Turbulent Flow Mode Research Articles

Comparison of characteristics of atmospheric pressure plasma jets using argon and helium working gases

Dielectric Barrier Discharge (DBD) plasma jets have been studied extensively in recent years because of their wide range of applications. In this work, we studied the performance of a DBD cold atmospheric pressure plasma jet with neutral argon gas experimentally and compared the produced DBD argon plasma with the helium jet in an equivalent operation. We investigated the effect of applied voltage, flow rate, and electrodes configurations on the argon and helium plume length and analyzed the physical parameters of the plasma plume, including discharge voltage, average gas, temperatures of the discharge gap, and jet velocity in plasma-on mode via Pitot glass tube as a speedometer. The results showed that by increasing any of the applied voltage or the flow rate at low values in different electrodes arrangements, plumes shape are more homogeneous and their length increases, but the helium plasma plume is transient from laminar to turbulent flow mode at higher flow rates. The temperatures of the discharge gap between the electrodes and their surroundings in the argon jet had a large increase over the helium jet in an equi-operational. Speedometer results show that plasma discharge affects the speed of argon jet more than helium.

Simulation of the mixing process in a slot channel with jet injection

The article presents the results of modeling in a two-dimensional formulation of the hydrodynamic process of mixing two streams of Newtonian fluids in a channel with jet injection. The mixing process takes place in a turbulent flow mode, without chemical and thermal interaction. The article considers the flow of a mixture in a flat slit channel when mixing two liquids. The first component of the mixture flows through the main wide channel. At a distance of 50 mm from the point of entry into the channel perpendicular to the main channel, there is a radial jet nozzle with a width of 1 mm, through which the second component enters the mixing channel. The article discusses several problems with different initial mixing rates of components. The characteristics corresponding to water and ethyl alcohol were taken as test components for mixing liquids. The simulation was carried out in the ANSYS-Fluent software package. The purpose of this work is to study the process of mixing two liquids in a slit channel. The article presents the results of modelling the mixing process.

Оценки равновесной температуры на поверхности сферического затупления спускаемого орбитального аппарата

The descent of a stabilized capsule from a low near-earth orbit along a ballistic trajectory is considered. The descent of a stabilized capsule from a low Earth orbit along a ballistic trajectory is considered. A reliable preliminary estimate of the intensity of the thermal impact of the air flow, which enters the surface of the capsule, is the equilibrium temperature of this surface. The value of the equilibrium temperature follows from the condition of equality of the density of the total heat flux brought to the surface and the density of the heat flux, removed due to the own radiation of this surface. Based on the peculiarities of the heat exchange process on the surface of the spherical blunt capsule three sections of the trajectory are distinguished. At free-molecular flowing of the blunt, at the section of descent the intensity of heat exchange is assumed to be proportional to the energy of the molecule flow. At the section corresponding to the continuum flow regime, the intensity of convective and radiative heat transfer is estimated by empirical dependences obtained by processing the calculated and experimental results. When combining laminar and turbulent flow modes on the attenuation surface, the highest of two local values of convective heat flux density is taken. At the intermediate section of the descent, the Knudsen number is taken as an argument determining the intensity of convective heat transfer, and empirical dependences are used to estimate the radiation component. A quantitative analysis of the change in the time of descent of the maximum value of the equilibrium temperature on the blunted surface for a ball capsule and a capsule with a ball segment was carried out. The results of such analysis are the purpose of the present work and allow to identify the most heat-stressed areas of the descent trajectory on the blunt surface at the preliminary stage of the design and to choose the acceptable materials of the thermal protection coating

Сapillatry effects manifested in the process of draining a liquid film that evaporated into a gas cross flow

