Vortex Gas Research Articles

Investigation of plasma swirl dynamics and effects of secondary gas injection in a vortex gas stabilized DC arc plasma

A numerical analysis of plasma gas swirl is performed to gain an understanding of fluid motion and the contact pattern of the plasma with a secondary gas. The secondary gas is injected into the plasma torch to either quench the hot plasma or alternatively to facilitate the heating of this secondary gas. A three-dimensional, transient fluid model is implemented in the OpenFOAM solver to examine the arc and plasma dynamics. The gases (plasma gas as well as secondary quench gases) are injected via 2 sets of 4 tangential ports spaced apart at a specified distance. The tangential injection create a plasma gas vortex that facilitates the swirling of the arc inside the torch. The plasma swirl propagates in the downstream direction where it mixes with the quench gas. The simulation results reveal that the operating arc current has a significant impact on the decay of plasma gas swirl in the downstream direction. At high arc currents, the rise in the joule heating enhances the cathode jet which suppresses the velocity gradients of the incoming swirl. A high inlet gas flow rates (e.g. Q = 18 slpm) have been observed to retain the inlet swirl intensity after passing through the arc column compared to the case with low gas flow rate (e.g. Q = 4.5 slpm). We have also examined the impact of secondary gas swirl on the quenching behaviour plasma torch plume by feeding the secondary gas in a co-current and counter-current with respect to the plasma gas rotational direction. The present simulation results help in understanding the effect of key process parameters controlling the flow and mixing behaviours in vortex stabilized plasma torch.

Rotation of the polarization of light as a tool for investigating the collisional transfer of angular momentum from rotating molecules to macroscopic gas flows

We present a detailed theoretical and experimental study of the rotation of the plane of polarization of light traveling through a gas of fast-spinning molecules. This effect is similar to the polarization drag phenomenon predicted by Fermi a century ago and it is a mechanical analog of the Faraday effect. In our experiments, molecules were spun up by an optical centrifuge and brought to the super-rotor state that retains its rotation for a relatively long time. Polarizability properties of fast-rotating molecules were analyzed considering the rotational Doppler effect and Coriolis forces. We used molecular dynamics simulations to account for intermolecular collisions. We found, both experimentally and theoretically, a nontrivial nonmonotonic time dependence of the polarization rotation angle. This time dependence reflects transfer of the angular momentum from rotating molecules to the macroscopic gas flow, which may lead to the birth of gas vortices. Moreover, we show that the long-term behavior of the polarization rotation is sensitive to the details of the intermolecular potential. Thus, the polarization drag effect appears as a novel diagnostic tool for the characterization of intermolecular interaction potentials and studies of collisional processes in gases.

Hydrodynamic modeling of non-swirling and swirling gas-particle two-phase turbulent flow using large eddy simulation

Flow structures of non-swirling and swirling particle-laden turbulence are modeled using the large eddy simulation. Four-way coupling to reveal the interaction mechanism of gas to particle, particle to gas and particle-particle collisions is adopted by a new proposed subgrid scale (SGS) particle kinetic energy-granular temperature model. Evolution of particle vortices, particle coherent structures and particle dynamics are predicted numerically, and comparisons are also conducted. Predictions are in good agreement with experimental measurements. For non-swirling flows, the typical coherent structures, and larger vortices at downward region of gas flow are dominantly. However, particle transport characteristics are significantly different, distinctly coherent structures and more vortices have not been generated in additions to fewer ones at far distance from entrance. Meanwhile, fewer coherent structures and vortices that dispersed at corner recirculation are observed for swirling flow. Compared to gas flow, the number of particle vortices is less and Reynolds stress and kinetic energy transport behaviors of particle flow are relatively independent due to inertia. Gas fluctuations of shear stresses are approximately twice greater than those of particle in non-swirling flows. A novelty finding is that turbulent fluctuations and instantaneous gas vortices of non-swirling flow are stronger than swirling case.

Large eddy simulation of aerosol particle dispersion mechanism in aircraft exhaust plume

Aerosol Infrared Stealth Technology is a novel method to suppress the infrared by releasing aerosol particles around the hot exhaust plume. However, the Infrared Radiation (IR) suppression rate is much lower in flight tests, which is caused by the unclear particle dispersion mechanism in high-speed non-isothermal shear flow. Hence, the large eddy simulation (LES) and particle discrete phase model (DPM) are coupled to solve the turbulent shear flow and the aerosol particle motion. The effects of aircraft flight speed on the turbulent flow characteristics and particle motion behaviors are systematically studied. It is found that the Mach rings appear in flight conditions, and they are getting blurry when increasing the flight speed. Both γ-like vortex and horseshoe-like vortex are observed when Ma < 1, and the horse-like could disappear when Ma > 1. The aerosol particle distribution is closely related to the turbulent gas vortices. The particle lateral dispersion is obviously restricted with the rising flight speed. Besides, the drag force plays a dominant role, the Saffman force and the thermophoretic force are of the same order of magnitude, all of these three forces are much greater than the gravity.

