel with internal rarefied gas flows is not explored in the literature. In this work, we present three-dimensional numerical simulations in a microchannel containing ridges arranged transverse to the flow direction. The length of the ridges is varied with respect to the pitch of the ridges; this ratio being christened as ridge fraction (δ). The effect of δ, ridge height ratio (h/H), Reynolds number (Re), Knudsen number (Kn) and Tangential momentum accommodation coefficient (αv) on the flow friction in terms of Poiseuille number (fRe) is investigated. In the continuum and slip regimes, it is found that fRe decreases with an increase in δ and h and a significant reduction in flow friction was shown with respect to a straight microchannel. There exists a critical height after which decrease in fRe does not occur. Further, fRe was observed to be a strong function of Re at high δ, which is attributed to the intensity of acceleration–deceleration experienced by the gas during the flow. Also, fRe is established to be directly proportional to αv. The various flow characteristics are scrutinised where vortices are found trapped inside the ridge affecting the dynamics of flow in the ridges, whose size increases on increasing the gas rarefaction. This comprehensive investigation of behaviour of slip flow in complex microchannels will be useful in designing micro-devices with reduced friction.

Radiation Hydrodynamical Turbulence in Protoplanetary Disks: Numerical Models and Observational Constraints

Abstract Planets are born in protostellar disks, which are now observed with enough resolution to address questions about internal gas flows. Magnetic forces are possibly drivers of the flows, but ionization state estimates suggest that much of the gas mass decouples from magnetic fields. Thus, hydrodynamical instabilities could play a major role. We investigate disk dynamics under conditions typical for a T Tauri system, using global 3D radiation-hydrodynamics simulations with embedded particles and a resolution of 70 cells per scale height. Stellar irradiation heating is included with realistic dust opacities. The disk starts in joint radiative balance and hydrostatic equilibrium. The vertical shear instability (VSI) develops into turbulence that persists up to at least 1600 inner orbits (143 outer orbits). Turbulent speeds are a few percent of the local sound speed at the midplane, increasing to 20%, or 100 m s−1, in the corona. These are consistent with recent upper limits on turbulent speeds from optically thin and thick molecular line observations of TW Hya and HD 163296. The predominantly vertical motions induced by the VSI efficiently lift particles upward. Grains 0.1 and 1 mm in size achieve scale heights greater than expected in isotropic turbulence. We conclude that while kinematic constraints from molecular line emission do not directly discriminate between magnetic and nonmagnetic disk models, the small dust scale heights measured in HL Tau and HD 163296 favor turbulent magnetic models, which reach lower ratios of the vertical kinetic energy density to the accretion stress.

Assessment and development of the gas kinetic boundary condition for the Boltzmann equation

Gas–surface interactions play important roles in internal rarefied gas flows, especially in micro-electro-mechanical systems with large surface area to volume ratios. Although great progress has been made to solve the Boltzmann equation, the gas kinetic boundary condition (BC) has not been well studied. Here we assess the accuracy of the Maxwell, Epstein and Cercignani–Lampis BCs, by comparing numerical results of the Boltzmann equation for the Lennard–Jones potential to experimental data on Poiseuille and thermal transpiration flows. The four experiments considered are: Ewartet al.(J. Fluid Mech., vol. 584, 2007, pp. 337–356), Rojas-Cárdenaset al.(Phys. Fluids, vol. 25, 2013, 072002) and Yamaguchiet al.(J. Fluid Mech., vol. 744, 2014, pp. 169–182; vol. 795, 2016, pp. 690–707), where the mass flow rates in Poiseuille and thermal transpiration flows are measured. This requires that the BC has the ability to tune the effective viscous and thermal slip coefficients to match the experimental data. Among the three BCs, the Epstein BC has more flexibility to adjust the two slip coefficients, and hence for most of the time it gives good agreement with the experimental measurements. However, like the Maxwell BC, the viscous slip coefficient in the Epstein BC cannot be smaller than unity but the Cercignani–Lampis BC can. Therefore, we propose to combine the Epstein and Cercignani–Lampis BCs to describe gas–surface interaction. Although the new BC contains six free parameters, our approximate analytical expressions for the viscous and thermal slip coefficients provide useful guidance to choose these parameters.