The aim of the work is to determine the regularities of changes in the width of the flowing liquid film evaporating into a gas cross flow resulting from thermocapillary and concentration-capillary effects (Marangoni effect).
 The research was carried out experimentally employing an installation with a heated (working) surface, on which a liquid film (water or aqueous glycerol solution) was flowing. A flow of air was fed into the space between the working surface and the enclosing wall located at a distance from it, evenly in height and perpendicularly to the film movement. The film width was measured along the height of the working surface and the obtained dependencies were presented as graphs for different parameters of the flowing process.
 In all experiments, the graphs have an initial section with a sharp decrease in the film width, below which its change occurs smoothly. The degree of reduction in the film width depending on the parameters of the flowing process can reach 80%. In laminar mode, the reduction of the film width is more uniform than in turbulent mode, when some rise of the curves beyond the initial section is observed and can be explained by intensive mixing of the liquid. As the initial liquid flow increases, the degree of reduction in the film width decreases. Significant influence on the change in the film width is exerted by the temperature of the working surface during the flow of water: with increasing temperature, the film width decreases. At values of this temperature greater than 100℃, the rise of the curves is observed, possibly associated with the transition to volumetric evaporation.
 When the glycerol solution is flowing, the temperature of the working surface less influences the change in film width, although its decrease along the height of the working surface is greater than for water. The process parameters such as initial liquid temperature, initial solution concentration, and velocity and air temperature have relatively little effect on the change in film width. To correct the area of the film evaporation surface, determined by technological calculation, the width of the average evaporation surface is introduced under capillary effects. Its value increases with the initial liquid flow. For water, this dependence is characterized by a jump-like decrease in the film width in the transition from laminar to turbulent flow mode. This effect is absent on the dependence for glycerol solution, which can be explained by its higher viscosity. As the temperature of the working surface increases, the width of the average evaporation surface of the film decreases, some decrease being also observed with an increase in air velocity. Empirical equations for estimation of the correction factor were obtained. Capillary effects in this case are expressed by decrease in the width of the film at the height of its runoff, which leads to the need to correct the area of the film evaporation surface determined by technological calculation.

Drag Reduction Performance and Stability of an Organic Polymer, Surfactant, and Their Complexes

AbstractLong‐chained polymeric drag‐reducing agents suffer from mechanical degradation when exposed to a high‐shearing environment during the transportation of liquids in turbulent flow mode. Improving the polymer's resistance to shear forces using chemical methods is time‐consuming and associated with high costs. The drag reduction performance and stability of xanthan gum polymer, cetyltrimethylammonium bromide surfactant, and their physically formed complexes were investigated using a high‐shear rotating‐disk apparatus. The results demonstrated that all the examined additives could act as effective drag‐reducing agents, and their complexes showed modified mechanical degradation and drag reduction performances.

Supergranule aggregation for constant heat flux-driven turbulent convection

Turbulent convection processes in nature are often found to be organized in a hierarchy of plume structures and flow patterns. The gradual aggregation of convection cells or granules to a supergranule which eventually fills the whole horizontal layer is reported and analysed in spectral element direct numerical simulations of three-dimensional turbulent Rayleigh-B\'{e}nard convection at an aspect ratio of $60$. The formation proceeds over a time span of more than $10^4$ convective time units for the largest accessible Rayleigh number and occurs only when the turbulence is driven by a constant heat flux which is imposed at the bottom and top planes enclosing the convection layer. The resulting gradual inverse cascade process is observed for both temperature variance and turbulent kinetic energy. An additional analysis of the leading Lyapunov vector field for the full turbulent flow trajectory in its high-dimensional phase space demonstrates that turbulent flow modes at a certain scale continue to give rise locally to modes with longer wavelength in the turbulent case. As a consequence successively larger convection patterns grow until the horizontal extension of the layer is reached. This instability mechanism, which is known to exist near the onset of constant heat flux-driven convection, is shown here to persist into the fully developed turbulent flow regime thus connecting weakly nonlinear pattern formation with the one in fully developed turbulence. We discuss possible implications of our study for observed, but not yet consistently numerically reproducible, solar supergranulation which could lead to improved simulation models of surface convection in the Sun.

On finding a generalized analytical solution to the Hagen-Poiseuille problem for a turbulent flow mode

On finding a generalized analytical solution to the Hagen-Poiseuille problem for a turbulent flow mode

Application of the Correlation Velocity Measurements for Hydrodynamic Investigations of Turbulent Coolant Flow in Nuclear Reactor Elements

The method of correlation measurement of the coolant flow rate, widely used for operational diagnostics of nuclear power plants, can be extensively used in research practice. The aim of this work was to apply a correlation method based on the conductometric measurement system with wire-mesh sensors for measuring a coolant flow rate.Insignificant concentration of a salt solution (NaCl or Na2SO4 ) creates a gradient of the conductivity in the flow, which is used as a passive scalar measured by the system. Authors used turbulent pulsations at the interface of two concurrent flows with identical velocities in a square channel as a signal source for the correlation method. The paper presents the methodology of the tests, test facility description, signalto-noise ratio estimation, the results of digital signal processing and comparison of the measured velocities in the model with the flowrate‒averaged velocity determined by the use of flowmeters. The measured velocity values give acceptable agreement for the turbulent flow modes. It was shown that the measurement accuracy drops sharply for low-Reynolds flows.The obtained results were used for flowrate measurements in core-imitator channels of the nuclear reactor test model. The presented paper is an approbation of this approach for its application as part of an test model of a nuclear reactor in order to determine the each duct flow rates in the channels of the core simulator using wire mesh sensors.