Turbulent Relaxation to Equilibrium in a Two-Dimensional Quantum Vortex Gas

We experimentally study emergence of microcanonical equilibrium states in the turbulent relaxation dynamics of a two-dimensional chiral vortex gas. Same-sign vortices are injected into a quasi-two-dimensional disk-shaped atomic Bose-Einstein condensate using a range of mechanical stirring protocols. The resulting long-time vortex distributions are found to be in excellent agreement with the meanfield Poisson-Boltzmann equation for the system describing the microcanonical ensemble at fixed energy $\cal{H}$ and angular momentum $\cal{M}$. The equilibrium states are characterized by the corresponding thermodynamic variables of inverse temperature $\hat{\beta}$ and rotation frequency $\hat{\omega}$. We are able to realize equilibria spanning the full phase diagram of the vortex gas, including on-axis states near zero-temperature, infinite temperature, and negative absolute temperatures. At sufficiently high energies the system exhibits a symmetry-breaking transition, resulting in an off-axis equilibrium phase at negative absolute temperature that no longer shares the symmetry of the container. We introduce a point-vortex model with phenomenological damping and noise that is able to quantitatively reproduce the equilibration dynamics.

Point-vortex dynamics in three-dimensional ageostrophic balanced flows

Geophysical turbulent flows, characterized by rapid rotation, quantified by small Rossby number, and stable stratification, often self-organize into a collection of coherent vortices, referred to as a vortex gas. The lowest-order asymptotic expansion in Rossby number is quasigeostrophy, which has purely horizontal velocities and cyclone–anticyclone antisymmetry. Ageostrophic effects are important components of many geophysical flows and, as such, these phenomena are not well modelled by quasigeostrophy. The next-order correction in Rossby number, which includes ageostrophic effects, is the so-called balanced dynamics. Balanced dynamics includes ageostrophic vertical velocity and breaks the geostrophic cyclone–anticyclone antisymmetry. Point-vortex solutions are well known in two-dimensional and quasigeostrophic dynamics and are useful for studying the vortex-gas regime of geophysical turbulence. Here, we find point-vortex solutions in fully three-dimensional continuously stratified QG $^{+1}$ dynamics, a particular formulation of balanced dynamics. Simulations of QG $^{+1}$ point vortices show several interesting features not captured by quasigeostrophic point vortices including significant vertical transport on long time scales. The ageostrophic component of QG $^{+1}$ point vortex point-vortex dynamics renders them useful in modelling flows where quasigeostrophy filters out important physical processes.

Сравнение современных моделей турбулентности для течения Тейлора-Куэтта

Swirling flows of fluids and gases are an integral part of many complex flows which are widely encountered in nature and technology. The working process of numerous technical devices (cyclones, vortex combustion chambers, air separators, gas and steam turbines, electric machines and generators, etc.) is generally determined by the laws of hydrodynamics and heat exchange of rotating flows. The problem of deriving general laws for a turbulent flow in the field of centrifugal forces provokes considerable scientific interest since it belongs to an underdeveloped field of hydromechanics. Therefore, mathematical modeling of swirling turbulent flows is still an urgent problem. In this paper, a comparative analysis of the advanced turbulence models for the Taylor -Couette flow is carried out. For this purpose, the linear turbulence models (SARC and SST-RC), the Reynolds stress method SSG/LRR-RSM-w2012, and a two-fluid model are used. The results obtained using these models are compared with each other and with known experimental data and direct numerical simulation results. The numerical results calculated with the use of turbulence models for the Taylor-Couette flow confirm that almost all the models adequately describe velocity profiles. However, they yield different turbulent viscosity values and, as a result, different friction coefficients. A comparison of the numerical results shows that the friction coefficient calculated using a two-fluid turbulence model is the closest to that obtained experimentally.