Molecular tagging velocimetry by direct phosphorescence in gas microflows: Correction of Taylor dispersion

Molecular tagging velocimetry is a little-intrusive technique based on the properties of specific molecules able to emit luminescence once properly excited. Several variants of this technique have been successfully developed for analyzing external gas flows or internal gas flows in large systems. There is, however, very few experimental data on molecular tagging velocimetry for gas flows in mini or microsystems, and these data are strongly affected by the molecular diffusion of the tracer molecules. In the present paper, it is demonstrated that the velocity field in gas microflows cannot be directly deduced from the measured displacement field without taking into account Taylor dispersion effects. For that purpose, the benchmark case of a Poiseuille gas flow through a rectangular channel 960μm in depth is experimentally investigated by micro molecular tagging velocimetry using acetone vapor as a molecular tracer. An appropriate reconstruction method based on the advection diffusion equation is used to process the data and to correctly extract the velocity profiles. The comparison of measured velocities with flowrate data and with theoretical velocity profiles shows a good agreement, whereas when the reconstruction method is not implemented, the extracted velocity field exhibits qualitative and quantitative anomalies, such as a non-physical slip at the walls. The robustness of the reconstruction method is demonstrated on flows with light and heavy molecular species, namely helium and argon, at atmospheric as well as at lower pressure conditions, for which diffusion effects are stronger. The obtained results represent an encouraging step for future analysis of rarefied gas microflows.

Response to “Comment on ‘Data on Internal Rarefied Gas Flows”’ [J. Phys. Chem. Ref. Data 44, 036101 (2015)

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Comment on “Data on Internal Rarefied Gas Flows” [J. Phys. Chem. Ref. Data 27, 657 (1998)

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Multiscale simulation of non-isothermal microchannel gas flows

This paper describes the development and application of an efficient hybrid continuum-molecular approach for simulating non-isothermal, low-speed, internal rarefied gas flows, and its application to flows in Knudsen compressors. The method is an extension of the hybrid continuum-molecular approach presented by Patronis et al. (2013) [4], which is based on the framework originally proposed by Borg et al. (2013) [3] for the simulation of micro/nano flows of high aspect ratio. The extensions are: 1) the ability to simulate non-isothermal flows; 2) the ability to simulate low-speed flows by implementing a molecular description of the gas provided by the low-variance deviational simulation Monte Carlo (LVDSMC) method; and 3) the application to three-dimensional geometries. For the purposes of validation, the multiscale method is applied to rarefied gas flow through a periodic converging-diverging channel (driven by an external acceleration). For this flow problem it is computationally feasible to obtain a solution by the direct simulation Monte Carlo (DSMC) method for comparison: very close agreement is observed.The efficiency of the multiscale method, allows the investigation of alternative Knudsen-compressor channel configurations to be undertaken. We characterise the effectiveness of the single-stage Knudsen-compressor channel by the pressure drop that can be achieved between two connected reservoirs, for a given temperature difference. Our multiscale simulations indicate that the efficiency is surprisingly robust to modifications in streamwise variations of both temperature and cross-sectional geometry.

Shrouded plasma spray of Ni–20Cr coatings utilizing internal shroud film cooling

It is widely reported that oxidation during plasma thermal spraying leads to degradation in the coating corrosion and wear resistant properties. A number of shrouding systems have been developed to minimize oxidation to enable production of low oxide content coatings while spraying in air. The shroud design in this work was based on a detailed review of the mechanisms of oxidation during plasma spraying and the result of past shrouded plasma spray trials and simulations. It incorporated a novel film cooling concept within the shroud which was intended to cool the inner shroud wall and reduce energy loss from the plasma jet, while at the same time causing vaporized material to condense in the gas film and flow out of the shroud rather than building up on the shroud walls. Coatings were sprayed without the shroud, with the solid shroud alone (no shroud gas flow) and with various settings of internal and external shroud gas flows. The effect of these variables on the coating quality and oxide content was assessed by XRD, SEM and microhardness testing. The solid shroud was found to be effective in reducing in-flight oxidation but less effective in reducing layer to layer oxidation during spraying. High external shroud gas flows had the most significant effect on reducing coating oxidation. The internal shroud gas flow used to generate the film cooling effect was ineffective in significantly reducing internal dust build-up within the shroud and had no definitive effect on the coating properties. The shroud operation and mechanisms of oxidation are discussed to highlight areas of improvement for future designs.