The heat transfer of round impinging jets

Heat transfer of round impinging jets has been investigated in a wide range of Reynolds numbers (Re=40-21000). A jet flowing out of a profiled nozzle and a long tube at a constant wall temperature and at a distance to the obstacle h/d=1-20 was studied. In experiments at Re<3000 for an outflow from the tube, a noticeable increase in heat transfer by 200-400% is observed, compared with the case when the jet flows out of the nozzle. At Re>3000, the difference in heat transfer for the two variants of jet formation (from the tube and from the nozzle) decreases asymptotically. For laminar and turbulent flow modes, the results of numerical calculations are consistent with the experimental data.

Study of two positive current pulses atmospheric pressure cold plasma jet

Simple argon atmospheric pressure cold plasma jet has been designed and characterized. The spectral and electrical characteristics of the generated plasma jet were investigated. The effects of applied voltage and flow rate on plasma jet length were studied. A sinusoidal high voltage waveform was applied to ignite the plasma jet. Two current pulses were generated per each half cycle of the positive applied voltage while one current pulse was generated in the negative halves of the voltage waveforms. The time interval between the two positive current pulses decreased with increasing the applied voltage. The emission spectra of the plasma specified that the plasma included active species of O• and OH•. In addition, the estimated gas temperature of the generated plasma jet was in the range of 330 K, which nominated this jet to be used to treat heat sensitive materials. The obtained results showed that the length of the plasma jet increased by increasing any of the applied voltage or the flow rate at low values, whereas the length approached steady-state values at higher applied voltages and transient from laminar to turbulent flow mode at higher flow rates. This work is highly useful for applications of the required exposure to active species, charged ions, and UV photons at low operating temperatures and applied voltages.

Wave Structure of Liquid Films during the Transition to the Turbulent Flow Mode

The results of an experimental study of the waves on the surface of a vertically falling liquid film in the range of the Reynolds numbers of film flow 80 < Re < 420 have been presented. The experiments have been carried out with the use of the field optical diagnostic methods with high spatial and time resolution. Generation of capillary ripples with a small wavelength has been detected at the crests of large three-dimensional waves. It is in these areas of three-dimensional waves where so-called “turbulence spots,” the occurrence of which is associated with a transition to a turbulent flow mode, are observed, as noted in the literature. Long-lived rounded depressions with characteristic transverse dimensions of 1–3 mm, which are formed in the region of the interaction of capillary precursors and then propagate as independent structural elements of the flow, have also been recorded.

Viscous stress tensor and stability of laminar contravortical flows

Introduction: coaxial layers in contravortical flows rotate in the opposite directions. This determines their complicated spatial structure. The relevance of the subject is in the uniquely effective mixing of the moving medium. This property has a great potential of application from microbiology and missile building for obtaining highly dispersed mixtures to heat engineering for increasing the intensity of heat transfer. However, contravortical flows have a high degree of hydrodynamic instability. This hinders effective development of these technologies. Contravortical flows are observed behind Francis hydroturbines, whose derated operation causes modes with a significant increase of hydraulic unit vibrations up to destruction of the units. The purpose of the study is to identify physical laws of the contravortical flow hydrodynamics, common for both laminar and turbulent fluid flow modes.&#x0D; Materials and methods: theoretical analysis of the viscous stress tensor and local stability zones of contravortical laminar flows.&#x0D; Results: the article provides a mathematical description of the tensor of viscous tangential (τij) and normal (σii) stresses as well as local stability zones of the flow according to Rayleigh (Ra) and Richardson (Ri) criteria. The graphs of the radial-axial distributions of the viscous stress components are given, local stability zones are shown and the point of “vortex breakdown” is indicated. The solutions are obtained in the form of Fourier – Bessel series. The hydrodynamic structure of the flow is analysed.&#x0D; Conclusions: it is established that the most significant viscous stresses are observed at the beginning of the interaction zone of contrarotating layers. It is established that the areas with the most unstable flow are localized in the flow vortex core. Three zones can be distinguished in the vortex core: a zone of weak instability with local Richardson numbers to Ri = –1, passing into a zone of flow destabilization with high negative values of Richardson numbers Ri = –10 to –100, in turn, transforming into a zone with rapidly increasing instability up to Ri = –1000. This is a zone of loss of flow stability, culminating in the “ortex breakdown”.