Process intensification of SO2/CO2 co-capture using microscale vortex flow contactor: Mass transfer behaviors, performance modeling, and flow simulation

To achieve the simultaneous desulfurization and decarbonization by a simple, compact, and low-energy-consumption way, a microscale vortex flow contactor was developed to determine the mass transfer performances of SO2/CO2 co-capture with NaOH solution. Their mass transfer behavior in varying operating parameters was examined in terms of the overall gas-phase mass transfer coefficient (Kga). It was found that the Kga ranges from 3.04 × 10−3 − 1.46 × 10−2 kmol/m3·s·kPa for SO2 capture and from 8.80 × 10−4 − 3.81 × 10−3 kmol/m3·s·kPa for CO2 capture, respectively. The Kga was mainly affected by the absorbent concentration, CO2 concentrations, and liquid–gas flow rates. Furthermore, a semi-empirical correlation was proposed to model their mass transfer performances. It showed the agreement between the measurement and prediction with R2 = 0.9851 for SO2 capture and R2 = 0.9904 for CO2 capture. Finally, the gas flow pattern inside the contactor was simulated using the computational fluid dynamics approach to address the role of gas vortex flow in mass transfer enhancement.

Simulation of 3D vortex jets in plasma torch application

The gas tunnel type plasma jet is an effective heat source for thermal processing applications such as plasma spraying. The key concept of gas tunnel plasma is its torch configuration, especially the role of the vortex gas flow. This is very important for the stability and energy density of the plasma jet produced. This work studied the flow of gas vortex in 3 dimensions using a finite element simulation. The simulation is based on solving partial differential equations where the incompressible Navier-Stokes equation is used as a governing equation that describes the laminar flow. The geometry of the plasma torch investigated is based on the design by A. Kobayashi. Key parameters investigated were gas pressure, velocity and profile of the vortex. It can be shown that the simulation produced results that are better matched to the experimental result than the calculation done in previous work. The simulation can also show detailed pictures of the vortex and its properties within the plasma chamber. This study could be useful in the design optimization of the plasma torch in the future.

Nitrogen fixation in an electrode-free microwave plasma

<h2>Summary</h2> Plasma-based gas conversion has great potential for enabling carbon-free fertilizer production powered by renewable electricity. Sustaining an energy-efficient plasma process without eroding the containment vessel is currently a significant challenge, limiting scaling to higher powers and throughputs. Isolation of the plasma from contact with any solid surfaces is an advantage, which both limits energy loss to the walls and prevents material erosion that could lead to disastrous soil contamination. This paper presents highly energy-efficient nitrogen fixation from air into NO_x by microwave plasma, with the plasma filament isolated at the center of a quartz tube using a vortex gas flow. NO_x production is found to scale very efficiently when increasing both gas flow rate and absorbed power. The lowest energy cost recorded of ∼2 MJ/mol, for a total NO_x production of ∼3.8%, is the lowest reported up to now for atmospheric pressure plasmas.

Streaming instability with multiple dust species – II. Turbulence and dust–gas dynamics at non-linear saturation

ABSTRACT The streaming instability is a fundamental process that can drive dust–gas dynamics and ultimately planetesimal formation in protoplanetary discs. As a linear instability, it has been shown that its growth with a distribution of dust sizes can be classified into two distinct regimes, fast- and slow-growth, depending on the dust-size distribution and the total dust-to-gas density ratio ϵ. Using numerical simulations of an unstratified disc, we bring three cases in different regimes into non-linear saturation. We find that the saturation states of the two fast-growth cases are similar to its single-species counterparts. The one with maximum dimensionless stopping time τs,max = 0.1 and ϵ = 2 drives turbulent vertical dust–gas vortices, while the other with τs,max = 2 and ϵ = 0.2 leads to radial traffic jams and filamentary structures of dust particles. The dust density distribution for the former is flat in low densities, while the one for the latter has a low-end cut-off. By contrast, the one slow-growth case results in a virtually quiescent state. Moreover, we find that in the fast-growth regime, significant dust segregation by size occurs, with large particles moving towards dense regions while small particles remain in the diffuse regions, and the mean radial drift of each dust species is appreciably altered from the (initial) drag-force equilibrium. The former effect may skew the spectral index derived from multiwavelength observations and change the initial size distribution of a pebble cloud for planetesimal formation. The latter along with turbulent diffusion may influence the radial transport and mixing of solid materials in young protoplanetary discs.

