Inhomogeneous Gas Flow Research Articles

Modeling of gas jet flow into a liquid bath using the inhomogeneous poly-dispersed method

Precise modeling of gas injection into a liquid bath is essential for both determining the enormous interaction mechanisms between phases and promoting its better utilization in nuclear industry. For this purpose, an investigation on three CFD methods was performed and compared with the experimental results from air injected vertically into a water pool through a circular nozzle with diameter of 1.58 mm in this work. An inhomogeneous poly-dispersed method based on a bubble-size-dependent lift force model was integrated into the traditional gas-liquid two-fluid model for the simulation of gas-liquid jet flow with air injected vertically in a water bath. Results show that the computer resources consumption can be kept at a low level using the method. Compared with the mono-dispersed and homogeneous poly-dispersed models, the predicted results of the void fraction distribution obtained by using the inhomogeneous poly-dispersed model considering the adjustable coefficients for the break up and coalescence models show good agreements with those of the experiment, with a maximum deviation less than 20%. Hence, the model should be more reliable to predict two-phase flow parameters. Bubble distributions analyses of varying scales were also demonstrated, indicating that the adoption of both inhomogeneous gas flow field and lift force model able to change sign of its coefficient play an important role for predicting the gas injection into liquid bath correctly.

THz gas discharge in nitrogen as a source of ultraviolet radiation

This work presents the results of investigation of THz gas discharge in inhomogeneous flow of nitrogen. As a source of sub-millimetre radiation we used 40 kW@670 GHz gyrotron created in IAP RAS. It is shown that, despite the tendency of the discharge in electromagnetic wave beams to propagate towards the heating radiation from the breakdown region, in the case of a strongly inhomogeneous gas flow (nitrogen in our case), conditions can be created under which the discharge practically does not propagate and becomes point-like. The features of the glow of a THz discharge in a nitrogen flow in a wide range of background gas pressures are studied. The presence of a powerful afterglow in some bands of the second positive system of nitrogen at high pressures was demonstrated. The prospects of using such a discharge as a source of UV radiation are discussed.

Applications of the gas discharge sustained by the powerful radiation of THz gyrotrons

The terahertz frequency range currently remains the least studied from the point of view of gas-discharge physics. Recent progress in its mastering is associated, first of all, with the creation of powerful sources of terahertz radiation - gyrotrons. Nevertheless, the discharge sustained by the powerful radiation of the terahertz frequency range is of interest not only from the fundamental point of view (because of its lack of knowledge), but also from the applied one. This paper presents results of investigations aimed at studying the point-like THz discharge as a source of vacuum ultraviolet radiation and at research of CW THz plasma torch existing at atmospheric pressure and therefore having promising applications in various fields: plasma medicine, plasma chemistry, material processing etc. It is shown that it is possible to create a point-like plasma in an inhomogeneous gas flow with the density above the cut-off one (4·1015 cm−3 for 0.67 THz). It has been demonstrated that such plasma is an effective source of VUV and EUV radiation. Investigations of the atmospheric plasma torch, sustained by CW sub-terahertz radiation (1 kW @ 0.26 THz), demonstrated the possibility of the creation dense non-equilibrium plasma (2·1015 cm−3) with electron temperature in the range from 1 to 2 eV.

Waves in inhomogeneous plasmas and liquid and gas flows. Analogies between electro- and gas-dynamic phenomena

Two rather closely analogous classes of phenomena — plasma and hydrodynamic — are reviewed. Topics covered include: surface waves in an inhomogeneous plasma layer and in an inhomogeneous incompressible fluid flow; the instability of the tangential discontinuity of an inhomogeneous gas flow and the negative mass instability of an electron-beam-carrying plasma; radiative Cherenkov instabilities of an electron beam in a plasma and of a gas flow in a gas; the instability of plasma flows and of incompressible fluid flows with inhomogeneous velocity profiles. Special attention is given to the collisionless damping of plasma surface waves. It is shown that no such damping occurs for surface waves in gas flows.

Plasma density in discharge sustained in inhomogeneous gas flow by high-power radiation in the terahertz frequency range

We have measured the density of plasma (electron concentration) in discharge maintained in inhomogeneous argon flow under the action of high-power pulsed radiation of gyrotron (frequency, 0.67 THz; power 40 kW; pulse duration, 20–30 μs) in a range of background gas pressures in the discharge chamber from 10–3 to 300 Torr. The electron concentration at low pressures (10–3 to 7 Torr) was determined using Starkeffect induced broadening of the Hα atomic emission line (656.3 nm) of hydrogen present in discharge as a small impurity in residual gases. The maximum observed Stark broadening of the Hα line corresponded to a plasma density on the order of 2 × 1016 cm–3, which exceeded the critical value for the given frequency of radiation sustaining the discharge. At background pressures above 7 Torr, the plasma density was estimated from analysis of the spatiotemporal patterns and waveforms of discharge glow in the visible spectral range. These estimations gave electron concentrations on the level of (1–2) × 1015 cm–3.