Laminar and turbulent flow modes of cold atmospheric pressure argon plasma jet

Laminar and turbulent flow modes of a cold atmospheric pressure argon plasma jet are investigated in this work. The effects of the gas flow rate, applied voltage, and frequency on each plasma mode and on intermodal transitions are characterized using photographic, electrical, and spectroscopic techniques. Increasing the gas flow rate increases the plasma jet length in the laminar mode. Upon transition to the turbulent mode, increasing the gas flow rate leads to a decrease in the plasma jet length. The flow rate at which the jet transitions from laminar to turbulent increases with the applied voltage. The presence of nitric oxide (NO) radicals is indicated by the emission spectra of the turbulent plasmas only, while excited Ar, N2, OH, and O excited species are produced in both laminar and turbulent modes. With no distinctive behavior observed upon transition between the two operating modes, the power consumption was found to be insensitive to gas flow rate variation, while the energy density was found to decrease exponentially with the gas flow rate. Rotational and vibrational temperature measurements of the two plasma modes indicated that they are of the non-thermal equilibrium plasma type. Since they offer NO radicals while maintaining the benefits of the laminar plasma jet, the turbulent plasma jet is more useful than its laminar counterpart in biomedical applications.

Особенности пожаротушения в замкнутом объеме тонкораспыленной водой

Enclosure fire fighting by finely-dispersed water capability without direct water droplet flow effect from a sprayer upon a fire is shown. Fire fighting requirement droplet flow investigation experience points at successful application of Zauter’s diameter (volume of all droplets to their surface relation) while describing the results of experimental studies and theoretical calculations. With presence of enclosure eddy medium motion on account of ascending combustion product flow and interaction with a sprayer droplet flow in a finely-dispersed droplet ensemble there appear eddy diffusion, migration, deposition, and Magnus effect phenomena. With turbulent gas flow mode the particle deposition velocity on pipe and channel walls in most cases surpasses Brownian deposition velocity of the same particles in eddy-free motion. The above phenomena are typical of fine droplets up to 150 μm in Zauter’s diameter. Droplets of 200 μm and larger diameter are not subjected to eddy gas motion effect, just depositing under force of gravity and initial impulse. A particle motion in gaseous medium follows its pulsations with this or that phase or amplitude lagging, i. e. asymmetrically, but with certain velocity excess to the side of lower magnitudes of pulsating medium velocity. Fine water droplet motion study in eddying gaseous medium allows to reveal processes facilitating fire suppression in the whole enclosure without direct water droplet flow effect from a sprayer upon a fire. The experimental studies performed on enclosure Class A and B fire suppression have demonstrated enclosure fire suppression capacity by finely-dispersed water with droplets of up to 50 μm in diameter that was predicted under theoretical consideration of fine droplets motion in eddying gas medium.

Течение гелия-II в канале с пористой вставкой при безвихревом сверхтекучем движении

The heat and mass transfer processes associated with flow of superfluid helium (He-II) in a channel filled with porous medium in a certain part of its length are analyzed. The heat flux is directed along the channel axis so that a vapor plug is generated near the heater. The steady-state heat-and-mass transfer processes at the interfaces are calculated using the molecular kinetic theory methods. The motion of He-II in pores is described by equations that take into account the specific features of heat and mass transfer in a quantum fluid. A map of flow modes for normal and superfluid components for a monodisperse porous fill is plotted. The normal component’s laminar and turbulent flow modes for nonvortex superfluid motion are considered. Formulas correlating the porous insert length with the fluid velocity are derived for these modes. It is found that He-II flow toward the heater is possible when the porous insert length exceeds a certain value. The dependences of this reversible length on the porous fill structural features and heat flux density are presented. For the laminar mode of normal motion, the reversible length is determined by the diameter of monodisperse fill particles at a given temperature, whereas in the turbulent mode this length also depends on the heat flux. The influence of liquid temperature is studied. Reversible length calculation examples are given. The results of the performed calculations are analyzed by comparing them with the previously obtained data for He-II flow in a single capillary.