Arc dynamics in a vortex-stabilized non-transferred plasma torch with a tangential gas feed

The mode of anode arc attachments in a thermal plasma torch, such as constricted arc, multi-arc or diffused arc attachment, has operational as well as economic consequences on the use of plasma torches in industrial applications. The mode of arc attachment is strongly dependent on the gas flow dynamics, gas vortex formation and wall energy exchange. To gain more insight into arc attachment behaviour, this article presents a transient flow plasma model implemented using OpenFOAM computational fluid dynamics code to study the effect of gas swirl and arc rotation on modes of arc attachments. A tangentially fed plasma gas creates a gas swirl that helps to induce the arc rotation. The investigations were carried out at arc currents between 200–450 A and input gas flow rates in the range of 5–20 slpm. High gas swirl and low arc currents are found to favour constricted attachment on the anode surface. The local gas vortex driven by the plasma swirl creates hot spots inside the torch that dictate the arc attachment. The transition of arc attachment from the diffuse to constricted mode was simulated by imposing a step-change in the gas flow which shows the evolution of different arc attachment behaviour. During the transition from constricted to diffused attachment, the arc tends to undergo a transitionary multi-attachment mode at specific selected current and flow parameters.

Turbulence Theory: Imperfect, but Necessary

Key Points Provides a scaling theory for baroclinic turbulence Use a novel combination of vortex gas theory and theory of beta‐plane jets Good agreement with the simulations presented

Cracking Failure Analysis of Pipe to Flange Weld Joint

Cracking failures occurred frequently in blowdown pipe to flange weld joint, and the root crack appeared on the blowdown pipe side of the weld joint and extend into the base metal of pipe. To explore the reason why cracking failures occurred on the blowdown pipe side of the weld joint, the weld joint was analyzed via macroscopic and metallographic structure observation, and blowdown pipe was tested by physical and chemical property testing, and corrosion scale was analyzed by SEM, EDS, and XRD. The dent and thinner wall thickness of pipe are found in the internal pipe to flange weld joint. Widmanstatten metallographic structure is found in heat affected zone (HAZ), fusion zone (FZ) and weld metal zone (WZ) of weld joint. Stress concentration form in the weld joint at blowdown pipe side during welding due to the dent and different wall thickness. Under the action of gas vortex in the dent of weld joint, corrosive droplets aggravate the localized corrosion of the weld joint. Stress corrosion cracking (SCC) were initiated in FZ and WZ from the surface of weld joint and propagated into the base metal of pipe under the combined actions of stress resulting from weld process and during operation, and corrosion.

Blow-off characteristics of a premixed methane/air flame response to acoustic disturbances in a longitudinal combustor

In this work, we conducted systematic studies on a premixed methane/air flame response to acoustic disturbances in a longitudinal combustor with emphasis on the blow-off characteristics. Both experimental and numerical studies are performed to shed lights on the premixed flame-acoustics interaction in an acoustic resonant combustor. It is experimentally found that the flame separation and flame height increment are captured during the blow-off process. The vortices on the unburnt gas are revealed by the schlieren images, which suggest that the flame height increment is significantly affected by the evolution of the vortices in the unburnt gas. The blow-off sound pressure level is shown to be decreased significantly under an excitation frequency of 260 Hz. In this case, our PIV tests suggest that a weaker pressure disturbance can induce velocity fluctuations with a larger amplitude. To better understand the acoustics resonant dynamical response of the combustor and blow-off characteristics, numerical simulation is performed to investigate the combustor's acoustic modes. According to the acoustic modes of the combustor under various excitation frequencies, the flame is located in a low sound pressure region when the excitation frequency is 260 Hz, where the velocity amplitude is larger than that in other regions. The easily induced blow-off behavior of the flame at 260 Hz is caused by the critical velocity amplitude in this region. In general, the present work sheds light on the fundamental physics of a premixed flame response to acoustic disturbances.

Ti3C2Tx MXene-Loaded 3D Substrate toward On-Chip Multi-Gas Sensing with Surface-Enhanced Raman Spectroscopy (SERS) Barcode Readout.

Gas sensors lie at the heart of various fields ranging from medical to environmental sciences, and the demand of gas sensors is instantly expanding. However, in the face of complex gas samples, how to maintain high sensitivity while performing multiplex detection still puzzles the researchers. Here, by introducing Ti3C2Tx MXene into a microfluidic gas sensor with a three-dimensional (3D) transferable SERS substrate, a powerful gas sensor having both multiplex detecting ability and high sensitivity is demonstrated. The employ of MXene endows the sensor with a universal high adsorption efficiency for various gases while the generation of in situ gas vortices in the sophisticated nanomicro structure extends the molecule residence time in SERS-active area, both leading to the increased sensitivity. In the proof-of-concept experiment, a limit of detection (LOD) of 10-50 ppb was achieved for three typical volatile organic compounds (VOCs) according to the intrinsic SERS signals of gas molecules. Besides, the well-designed periodic 3D structure solves the general repeatability problem of SERS substrates. In addition, the detailed composition of gas mixture was revealed using classic least-square analysis (CLS) with an average accuracy of 90.6%. Further, a chromatic barcode was developed based on the results of CLS to read out the complex composition of samples visually.