Micro molecular tagging velocimetry for analysis of gas flows in mini and micro systems

A new micro molecular tagging velocimetry (μMTV) setup has been developed to analyze velocity fields in confined internal gas flows. MTV is a little-intrusive velocimetry technique. It relies on the properties of molecular tracers which can experience relatively long lifetime luminescence once excited by a laser beam with an appropriate wavelength. The technique has been validated for acetone seeded flows of argon inside a 1 mm depth rectangular minichannel, with a multilayer design offering two optical accesses. Velocity profiles have been obtained using a specific data reduction process, with a resolution in the order of 15 μm. The experimental data are compared to theoretical velocity profiles of compressible pressure-driven flows. A good agreement is observed, except close to the walls, where the accuracy would still need to be improved. Following these first results obtained at atmospheric pressure, the influence of pressure on the luminescence intensity of acetone molecules is analyzed. The obtained data lead to a discussion of MTV applicability to rarefied flows and its possible use for a direct measurement of velocity slip at the channel walls.

Hybrid continuum–molecular modelling of multiscale internal gas flows

We develop and apply an efficient multiscale method for simulating a large class of low-speed internal rarefied gas flows. The method is an extension of the hybrid atomistic–continuum approach proposed by Borg et al. (2013) [28] for the simulation of micro/nano flows of high-aspect ratio. The major new extensions are: (1) incorporation of fluid compressibility; (2) implementation using the direct simulation Monte Carlo (DSMC) method for dilute rarefied gas flows, and (3) application to a broader range of geometries, including periodic, non-periodic, pressure-driven, gravity-driven and shear-driven internal flows. The multiscale method is applied to micro-scale gas flows through a periodic converging–diverging channel (driven by an external acceleration) and a non-periodic channel with a bend (driven by a pressure difference), as well as the flow between two eccentric cylinders (with the inner rotating relative to the outer). In all these cases there exists a wide variation of Knudsen number within the geometries, as well as substantial compressibility despite the Mach number being very low. For validation purposes, our multiscale simulation results are compared to those obtained from full-scale DSMC simulations: very close agreement is obtained in all cases for all flow variables considered. Our multiscale simulation is an order of magnitude more computationally efficient than the full-scale DSMC for the first and second test cases, and two orders of magnitude more efficient for the third case.

Velocity field measurements in gas phase internal flows by molecular tagging velocimetry

The goal of the present work is to implement Molecular Tagging Velocimetry (MTV) for the analysis of internal gas flows in mini-channels. A MTV experimental setup has been designed. Tagging and detecting steps are respectively insured by a UV laser and a CCD camera coupled to intensified relay optics. A specific channel with 1 × 5 mm2 rectangular cross section has been designed and equipped with integrated temperature sensors along its 20 cm length. It has been manufactured in PEEK (PolyEtherEther-Ketone) and Suprasil® optical windows have been integrated for the tagging access. Image processing allows extraction of velocity profiles for a pressure driven steady flow of argon through this channel. These profiles are compared to the theoretical profiles of laminar flows and the accuracy of the method is discussed. The MTV potential for the analysis of internal gaseous flows is commented on, with a discussion on perspectives for velocity measurement in rarefied flows and direct access to slip velocity at the walls.

Convective heat transfer in pressure-driven nitrogen slip flows in long microchannels: The effects of pressure work and viscous dissipation

An analytical and numerical study is carried out to examine the convective heat transfer in two-dimensional pressure-driven nitrogen slip flows in long microchannels, whose length-to-height ratios are above 500. The momentum and the energy equations are solved, where variable properties, rarefaction that involves velocity slip, thermal creep and temperature jump, pressure work, and viscous dissipation are all taken into account. Nitrogen is assumed to be a perfect gas. The effects of pressure work and viscous dissipation, which are particularly significant for long microchannels, are examined by analyzing the uniform wall temperature and the uniform wall heat flux cases. It is found that the degree of rarefaction, which is characterized by the Knudsen number, is the key factor that determines the relative importance of pressure work and viscous dissipation. It is demonstrated that, for perfect gases, rarefaction promotes the conversion of internal energy to mechanical energy. Specifically, regardless of the fluid field development, pressure work and viscous dissipation cancel out in the absence of rarefaction, while pressure work is greater than viscous dissipation with rarefaction and its dominance increases as the Knudsen number increases. It is shown that the combination of pressure work and viscous dissipation makes a significant impact on the Nusselt number in both the continuum and the rarefaction cases. Therefore, it is concluded that for convective heat transfer in internal gas flows, both pressure work and viscous dissipation need to be considered in analysis.