The influence of secondary flow in a two-phase gas-solid system in straight channels with a non-circular cross-section

The paper considers two-phase gas-solid turbulent flow of pneumatic transport in straight horizontal channels with a non-circular cross-section. During turbulent flow, a specific flow phenomenon, known as secondary flow, occurs in these channels in the cross-sectional plane. The existence of strong temperature gradients in the cross-sectional plane of the channel or the cases of curved channels result in the appearance of the secondary flow of the first kind. However, in straight channels with a non-circular cross-section, in the developed turbulent flow mode, a secondary flow, known as Prandtl?s secondary flow of the second kind, is induced. The paper presents a numerical simulation of a developed two-phase turbulent flow by using the PHOENICS 3.3.1 software package. Reynolds stress model was used to model the turbulence. The paper provides the data on the changes in turbulent stresses in the channel cross-section as well as the velocities of solid particles transported along the channel.

English

Transporting viscous liquids in pipelines is considered as one of the most energy consuming sectors in the industry due to the turbulent flow mode associated. High molecular weight polymers are effective drag reduction agents for enhancing liquid flow through pipelines, but they typically break apart (mechanical degradation) when subjected to high shear forces. Introducing a surfactant to the polymer to form a polymer-surfactant complex is a known technique to minimize the mechanical degradation. However, most available polymer-surfactant complexes formed of oppositely charged additives tend to exhibit low drag reduction performance. In the present study, we chose a different approach by investigating the drag reduction performance and mechanical degradation resistance of a similarly charged polymersurfactant complex using a rotating disk apparatus (RDA). Additionally, transmission electron microscopy (TEM) was used to gain an in-depth perspective into the polymer-surfactant complex structure. The experimental results showed that rigid complexes are successfully formed and exhibit improved drag reduction performance and mechanical degradation resistance. © 2015 Pushpa Publishing House, Allahabad, India.

The mathematical model of heat transfer in channels containing packed and granular layers

The Prandtl turbulent boundary layer model is considered for determining heat-transfer coefficients. Equations for calculating the momentumand heat-transfer coefficients in a turbulent flow mode are obtained. The boundary layer parameters on packing elements are determined. Examples of calculating the Nusselt numbers for a channel containing fine packing elements in a wide range of Reynolds number are presented. A comparison with the results obtained by other researchers is given.

Turbulent flow mechanisms in mixing T-junctions by Large Eddy Simulations

We consider the turbulent mixing process in two T-junction geometries as simplified models for mixing in the intake manifolds of Internal Combustion (IC) engines. These junctions have square and circular cross-sections, respectively. The turbulent flow structures and modes are analyzed by Large Eddy Simulations (LES). A grid sensitivity study is performed and the velocity field and the mixing scalar are compared to experimental data. The agreement is good for high enough mesh resolutions. Furthermore, the LES results are compared to unsteady Reynolds averaged Navier–Stokes (URANS) results, in order to gain an understanding of the shortcomings associated with URANS. The secondary structures found in both geometries include Dean-like vortices due to flow curvature in the region of the junction. Further downstream of the junction, these vortices are dissipated and due to an upward motion of the bulk flow, new vortical structures are generated. These downstream vortical structures rotate in the opposite direction relative to the upstream ones and govern the mean scalar distribution far downstream of the junction. We find also that the URANS results show qualitatively different flow structures leading to different scalar distributions as compared to experimental and LES results. The mixing quality is studied using a uniformity index showing a more uniform and faster mixing in the circular cross-section case. Spectral analysis of the LES data show for both geometries a shear layer instability with a dimensionless frequency in the order of unity. Additionally to that, vortex-shedding phenomena are observed in the circular case at St≈0.5.

Relaminarization in supersonic microjets at low Reynolds numbers

Results of measuring the length of the supersonic portion of the air jets that flow out of axisymmetric sonic nozzles 10.4 μm-1 mm in diameter are presented. The measurements are carried out in a range of degree of jet noncalculation of 1–30 and in a wide Reynolds number range, including the laminar and turbulent flow modes. It is shown that the Reynolds number calculated from the nozzle diameter and the outlet parameters of gas is the parameter that governs jet flow. It is found that, for a laminar jet mixing layer, the length of the supersonic portion sharply increases. When the jet mixing layer becomes turbulent, the length of the supersonic portion decreases. The effect of increasing the length of the supersonic portion after its decrease due to the turbulization of flow in a jet and a growth in the Reynolds number is first discovered.