Influence of the shielding gas flow on the removal of process by-products in the selective laser melting process

Selective laser melting is a promising additive manufacturing technology for the production of complex metal components. The technique uses metallic powder as a starting material and a laser for melting and building-up parts layer by layer. One crucial factor influencing the process stability and therefore the part quality is the shielding gas flow. In addition to the shielding properties of the inert atmosphere the gas flow is responsible for the removal of process by-products like spatter and welding fumes originating from the process zone. Insufficient removal or inhomogeneous gas flow distribution may lead to increased interaction between laser and process by-products. Consequences are attenuation of the laser spot as well as redeposition of this by-products on surfaces which are exposed to the laser afterwards. Firstly, a conclusion on all known process by-products is drawn. Secondly, based on these considerations the uniformity of the gas flow is investigated by the width of single welds. Furthermore process deviations are provoked by unfavorable gas flow conditions. Thirdly, the impact of this deviations on building surface and part quality is investigated by 3D confocal microscopy, microsections and ultrasonic testing. Finally, theoretical approach for the formation of these process deviations and arising material defects is presented.

Thermoacoustic Cryocooler with Multi-cold Fingers for High Temperature Superconductor

The commercialization of high temperature superconductors (HTS) in the near future becomes the focal topic nowadays. Consequently, larger cooling capacity cryocoolers are in urgent need to support such technology. Considering the inhomogeneous gas flow and temperature distribution in regenerator caused by the enlargement in diameters, the pulse tube cryocooler (PTC), although providing high reliability and high efficiency, has encountered a bottleneck to further increase its cooling capacity. This paper designs a new system composed of three parallel identical cold fingers driven by an opposed linear compressor. Experimental results have shown that each of the three PTCs has good consistency, performing as well as the situation when working separately. With charge pressure of 2.5MPa and working frequency of 54Hz, the three cold fingers reached the minimum temperature of 36.5K, 37.3K and 36.3K respectively, and obtained total cooling capacity of 230W at liquid nitrogen temperature with the system overall relative Carnot efficiency reaching 13.2%. It successfully avoided the problems coming out in the traditional large capacity PTCs and will have hopeful prospects when applied to the occasion of large cooling capacity.

Acoustical Instability of Inhomogeneous Gas Flows With Distributed Heat Release

Acoustical instability of the inhomogeneous heat-emitted gas flow in dependence on the Mach number is investigated. Obtained acoustical equations for perturbations of velocity, density, pressure, and temperature have diff erent forms in inhomogeneous media. Growth increments for these perturbations diff er from each other as well. They are significantly dependent on the Mach number and the flow direction. The features of sub- and supersonic acoustics are considered.

Generalized potentiality of inhomogeneous media flows

A method for specifying a class of potential flows of inhomogeneous continuous media is developed. The general approach is based on expanding a medium material symmetry group to a special volume-preserving group, allowing us to obtain a law for the conservation of vorticity and, when there is no vorticity, to derive the unsteady Bernoulli equation. As illustrations, plane steady stationary flows of an inhomogeneous incompressible fluid and variable-entropy gas are considered. The problem of an inhomogeneous gas flow around a wedge yielding the formation of a shock wave is solved.

Non-Newtonian Viscosity of Complex-Plasma Fluids

Investigations of shear flows in three-dimensional complex-plasma fluids produced in a dc discharge were carried out. The shear was induced either by an inhomogeneous gas flow or by a laser beam. The viscosity of complex plasmas was measured over a broad range of shear rates, up to the hydrodynamic limit when the discreteness becomes important. Analysis of the measurements reveals non-Newtonian behavior of complex plasmas accompanied by substantial shear thinning.

Decoupled and Inhomogeneous Gas Flows in S0 Galaxies

A recent analysis of the Einstein sample of early-type galaxies has revealed that at any fixed optical luminosity LB, S0 galaxies have lower mean X-ray luminosity LX per unit LB than elliptical galaxies. Following a previous analytical investigation of this problem (Ciotti & Pellegrini), we have performed two-dimensional numerical simulations of the gas flows inside S0 galaxies in order to ascertain the effectiveness of rotation and/or galaxy flattening in reducing the LX/LB ratio. The flow in models without supernova (SNIa) heating is considerably ordered, and essentially all the gas lost by the stars is cooled and accumulated in the galaxy center. If rotation is present, the cold material settles in a disk on the galactic equatorial plane. Models with a time-decreasing SNIa heating host gas flows that can be much more complex. After an initial wind phase, gas flows in energetically strongly bound galaxies tend to reverse to inflows. This occurs in the polar regions, while the disk is still in the outflow phase. In this phase of strong decoupling, cold filaments are created at the interface between inflowing and outflowing gas. Models with more realistic values of the dynamical quantities are preferentially found in the wind phase with respect to their spherical counterparts of equal LB. The resulting LX of this class of models is lower than in spherical models with the same LB and SNIa heating. At variance with cooling flow models, rotation is shown to have only a marginal effect in this reduction, while the flattening is one of the driving parameters for such underluminosity, in accordance with the analytical investigation.

The use of the finite-element method to calculate acoustic fields in inhomogeneous gas flows

A numerical method for calculating linear acoustic fields in axisymmetric regions containing a subsonic irrotational gas flow is developed. The boundaries of the regions under consideration can contain portions corresponding to open space. The calculation scheme is based on the finite-element method, with the use of the variational principle. The time-averaged stationary parameters of the gas flow, necessary to calculate the acoustic fields excited in it, are found by solving the non-linear equations of gas dynamics using an iterative procedure similar to Newton's method.