Chemical evolution of bulges at high redshift

AbstractWe present a new class of hydrodynamical models for the formation of bulges (either massive elliptical galaxies or classical bulges in spirals) in which we implement detailed prescriptions for the chemical evolution of H, He, O and Fe. Our results hint toward an outside-in formation in the context of the supernovae-driven wind scenario. The build-up of the chemical properties of the stellar populations inhabiting the galactic core is very fast. Therefore we predict a non significant evolution of both the mass-metallicity and the mass-[α/Fe] relations after the first 0.5 − 1 Gyr. In this framework we explain how the observed slopes, either positive or negative, in the radial gradient of the mean stellar [α/Fe], and their apparent lack of any correlation with all the other observables, can arise as a consequence of the interplay between star formation and metal-enhanced internal gas flows.

Assessment of a Second-Moment Closure Model for Strongly Heated Internal Gas Flows

Both low- and high-Reynolds-number versions of the stress-ω model of Wilcox (Turbulence Modeling for CFD, 2nd ed., DCW Industries, Inc.) were used to predict velocity and heat transfer data in a high-heat-flux cylindrical tube for which fluid properties varied strongly with temperature. The results indicate that for accurate heat transfer calculations under the conditions considered in this study, inclusion of low-Reynolds-number viscous corrections to the model are essential. The failure of the high-Reynolds-number model to accurately predict the wall temperature was attributed to an overprediction of the near-wall velocity.

Zinc deposition experiments for validation of direct-simulation Monte Carlo calculations of rarefied internal gas flows

Specially designed experiments were performed for the validation of direct simulation Monte Carlo calculations of rarefied internal gas flows. Hot zinc vapor was released into a cold-wall deposition chamber filled with a background gas at low pressure in which two silicon wafers were mounted. The deposited zinc layers on these wafers have been measured in four different experiments, with varying background gas species (argon and helium) and background pressures (0.031–0.25Pa). The background gas conditions were chosen such that the Knudsen numbers based on the zinc mean free path range from 0.34 to 2.8. The experimental results have been compared to simulation results.

Numerical analysis of gas–surface scattering effect on thermal transpiration in the free molecular regime

The microscopic reversibility of gas–surface collisions is discussed and analyzed using the direct simulation Monte Carlo method for different scattering kernels. A testing procedure for the reversibility analysis is described. Then, the thermal transpiration phenomenon is studied numerically for free molecular flow through short and long capillaries using Maxwell, Cercignani–Lampis, and Epstein scattering kernels. Comparison with experimental data shows that the Cercignani–Lampis and Epstein kernels allow a more accurate description of non-isothermal internal rarefied gas flows than the commonly used Maxwell model.

New directions in fluid dynamics: non-equilibrium aerodynamic and microsystem flows.

Fluid flows that do not have local equilibrium are characteristic of some of the new frontiers in engineering and technology, for example, high-speed high-altitude aerodynamics and the development of micrometre-sized fluid pumps, turbines and other devices. However, this area of fluid dynamics is poorly understood from both the experimental and simulation perspectives, which hampers the progress of these technologies. This paper reviews some of the recent developments in experimental techniques and modelling methods for non-equilibrium gas flows, examining their advantages and drawbacks. We also present new results from our computational investigations into both hypersonic and microsystem flows using two distinct numerical methodologies: the direct simulation Monte Carlo method and extended hydrodynamics. While the direct simulation approach produces excellent results and is used widely, extended hydrodynamics is not as well developed but is a promising candidate for future more complex simulations. Finally, we discuss some of the other situations where these simulation methods could be usefully applied, and look to the future of numerical tools for non-equilibrium flows.