Local flow dynamics in the motion of slug bubbles in a flowing mini square channel

For a slug flow flowing in a square capillary, the surrounding liquid flows through a thin film between the slug bubble and the wall and the corners of the channel and generates an asymmetric two-phase flow field with significant spatial variations. The current literature mainly discusses the bulk features of such a flow field, while local phenomena in these flows are not well known yet. In this study, the local dynamics of the liquid film and corner flows in a flowing square channel with a width of 3 mm were investigated by experiments and numerical simulations. Instantaneous velocity fields in the liquid flow were measured at the channel's central plane using particle tracking velocimetry (PTV). The obtained data were utilized to calculate the axial variations of the film and corner velocities. Three-dimensional numerical simulations with adaptive mesh refinement of the interface were performed for a series of flow conditions. Comparing the film velocity obtained from PTV and simulations demonstrates an overall good agreement between the results, with discrepancies close to the solid walls, where the PTV data is sporadic. Film and corner flows show rapid velocity changes adjacent to the minimum film area. Both experiments and simulations elucidate the backflow phenomena in the analyzed flow fields, where the liquid locally flows in the opposite streamwise direction. The backflow region shrinks as the liquid flow rate and speed of the bubble increase. Film flow shows significant pressure drops as it passes through the minimum film area. On the contrary, the corner flow does not demonstrate any rapid pressure fluctuations. The vortex structures’ identification revealed dominant vortex loops inside the gas and small-scale vortex filaments along the channel walls, generated from the liquid film rupture.

Experimental study of energy separation in a Ranque-Hilsch tube with a screw vortex generator

The paper considers the process of energy separation in a vortex tube with an original screw-type vortex generator, which was not previously used for this purpose. The effects of inlet pressure, pressure at hot and cold outlets of the tube, and mass fraction of the cold stream in the total flow on the temperature separation and process efficiency have been investigated. It is shown that increasing inlet pressure, the effect of the flow temperature separation increases, and an increase in the outlet pressure leads to a decrease in the effect. To determine the region of energy separation, the vortex gas flow in a screw swirl placed in a round tube and designed to swirl the flow in a Ranque-Hilsch vortex tube was experimentally studied. It has been experimentally shown that the temperature of the swirling gas flow leaving the screw swirl at the periphery increases with respect to the inlet gas temperature, and the temperature of the swirling gas flow in the axial region decreases. This indicates that energy separation occurs in the screw vortex generator.

Radial Fulde-Ferrell-Larkin-Ovchinnikov-like state in a population-imbalanced Fermi gas

The possibility of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state in a population imbalanced Fermi gas with a vortex is proposed. Employing the Bogoliubov-de-Gennes formalism we self-consistently determine the superfluid order parameter and the particle number density in the presence of a vortex. We find that as increasing population imbalance, the superfluid order parameter spatially oscillates around the vortex core in the radial direction, indicating that the FFLO state becomes stable. We find that the radial FFLO states cover a wide region of the phase diagram in the weak-coupling regime at $T=0$ in contrast to the conventional case without a vortex. We show that this inhomogeneous superfluidity can be detected as peak structures of the local polarization rate associated with the node structure of the superfluid order parameter. Since the vortex in the 3D Fermi gas with population imbalance has been already realized in experiments, our proposal is a promising candidate of the FFLO state in cold atom physics.

High-Efficiency and Energy-Saving Thermostat Combining Peltier Effect and Air Vortex Characteristics

This project uses the Peltier effect to optimize the temperature control system of the thermostat, uses semiconductors to generate temperature differences, and then forms a heat pump, uses the conversion between high and low taste energy to replace the compressor for refrigeration and temperature adjustment, and at the same time uses the gas vortex characteristics to exchange gas and Adapt and improve the heat dissipation and other aspects. The incubator designed in this project is more excellent in terms of energy saving, low noise and service life, and improves the utilization of energy and resources. At the same time, the temperature control of the incubator has the advantages of stability and accuracy, which can effectively solve the problem of food and medical the deterioration and loss of supplies.